diff --git a/docs/content/limitations/builtin-apis/streams-and-readline.mdx b/docs/content/limitations/builtin-apis/streams-and-readline.mdx
index f9d69e3db..561840268 100644
--- a/docs/content/limitations/builtin-apis/streams-and-readline.mdx
+++ b/docs/content/limitations/builtin-apis/streams-and-readline.mdx
@@ -3,7 +3,8 @@
   `readable.pipe(process.stdout)` and `readable.pipe(process.stderr)` compile and, as in Node, do not end the process stream.
   When an untyped value reaches `NodeJS.ReadableStream` or `NodeJS.WritableStream`, those two interfaces hold any value, so JavaScript stream objects (constructor-function streams, plain objects with `on` and `pipe`) pass through TypeScript functions typed with them and come back to JavaScript as the same objects.
   Member calls on such a value use the native stream when it is one and call the object's own members otherwise; `readable.pipe()` into a JavaScript destination follows Node's pipe protocol.
-  A `pipeline` stage and a `for await` source that arrive as untyped values must be native streams, or `Cipheriv` and `Decipheriv` values, at run time; other values throw a not-supported error.
+  The first `pipeline` stage may arrive as an untyped value that is a native stream, a `Cipheriv` or `Decipheriv` value, or an iterable: a string, an array, a generator object, or any value with a `Symbol.asyncIterator` or `Symbol.iterator` method, which the pipeline pumps into the next stage as Node does.
+  Later `pipeline` stages and `for await` sources that arrive as untyped values must be native streams, or `Cipheriv` and `Decipheriv` values, at run time; other values throw a not-supported error.
   Native streams expose no `emit` to JavaScript, so a JavaScript stream whose own `pipe()` emits on its destination (the legacy `Stream` pipe) cannot pipe into one.
   `Readable.from` accepts strings, Buffers, arrays, Sets, Maps, Map and Set iterators, generators, async generators, and class iterables and async iterables, and in JavaScript any iterable or async iterable.
   In JavaScript, a sync iterator value with a `then` method that is not a promise, and an object whose `[Symbol.asyncIterator]` or `[Symbol.iterator]` method is a generator method, are unsupported.
@@ -14,7 +15,7 @@
   `node:stream/consumers` and readline interfaces read object-mode streams of strings and Buffers; other values throw a not-supported error.
   `pipeline` accepts function stages anywhere among its streams: async generator functions (`async function* (source)`, iterating `source` with `for await`), an async generator source function (`async function* ({ signal })`), an async generator function as the destination, and an async function destination whose value resolves the promise form and reaches the callback's second argument.
   A stage is written inline, or named by a function declaration or a `const` bound to a function expression that nothing reassigns.
-  The source may also be an array, a string, a synchronous generator object, or a synchronous generator function without parameters; their elements reach the next stage on the same ticks as in Node.
+  The source may also be an array, a string, a `Set`, a `Map`, a generator or async generator object, or a synchronous generator function without parameters; their elements reach the next stage on the same ticks as in Node.
   Stages may take the `{ signal }` argument.
   With a function destination, `pipeline()` returns an object-mode PassThrough that receives an async generator destination's values, or an async function destination's value unless it is `null` or `undefined`.
   `@types/node` types that result as `NodeJS.WritableStream`, so `for await` over it needs the `PassThrough` type (`pipeline(...) as PassThrough`, or a JSDoc `@type`).
diff --git a/native/llvm-codegen/src/emit.cpp b/native/llvm-codegen/src/emit.cpp
index 9cae164d3..14bcd1522 100644
--- a/native/llvm-codegen/src/emit.cpp
+++ b/native/llvm-codegen/src/emit.cpp
@@ -19,6 +19,7 @@
 #include "llvm/CodeGen/MachineFunction.h"
 #include "llvm/CodeGen/MachineFunctionPass.h"
 #include "llvm/CodeGen/MachineModuleInfo.h"
+#include "llvm/IR/DiagnosticHandler.h"
 #include "llvm/IR/DiagnosticInfo.h"
 #include "llvm/IR/DiagnosticPrinter.h"
 #include "llvm/IR/LegacyPassManager.h"
@@ -41,11 +42,13 @@
 #include "llvm/Support/SourceMgr.h"
 #include "llvm/Support/ThreadPool.h"
 #include "llvm/Support/Threading.h"
+#include "llvm/Support/JSON.h"
 #include "llvm/Support/raw_ostream.h"
 #include "llvm/Target/TargetMachine.h"
 #include "llvm/TargetParser/Triple.h"
 #include "llvm/Transforms/Utils/SplitModule.h"
 
+#include <atomic>
 #include <mutex>
 #include <optional>
 #include <string>
@@ -170,6 +173,108 @@ void limitProfileImports() {
   (void)Limited;
 }
 
+// How the --profile-use profile matched the functions it annotated, summed
+// over partitions: functions that received its counts, functions whose
+// control flow changed since it was collected (hash mismatch), and functions
+// it has no record of (new, or never run in training: the merged profile is
+// sparse). A partition reused from the cache without its counts is counted
+// in Unknown.
+struct ProfileMatch {
+  std::atomic<uint64_t> Annotated{0};
+  std::atomic<uint64_t> Mismatched{0};
+  std::atomic<uint64_t> Missing{0};
+  std::atomic<uint64_t> Unknown{0};
+
+  void add(const ProfileMatch &Other) {
+    Annotated += Other.Annotated;
+    Mismatched += Other.Mismatched;
+    Missing += Other.Missing;
+  }
+
+  std::string serialize() const {
+    return formatv("{0} {1} {2}", Annotated.load(), Mismatched.load(),
+                   Missing.load())
+        .str();
+  }
+
+  bool deserialize(StringRef Text) {
+    SmallVector<StringRef, 3> Parts;
+    Text.split(Parts, ' ');
+    uint64_t Values[3];
+    if (Parts.size() != 3)
+      return false;
+    for (size_t I = 0; I < 3; ++I)
+      if (Parts[I].getAsInteger(10, Values[I]))
+        return false;
+    Annotated += Values[0];
+    Mismatched += Values[1];
+    Missing += Values[2];
+    return true;
+  }
+};
+
+// Counts the profile annotation's hash-mismatch and missing-function
+// warnings instead of printing them. Every other diagnostic takes the
+// context's default handling.
+class ProfileMatchHandler : public DiagnosticHandler {
+public:
+  explicit ProfileMatchHandler(ProfileMatch &Match) : Match(Match) {}
+
+  bool handleDiagnostics(const DiagnosticInfo &Info) override {
+    if (Info.getKind() != DK_PGOProfile)
+      return false;
+    std::string Message;
+    raw_string_ostream Stream(Message);
+    DiagnosticPrinterRawOStream Printer(Stream);
+    Info.print(Printer);
+    if (StringRef(Message).contains("hash mismatch"))
+      ++Match.Mismatched;
+    else if (StringRef(Message).contains_insensitive("no profile data"))
+      ++Match.Missing;
+    else
+      return false;
+    return true;
+  }
+
+private:
+  ProfileMatch &Match;
+};
+
+// The annotation warns about functions the profile does not describe only
+// when asked; the match counts need those warnings. Process-wide, set once.
+void reportMissingProfiles() {
+  static const bool Reported = [] {
+    if (cl::Option *Option =
+            cl::getRegisteredOptions().lookup("pgo-warn-missing-function"))
+      Option->addOccurrence(0, "pgo-warn-missing-function", "true");
+    return true;
+  }();
+  (void)Reported;
+}
+
+// Functions of `M` that the profile annotated with counts.
+uint64_t annotatedFunctions(const Module &M) {
+  uint64_t Count = 0;
+  for (const Function &F : M) {
+    if (F.isDeclaration())
+      continue;
+    std::optional<Function::ProfileCount> Entry = F.getEntryCount();
+    if (Entry && !Entry->isSynthetic())
+      ++Count;
+  }
+  return Count;
+}
+
+// Prints the profile match as the successful emit's one line of JSON.
+void reportProfileMatch(const ProfileMatch &Match) {
+  json::Object Profile{{"annotated", Match.Annotated.load()},
+                       {"mismatched", Match.Mismatched.load()},
+                       {"missing", Match.Missing.load()},
+                       {"unknown_partitions", Match.Unknown.load()}};
+  json::Object Response{{"ok", true}, {"profile", std::move(Profile)}};
+  outs() << formatv("{0}\n", json::Value(std::move(Response)));
+}
+
 // Reads the profile's header once, before any partition needs it: it must
 // hold IR-level counts.
 std::optional<EmitFailure> inspectProfile(const EmitOptions &Options) {
@@ -191,6 +296,7 @@ std::optional<EmitFailure> inspectProfile(const EmitOptions &Options) {
 
 // Owns the analysis managers a pipeline needs for one module.
 struct Pipeline {
+  PassInstrumentationCallbacks Callbacks;
   LoopAnalysisManager LAM;
   FunctionAnalysisManager FAM;
   CGSCCAnalysisManager CGAM;
@@ -199,13 +305,26 @@ struct Pipeline {
 
   Pipeline(TargetMachine &Machine, OptimizationLevel Level,
            std::optional<PGOOptions> Profile = std::nullopt)
-      : PB(&Machine, tuningFor(Level), std::move(Profile)) {
+      : PB(&Machine, tuningFor(Level), std::move(Profile), &Callbacks) {
     PB.registerModuleAnalyses(MAM);
     PB.registerCGSCCAnalyses(CGAM);
     PB.registerFunctionAnalyses(FAM);
     PB.registerLoopAnalyses(LAM);
     PB.crossRegisterProxies(LAM, FAM, CGAM, MAM);
   }
+
+  // Adds to Count the functions the profile annotation gave counts to, as
+  // soon as that pass has run: inlining and dead code elimination later in
+  // the pipeline delete annotated functions.
+  void countAnnotated(std::atomic<uint64_t> &Count) {
+    Callbacks.registerAfterPassCallback(
+        [&Count](StringRef PassID, Any IR, const PreservedAnalyses &) {
+          if (PassID != "PGOInstrumentationUse")
+            return;
+          if (const auto *M = llvm::any_cast<const Module *>(&IR))
+            Count += annotatedFunctions(**M);
+        });
+  }
 };
 
 std::optional<EmitFailure> verify(Module &M, StringRef Code) {
@@ -387,8 +506,15 @@ std::optional<EmitFailure> publish(StringRef TemporaryPath,
 std::optional<EmitFailure>
 emitModule(Module &M, TargetMachine &Machine, OptimizationLevel Level,
            std::optional<PGOOptions> Profile, CodeGenFileType Type,
-           StringRef OutputPath, std::vector<SmallString<256>> &Temporaries) {
+           StringRef OutputPath, std::vector<SmallString<256>> &Temporaries,
+           ProfileMatch &Match) {
+  bool Annotating = Profile && Profile->Action == PGOOptions::IRUse;
+  if (Annotating)
+    M.getContext().setDiagnosticHandler(
+        std::make_unique<ProfileMatchHandler>(Match));
   Pipeline P(Machine, Level, std::move(Profile));
+  if (Annotating)
+    P.countAnnotated(Match.Annotated);
   P.PB.buildPerModuleDefaultPipeline(Level).run(M, P.MAM);
   if (std::optional<EmitFailure> Failure =
           verify(M, "post_optimization_verification_failed"))
@@ -555,20 +681,32 @@ std::optional<EmitFailure>
 compileOptimizedPartitions(const EmitOptions &Options,
                            const PartitionPlan &Plan, OptimizationLevel Level,
                            StringRef Salt, const ArtifactCache &Cache,
-                           std::vector<SmallVector<char, 0>> &Objects) {
+                           std::vector<SmallVector<char, 0>> &Objects,
+                           ProfileMatch &Match) {
   size_t Count = Plan.Modules.size();
+  bool Annotating = !Options.ProfileUse.empty();
   std::vector<std::unique_ptr<MemoryBuffer>> Summaries(Count);
   SharedFailure Failures;
   forEachPartition(Count, [&](size_t I) {
     std::string Key = hashParts({Salt, "summary", Plan.Modules[I]});
-    if ((Summaries[I] = Cache.lookup(Key, "bc")))
+    if ((Summaries[I] = Cache.lookup(Key, "bc"))) {
+      if (Annotating) {
+        std::unique_ptr<MemoryBuffer> Counts = Cache.lookup(Key, "match");
+        if (!Counts || !Match.deserialize(Counts->getBuffer()))
+          ++Match.Unknown;
+      }
       return;
+    }
     std::string LookupError;
     std::unique_ptr<TargetMachine> Machine =
         createTargetMachine(Options.Target, Options.OptLevel, LookupError);
     if (!Machine)
       return Failures.set({"target_machine_failed", LookupError});
     LLVMContext Context;
+    ProfileMatch PartitionMatch;
+    if (Annotating)
+      Context.setDiagnosticHandler(
+          std::make_unique<ProfileMatchHandler>(PartitionMatch));
     Expected<std::unique_ptr<Module>> Loaded =
         loadPartition(Options, Plan.Modules[I], I, Context, *Machine);
     if (!Loaded)
@@ -592,12 +730,19 @@ compileOptimizedPartitions(const EmitOptions &Options,
       // Profile instrumentation or annotation happens here, before the thin
       // link; the post-link optimization reads the annotated weights.
       Pipeline P(*Machine, Level, profileOptions(Options));
+      if (Annotating)
+        P.countAnnotated(PartitionMatch.Annotated);
       ModulePassManager MPM = P.PB.buildThinLTOPreLinkDefaultPipeline(Level);
       MPM.addPass(BitcodeWriterPass(Stream, /*ShouldPreserveUseListOrder=*/false,
                                     /*EmitSummaryIndex=*/true,
                                     /*EmitModuleHash=*/true));
       MPM.run(Mod, P.MAM);
     }
+    if (Annotating) {
+      Match.add(PartitionMatch);
+      // Stored before the summary, so a reused summary finds its counts.
+      Cache.store(Key, "match", PartitionMatch.serialize());
+    }
     StringRef Contents(Bitcode.data(), Bitcode.size());
     Cache.store(Key, "bc", Contents);
     Summaries[I] = MemoryBuffer::getMemBufferCopy(Contents);
@@ -744,7 +889,8 @@ compileOptimizedPartitions(const EmitOptions &Options,
 
 std::optional<EmitFailure>
 compilePartitions(const EmitOptions &Options, const PartitionPlan &Plan,
-                  std::vector<SmallVector<char, 0>> &Objects) {
+                  std::vector<SmallVector<char, 0>> &Objects,
+                  ProfileMatch &Match) {
   // Everything besides a partition's own bytes that determines its
   // artifacts. The caller's cache directory already identifies the exact
   // helper build; source content and partition count arrive per module.
@@ -778,7 +924,7 @@ compilePartitions(const EmitOptions &Options, const PartitionPlan &Plan,
     return compileUnoptimizedPartitions(Options, Plan, Salt, Cache, Objects);
   return compileOptimizedPartitions(Options, Plan,
                                     optimizationLevel(Options.OptLevel), Salt,
-                                    Cache, Objects);
+                                    Cache, Objects, Match);
 }
 
 // Publishes in-memory partition objects through private sibling files.
@@ -819,6 +965,8 @@ int emitPartitions(const EmitOptions &Options) {
   std::vector<SmallVector<char, 0>> Objects;
   std::optional<EmitFailure> Failure;
   bool Compiled = false;
+  // A failed attempt's counts are discarded with its objects.
+  auto Match = std::make_unique<ProfileMatch>();
   {
     ErrorOr<std::unique_ptr<MemoryBuffer>> Input =
         MemoryBuffer::getFile(Options.Input, /*IsText=*/false,
@@ -830,15 +978,16 @@ int emitPartitions(const EmitOptions &Options) {
         Input->reset();
         PartitionPlan Plan{std::move(Text->Sources),
                            std::move(Text->PublicSymbols)};
-        Compiled = !compilePartitions(Options, Plan, Objects);
+        Compiled = !compilePartitions(Options, Plan, Objects, *Match);
       }
     }
   }
   if (!Compiled) {
+    Match = std::make_unique<ProfileMatch>();
     PartitionPlan Plan;
     Failure = splitParsedModule(Options, Plan);
     if (!Failure)
-      Failure = compilePartitions(Options, Plan, Objects);
+      Failure = compilePartitions(Options, Plan, Objects, *Match);
   }
   std::vector<SmallString<256>> Temporaries;
   if (!Failure)
@@ -851,6 +1000,8 @@ int emitPartitions(const EmitOptions &Options) {
     return reportError(Failure->Code, Failure->Message,
                        Options.DiagnosticFormat);
   }
+  if (!Options.ProfileUse.empty())
+    reportProfileMatch(*Match);
   return 0;
 }
 
@@ -895,6 +1046,8 @@ int emit(const EmitOptions &Options) {
     return reportError("invalid_import",
                        "runtime bitcode import requires an optimized build",
                        Options.DiagnosticFormat);
+  if (!Options.ProfileUse.empty())
+    reportMissingProfiles();
   if (std::optional<EmitFailure> Failure = inspectProfile(Options))
     return reportError(Failure->Code, Failure->Message,
                        Options.DiagnosticFormat);
@@ -957,9 +1110,10 @@ int emit(const EmitOptions &Options) {
                              ? CodeGenFileType::ObjectFile
                              : CodeGenFileType::AssemblyFile;
   std::vector<SmallString<256>> Temporaries;
+  ProfileMatch Match;
   std::optional<EmitFailure> Failure =
       emitModule(*Mod, *Machine, Level, profileOptions(Options), Type,
-                 Options.Outputs[0], Temporaries);
+                 Options.Outputs[0], Temporaries, Match);
   if (!Failure)
     Failure = publish(Temporaries[0], Options.Outputs[0]);
   if (Failure) {
@@ -968,6 +1122,8 @@ int emit(const EmitOptions &Options) {
     return reportError(Failure->Code, Failure->Message,
                        Options.DiagnosticFormat);
   }
+  if (!Options.ProfileUse.empty())
+    reportProfileMatch(Match);
   return 0;
 }
 
diff --git a/packages/cli/test/native-link-info.test.ts b/packages/cli/test/native-link-info.test.ts
index 8ac579449..5e862e126 100644
--- a/packages/cli/test/native-link-info.test.ts
+++ b/packages/cli/test/native-link-info.test.ts
@@ -72,7 +72,7 @@ describe.runIf(supported)("macOS arm64 native link info", () => {
         minimum_os: "14.0",
       },
       program: { object, entry_symbol: "main" },
-      runtime_abi: { version: 37, marker: "scr_runtime_abi_v37" },
+      runtime_abi: { version: 42, marker: "scr_runtime_abi_v42" },
       runtime_pack: {
         kind: "precompiled",
         package: "@scriptc/runtime-darwin-arm64",
@@ -165,6 +165,6 @@ describe.runIf(supported)("macOS arm64 native link info", () => {
       () => null,
       (failure: { stderr?: string }) => failure,
     );
-    expect(error?.stderr).toContain("scr_runtime_abi_v37");
+    expect(error?.stderr).toContain("scr_runtime_abi_v42");
   });
 });
diff --git a/packages/compiler/ambient/scriptc-overrides.d.ts b/packages/compiler/ambient/scriptc-overrides.d.ts
index 2dafbcdf0..376eba9d5 100644
--- a/packages/compiler/ambient/scriptc-overrides.d.ts
+++ b/packages/compiler/ambient/scriptc-overrides.d.ts
@@ -70,3 +70,24 @@ interface PromiseConstructor {
     reject: (reason: Error) => void;
   };
 }
+
+/* Carriers of the types the lowerer gives untyped JavaScript collections
+ * whose uses prove them (js-collection-types.ts). Nothing reads them at run
+ * time. */
+declare namespace __scriptcInferredCollections {
+  const map_string_string: Map<string, string>;
+  const map_string_number: Map<string, number>;
+  const map_string_boolean: Map<string, boolean>;
+  const map_number_string: Map<number, string>;
+  const map_number_number: Map<number, number>;
+  const map_number_boolean: Map<number, boolean>;
+  const map_boolean_string: Map<boolean, string>;
+  const map_boolean_number: Map<boolean, number>;
+  const map_boolean_boolean: Map<boolean, boolean>;
+  const set_string: Set<string>;
+  const set_number: Set<number>;
+  const set_boolean: Set<boolean>;
+  const array_string: string[];
+  const array_number: number[];
+  const array_boolean: boolean[];
+}
diff --git a/packages/compiler/src/backend/llvm/emitter.ts b/packages/compiler/src/backend/llvm/emitter.ts
index 7c4a8aed1..1fcb1b803 100644
--- a/packages/compiler/src/backend/llvm/emitter.ts
+++ b/packages/compiler/src/backend/llvm/emitter.ts
@@ -81,6 +81,7 @@ import {
 import { specializeNumericCalls } from "../../ir/numeric-call-specialization.js";
 import { versionLoopBoundParameters } from "../../ir/parameter-versions.js";
 import { inlineExpressionFunctions } from "../../ir/expression-inlining.js";
+import { pairRowSums } from "../../ir/row-pairs.js";
 import {
   allocatedLiteral,
   fillAllocationLength,
@@ -582,14 +583,16 @@ function moduleEmitter(input: IrModule, options: LlvmTargetOptions): LlEmitter {
                 scalarizeNumericRecords(
                   specializeCallbackCalls(
                     specializeNumericCalls(
-                      inlineExpressionFunctions(
-                        foldConstantGlobals(
-                          copyStableCaptures(
-                            resolveSelfCaptures(
-                              hoistFinalFieldReads(
-                                freezeInitOnlyGlobals(
-                                  holdCharLocalsAsUnits(
-                                    dropUnwrittenPropertyBags(runPairedWhileLoopsAsFor(mod)),
+                      pairRowSums(
+                        inlineExpressionFunctions(
+                          foldConstantGlobals(
+                            copyStableCaptures(
+                              resolveSelfCaptures(
+                                hoistFinalFieldReads(
+                                  freezeInitOnlyGlobals(
+                                    holdCharLocalsAsUnits(
+                                      dropUnwrittenPropertyBags(runPairedWhileLoopsAsFor(mod)),
+                                    ),
                                   ),
                                 ),
                               ),
diff --git a/packages/compiler/src/backend/llvm/lib-dispatch.ts b/packages/compiler/src/backend/llvm/lib-dispatch.ts
index d8ca5add8..95cee3860 100644
--- a/packages/compiler/src/backend/llvm/lib-dispatch.ts
+++ b/packages/compiler/src/backend/llvm/lib-dispatch.ts
@@ -10,7 +10,7 @@ import { FN_ATTRS, arrNewCall, traceArg, vAdapters } from "./shapes.js";
 import { InternalCompilerError } from "../../errors.js";
 
 import { MAY_THROW_LIB_FNS } from "../../ir/builtin-effects.js";
-import { F64, type IrExpr, type IrType } from "../../ir/ir.js";
+import { BYTES_ELEMENT_SIZE, F64, type IrExpr, type IrType } from "../../ir/ir.js";
 import { LlvmUnsupportedError } from "./unsupported.js";
 import { emitJsLibCall } from "./jsval.js";
 import type { LlvmEmitterContext, LibCallExpr, LibCallPrefix, LlValue } from "./expr-context.js";
@@ -936,6 +936,57 @@ export function emitLibCall(host: LlvmEmitterContext, e: LibCallExpr): LlValue {
     return { name: t, type: e.type };
   }
   if (e.fn === "bytes.absent") return host.own({ name: "null", type: e.type });
+  if (e.fn === "bytes.disjoint") {
+    // [data, data + len * size) of each view, compared as addresses. A view
+    // that is not there (a binding the nest never assigned, NULL) answers
+    // false without reading its header, so the caller keeps its loop.
+    const B = host.B;
+    const values = e.args.map((arg) => ({ arg, value: host.emitReadReceiver(arg) }));
+    const present = values.map(({ value }) => {
+      const t = B.tmp();
+      B.line(`${t} = icmp ne ptr ${value.name}, null`);
+      return t;
+    });
+    const both = B.tmp();
+    B.line(`${both} = and i1 ${present[0]}, ${present[1]}`);
+    const compare = B.newLabel("disjoint.compare"),
+      done = B.newLabel("disjoint.done");
+    const from = B.currentLabel();
+    B.condBr(both, compare, done);
+    B.startBlock(compare);
+    const range = (arg: IrExpr, pointer: string): { start: string; end: string } => {
+      const size = arg.type.kind === "bytes" ? BYTES_ELEMENT_SIZE[arg.type.elem] : 1;
+      const dataSlot = B.tmp(),
+        data = B.tmp(),
+        lenSlot = B.tmp(),
+        len = B.tmp(),
+        bytes = B.tmp(),
+        start = B.tmp(),
+        end = B.tmp();
+      B.line(`${dataSlot} = getelementptr inbounds %ScrBytes, ptr ${pointer}, i64 0, i32 3`);
+      B.line(`${data} = load ptr, ptr ${dataSlot}`);
+      B.line(`${lenSlot} = getelementptr inbounds %ScrBytes, ptr ${pointer}, i64 0, i32 1`);
+      B.line(`${len} = load ${host.sizeType}, ptr ${lenSlot}`);
+      B.line(`${bytes} = mul ${host.sizeType} ${len}, ${size}`);
+      B.line(`${start} = ptrtoint ptr ${data} to ${host.sizeType}`);
+      B.line(`${end} = add ${host.sizeType} ${start}, ${bytes}`);
+      return { start, end };
+    };
+    const left = range(values[0]!.arg, values[0]!.value.name),
+      right = range(values[1]!.arg, values[1]!.value.name);
+    const before = B.tmp(),
+      after = B.tmp(),
+      apart = B.tmp(),
+      t = B.tmp();
+    B.line(`${before} = icmp ule ${host.sizeType} ${left.end}, ${right.start}`);
+    B.line(`${after} = icmp ule ${host.sizeType} ${right.end}, ${left.start}`);
+    B.line(`${apart} = or i1 ${before}, ${after}`);
+    const compared = B.currentLabel();
+    B.br(done);
+    B.startBlock(done);
+    B.line(`${t} = phi i1 [ false, %${from} ], [ ${apart}, %${compared} ]`);
+    return { name: t, type: e.type };
+  }
   if (e.fn === "record.isFrozen") {
     // The frozen bit of the record's rc word (scr_rc.h SCR_RC_FROZEN) is
     // its second-highest bit; an immortal word sets the highest one too.
diff --git a/packages/compiler/src/backend/llvm/lib-shared.ts b/packages/compiler/src/backend/llvm/lib-shared.ts
index de3a56c3a..5f6c56e28 100644
--- a/packages/compiler/src/backend/llvm/lib-shared.ts
+++ b/packages/compiler/src/backend/llvm/lib-shared.ts
@@ -802,6 +802,7 @@ export const LIB_FN_SYMS: Record<string, string> = {
   "procStream.write": "scr_proc_stream_write",
   "stream.stdioWritable": "scr_stream_stdio_writable",
   "stream.unboxDyn": "scr_stream_unbox_dyn",
+  "stream.dynIterableSource": "scr_stream_dyn_iterable_source",
   "stream.handleStream": "scr_stream_handle",
   "stream.pipeDyn": "scr_stream_pipe_dyn",
   "stream.unpipeDyn": "scr_stream_unpipe_dyn",
diff --git a/packages/compiler/src/backend/native-codegen.ts b/packages/compiler/src/backend/native-codegen.ts
index 5b91c60e7..413fb7e02 100644
--- a/packages/compiler/src/backend/native-codegen.ts
+++ b/packages/compiler/src/backend/native-codegen.ts
@@ -30,6 +30,8 @@ import { compilerReleaseVersion } from "../library/sidecar.js";
 import {
   nativeProfileHelperArgs,
   nativeProfileKey,
+  parseNativeProfileMatch,
+  type NativeProfileMatch,
   type NativeProfileOptions,
 } from "./native-profile.js";
 import { writeTextParts } from "../text-parts.js";
@@ -69,6 +71,8 @@ export interface NativeCodegenArtifact {
   dependencies: NativeArtifactDependency[];
   /** Every emitted file, starting with the requested output path. */
   outputPaths: string[];
+  /** How the build's profile matched, when it used one and the helper ran. */
+  profileMatch?: NativeProfileMatch;
 }
 
 const resolvedHelperCache = new Map<string, Promise<ResolvedHelper>>();
@@ -458,7 +462,7 @@ export async function emitNativeArtifact(
   try {
     if (isLlvmFile(options.llvm)) await copyFile(options.llvm.file, input);
     else await writeLlvmSource(input, options.llvm, 0o600);
-    await invoke(helper.binaryPath, [
+    const { stdout } = await invoke(helper.binaryPath, [
       "emit",
       "--input",
       input,
@@ -502,7 +506,9 @@ export async function emitNativeArtifact(
     for (let index = 0; index < stages.length; index++)
       await rename(stages[index]!, outputs[index]!);
     await pruneBuildCache(root);
-    return artifact;
+    const profileMatch =
+      options.profile?.use === undefined ? null : parseNativeProfileMatch(stdout);
+    return profileMatch === null ? artifact : { ...artifact, profileMatch };
   } finally {
     await Promise.all([
       ...stages.map((stage) => rm(stage, { force: true }).catch(() => undefined)),
diff --git a/packages/compiler/src/backend/native-profile.test.ts b/packages/compiler/src/backend/native-profile.test.ts
index 590c39eef..b2cb0c75d 100644
--- a/packages/compiler/src/backend/native-profile.test.ts
+++ b/packages/compiler/src/backend/native-profile.test.ts
@@ -1,6 +1,7 @@
 import { expect, test } from "vitest";
 import { executableLinkInputs } from "./link-plan-core.js";
 import {
+  parseNativeProfileMatch,
   nativeProfileHelperArgs,
   nativeProfileKey,
   nativeProfileRefusal,
@@ -71,3 +72,15 @@ test("only the instrumented link adds the profile runtime", () => {
     "-fprofile-generate",
   );
 });
+
+test("the helper's profile match is read from its output", () => {
+  expect(
+    parseNativeProfileMatch(
+      '{"ok":true,"profile":{"annotated":120,"mismatched":7,"missing":40,"unknown_partitions":1}}\n',
+    ),
+  ).toEqual({ annotated: 120, mismatched: 7, missing: 40, unknownPartitions: 1 });
+  // A helper that predates the report prints nothing.
+  expect(parseNativeProfileMatch("")).toBeNull();
+  expect(parseNativeProfileMatch('{"ok":true}\n')).toBeNull();
+  expect(parseNativeProfileMatch('{"ok":true,"profile":{"annotated":-1}}\n')).toBeNull();
+});
diff --git a/packages/compiler/src/backend/native-profile.ts b/packages/compiler/src/backend/native-profile.ts
index 4cd766319..904994a7d 100644
--- a/packages/compiler/src/backend/native-profile.ts
+++ b/packages/compiler/src/backend/native-profile.ts
@@ -44,6 +44,45 @@ export function nativeProfileHelperArgs(profile: NativeProfileOptions | undefine
   return [];
 }
 
+/** How an optimized build's profile matched the program's functions, as the
+ * helper reports it: functions that took the profile's counts, functions
+ * whose control flow changed since the profile was collected, functions the
+ * profile has no record of (new, or never run while it was collected), and
+ * partitions reused from the cache without their counts. */
+export interface NativeProfileMatch {
+  annotated: number;
+  mismatched: number;
+  missing: number;
+  unknownPartitions: number;
+}
+
+/** The profile match in a successful helper emit's output, or null when the
+ * helper reports none (a helper that predates the report). */
+export function parseNativeProfileMatch(stdout: string): NativeProfileMatch | null {
+  for (const line of stdout.split("\n")) {
+    let parsed: unknown;
+    try {
+      parsed = JSON.parse(line);
+    } catch {
+      continue;
+    }
+    const profile = (parsed as { profile?: Record<string, unknown> } | null)?.profile;
+    if (typeof profile !== "object" || profile === null) continue;
+    const count = (key: string): number | null => {
+      const value = profile[key];
+      return typeof value === "number" && Number.isInteger(value) && value >= 0 ? value : null;
+    };
+    const annotated = count("annotated");
+    const mismatched = count("mismatched");
+    const missing = count("missing");
+    const unknownPartitions = count("unknown_partitions");
+    if (annotated === null || mismatched === null || missing === null || unknownPartitions === null)
+      continue;
+    return { annotated, mismatched, missing, unknownPartitions };
+  }
+  return null;
+}
+
 /** Cache-key part; empty without a profile so other keys are unchanged. */
 export function nativeProfileKey(profile: NativeProfileOptions | undefined): string {
   if (profile?.generate !== undefined) return `profile-generate:${profile.generate}`;
diff --git a/packages/compiler/src/backend/runtime-abi.ts b/packages/compiler/src/backend/runtime-abi.ts
index 11bde6c2b..0eda18a5d 100644
--- a/packages/compiler/src/backend/runtime-abi.ts
+++ b/packages/compiler/src/backend/runtime-abi.ts
@@ -2,8 +2,8 @@
  * of the link contract: a program object keeps this symbol undefined and the
  * matching runtime defines it, so incompatible manual links fail before the
  * program can start. */
-export const RUNTIME_ABI_VERSION = 37 as const;
-export const RUNTIME_ABI_MARKER = "scr_runtime_abi_v37" as const;
+export const RUNTIME_ABI_VERSION = 42 as const;
+export const RUNTIME_ABI_MARKER = "scr_runtime_abi_v42" as const;
 
 /** Object consumption is useful today, but the complete scr_* surface may
  * still change before 1.0. The versioned marker prevents accidental mixing;
diff --git a/packages/compiler/src/backend/runtime-pack-targets.test.ts b/packages/compiler/src/backend/runtime-pack-targets.test.ts
index 9846e8d89..6f39c6d08 100644
--- a/packages/compiler/src/backend/runtime-pack-targets.test.ts
+++ b/packages/compiler/src/backend/runtime-pack-targets.test.ts
@@ -52,7 +52,7 @@ async function fixture(target: NativeTargetSpec, workers = false) {
       object_format: target.objectFormat,
       minimum_os: target.minimumOs,
     },
-    runtime_abi: { version: 37, marker: "scr_runtime_abi_v37" },
+    runtime_abi: { version: 42, marker: "scr_runtime_abi_v42" },
     compiler: { command: "fixture", identity: "fixture", target: target.llvmTriple },
     macros: { executable: [], excluded: ["SCR_LIB"], sanitizer: "external-toolchain-required" },
     flavors: {
diff --git a/packages/compiler/src/backend/runtime-pack.test.ts b/packages/compiler/src/backend/runtime-pack.test.ts
index e2b24f8ca..4561fad54 100644
--- a/packages/compiler/src/backend/runtime-pack.test.ts
+++ b/packages/compiler/src/backend/runtime-pack.test.ts
@@ -128,7 +128,7 @@ async function fixture(target: NativeTargetSpec = MACOS_ARM64_TARGET) {
       object_format: target.objectFormat,
       minimum_os: target.minimumOs,
     },
-    runtime_abi: { version: 37, marker: "scr_runtime_abi_v37" },
+    runtime_abi: { version: 42, marker: "scr_runtime_abi_v42" },
     compiler: {
       command: "clang",
       identity: "fixture clang",
diff --git a/packages/compiler/src/compile-types.ts b/packages/compiler/src/compile-types.ts
index c50931c6a..2a5cea5ea 100644
--- a/packages/compiler/src/compile-types.ts
+++ b/packages/compiler/src/compile-types.ts
@@ -2,6 +2,7 @@ import type { TargetCpu } from "./backend/target-cpu.js";
 import type { WindowsSubsystem } from "./backend/targets.js";
 import type { NativeLinkInfo } from "./backend/native-link-info.js";
 import type { NativeOptimization } from "./backend/optimization.js";
+import type { NativeProfileMatch } from "./backend/native-profile.js";
 import type { ScrDiagnostic } from "./diagnostics/diagnostic.js";
 import type { CoverageInput } from "./coverage/report.js";
 import type { TierMode } from "./frontend/tier-analysis.js";
@@ -100,6 +101,8 @@ export type CompileArtifact =
       path: string;
       translationUnitPath: string;
       backend: "llvm";
+      /** How a `profileUse` build's profile matched the program's functions. */
+      profileMatch?: NativeProfileMatch;
     };
 
 export type CompileFailure = {
diff --git a/packages/compiler/src/frontend/lowering/argument-counts.ts b/packages/compiler/src/frontend/lowering/argument-counts.ts
new file mode 100644
index 000000000..ec1753160
--- /dev/null
+++ b/packages/compiler/src/frontend/lowering/argument-counts.ts
@@ -0,0 +1,230 @@
+import * as ts from "../ts7/adapter.js";
+import { bodyReadsArguments } from "../arguments-usage.js";
+import { isJsSourceFile, locOf } from "../program.js";
+import { BOOL, F64, type IrExpr, type IrStmt, type IrType } from "../../ir/ir.js";
+import { varRef } from "../../ir/build.js";
+import { typeKey } from "../type-mapper.js";
+import type { Lowerer } from "./lowerer.js";
+import { isThisParameter } from "./call-signatures.js";
+import { buildFunctionAdapter } from "./coercions/builders.js";
+import { keyedHelperName } from "./stable-names.js";
+
+/** `arguments.length` in a top-level function declaration that the program
+ * also uses as a value. A direct call passes the declared parameters (tsc
+ * checks the arity), while a call through the value passes the parameter
+ * count of the function type the value was passed as (`[1].map(f)` calls f
+ * with 3). The value is an adapter that stores that count in a global slot
+ * before forwarding; the function reads the slot on entry and clears it,
+ * so a direct call (the cleared slot) sees the declared count. */
+
+/** The count slot of `decl`, or null when its `arguments.length` folds to
+ * the declared count (or the declaration does not qualify). */
+export function argumentCountSlot(lowerer: Lowerer, decl: ts.FunctionDeclaration): string | null {
+  const cached = lowerer.argumentCountSlots.get(decl);
+  if (cached !== undefined) return cached;
+  let slot: string | null = null;
+  if (
+    decl.body !== undefined &&
+    decl.name !== undefined &&
+    decl.asteriskToken === undefined &&
+    !decl.typeParameters?.nodes.length &&
+    lowerer.isTopLevelFnSymbol(decl.name) &&
+    decl.modifiers?.nodes.some((m) => m.kind === ts.SyntaxKind.AsyncKeyword) !== true &&
+    !isJsSourceFile(ts.getSourceFileOfNode(decl)) &&
+    fixedArity(decl) &&
+    bodyReadsArguments(decl) &&
+    usedAsValue(lowerer, decl)
+  ) {
+    const sig = lowerer.fnSigOf(decl.name);
+    if (sig) {
+      slot = `%g.argc.${sig.name}`;
+      lowerer.globalsList.push({ id: slot, name: "%argc", type: F64, mutable: true });
+    }
+  }
+  lowerer.argumentCountSlots.set(decl, slot);
+  return slot;
+}
+
+/** Reads `decl`'s count slot into a hidden local on entry and clears it; the
+ * local holds the declared count when the slot was clear. */
+export function declareArgumentCount(
+  lowerer: Lowerer,
+  decl: ts.FunctionDeclaration,
+  prologue: IrStmt[],
+): void {
+  const slot = argumentCountSlot(lowerer, decl);
+  if (slot === null) return;
+  const loc = locOf(decl);
+  const local = lowerer.declareHiddenLocal("%argc", F64);
+  const stored = varRef(slot, F64, loc);
+  prologue.push(
+    {
+      kind: "varDecl",
+      localId: local.id,
+      init: {
+        kind: "ternary",
+        cond: {
+          kind: "bin",
+          op: "===",
+          left: stored,
+          right: { kind: "numLit", value: 0, type: F64, loc },
+          type: BOOL,
+          loc,
+        },
+        then: { kind: "numLit", value: declaredCount(decl), type: F64, loc },
+        else_: stored,
+        type: F64,
+        loc,
+      },
+      loc,
+    },
+    { kind: "assign", localId: slot, value: { kind: "numLit", value: 0, type: F64, loc }, loc },
+  );
+  lowerer.ctx.argumentCount = local;
+}
+
+/** `decl` as a value at `reference`: the plain closure when every call
+ * through the expected function type passes the declared count, otherwise
+ * an adapter that stores the expected type's count in the slot first. */
+export function argumentCountValue(
+  lowerer: Lowerer,
+  decl: ts.FunctionDeclaration,
+  reference: ts.Expression,
+  closure: IrExpr & { type: IrType & { kind: "func" } },
+): IrExpr {
+  const slot = argumentCountSlot(lowerer, decl);
+  if (slot === null) return closure;
+  const count = contextualCallCount(lowerer, reference);
+  if (count === null || count === "varies") {
+    lowerer.unsupported(
+      "SC1090",
+      reference,
+      count === null
+        ? "functions that read 'arguments.length' used as values where no function type is expected (the count each call passes is not known)"
+        : "functions that read 'arguments.length' used as values where the expected function type has optional or rest parameters (the count varies per call)",
+    );
+  }
+  const declared = declaredCount(decl);
+  if (count <= declared) return closure;
+  const loc = locOf(reference);
+  const type = closure.type;
+  const key = `argc:${slot}:${count}:${typeKey(type)}`;
+  let name = lowerer.arrHofHelpers.get(key);
+  if (name === undefined) {
+    name = keyedHelperName(lowerer, "%argc.adapter", key);
+    lowerer.arrHofHelpers.set(key, name);
+    const params = type.params.map((t, i) => ({ localId: `p${i}.0`, name: `p${i}`, type: t }));
+    lowerer.liftedFns.push(
+      ...buildFunctionAdapter(
+        name,
+        type,
+        type,
+        params,
+        [],
+        [
+          {
+            kind: "assign",
+            localId: slot,
+            value: { kind: "numLit", value: count, type: F64, loc },
+            loc,
+          },
+          {
+            kind: "return",
+            value: {
+              kind: "callValue",
+              callee: varRef("f.0", type, loc),
+              args: params.map((p) => varRef(p.localId, p.type, loc)),
+              type: type.ret,
+              loc,
+            },
+            loc,
+          },
+        ],
+        loc,
+      ),
+    );
+  }
+  return { kind: "call", callee: name, args: [closure], type, loc };
+}
+
+/** The argument count every call through the function type `node` is
+ * expected as passes: that type's parameter count when it has one call
+ * signature with a fixed parameter list (a `this` parameter is type-only),
+ * "varies" when it has optional or rest parameters, null when there is no
+ * such type. */
+export function contextualCallCount(
+  lowerer: Lowerer,
+  node: ts.Expression,
+): number | "varies" | null {
+  const contextual = lowerer.checker.getContextualType(node);
+  if (contextual === undefined) return null;
+  const signatures = lowerer.checker.getCallSignatures(
+    lowerer.checker.getNonNullableType(contextual),
+  );
+  if (signatures.length !== 1) return null;
+  const declaration = signatures[0]!.declaration;
+  if (declaration === undefined) return null;
+  let count = 0;
+  for (const parameter of (declaration.parameters?.nodes ?? []) as ts.ParameterDeclaration[]) {
+    if (isThisParameter(parameter)) continue;
+    if (parameter.questionToken || parameter.initializer || parameter.dotDotDotToken)
+      return "varies";
+    count++;
+  }
+  return count;
+}
+
+/** Whether a program reference to the function declaration or method `fn`
+ * reads it as a value (`xs.map(f)`, `const g = f`, `xs.map(C.parse)`) rather
+ * than calling it. Export specifiers and default exports only name a
+ * function; the importing files' references resolve to it and are checked
+ * the same way. */
+export function usedAsValue(
+  lowerer: Lowerer,
+  fn: ts.FunctionDeclaration | ts.MethodDeclaration,
+): boolean {
+  const name = fn.name;
+  if (!name || !(ts.isIdentifier(name) || ts.isPrivateIdentifier(name))) return false;
+  const method = ts.isMethodDeclaration(fn);
+  const symbolAt = (n: ts.Node): ts.Symbol | null | undefined =>
+    method
+      ? lowerer.checker.getSymbolAtLocation(n)
+      : lowerer.resolveValueSymbol(n as ts.Identifier);
+  const symbol = symbolAt(name);
+  if (!symbol) return false;
+  let found = false;
+  for (const sf of lowerer.program.getSourceFiles()) {
+    if (sf.isDeclarationFile) continue;
+    ts.walkPreorder(sf, (n) => {
+      if (found) return "stop";
+      if (ts.isTypeNode(n)) return "skip";
+      if (n === name || !(ts.isIdentifier(n) || ts.isPrivateIdentifier(n))) return undefined;
+      if (n.text !== name.text || !n.parent) return undefined;
+      const parent = n.parent;
+      // A method is read through its member name (`C.parse`, `this.#m`).
+      const read = ts.isPropertyAccessExpression(parent) && parent.name === n ? parent : n;
+      if (method && read === n && !ts.isBindingElement(parent)) return undefined;
+      const use = read.parent;
+      if (use && ts.isCallExpression(use) && use.expression === read) return undefined;
+      if (ts.isExportSpecifier(parent) || ts.isExportAssignment(parent)) return undefined;
+      if (ts.isImportSpecifier(parent) || ts.isImportClause(parent)) return undefined;
+      if (symbolAt(n) === symbol) found = true;
+      return undefined;
+    });
+    if (found) break;
+  }
+  return found;
+}
+
+function fixedArity(fn: ts.FunctionLikeDeclaration): boolean {
+  return fn.parameters.nodes.every(
+    (p) =>
+      p.questionToken === undefined &&
+      p.initializer === undefined &&
+      p.dotDotDotToken === undefined,
+  );
+}
+
+function declaredCount(fn: ts.FunctionLikeDeclaration): number {
+  return fn.parameters.nodes.filter((p) => !isThisParameter(p)).length;
+}
diff --git a/packages/compiler/src/frontend/lowering/builtins/module-bindings.ts b/packages/compiler/src/frontend/lowering/builtins/module-bindings.ts
index 5465aeafa..4ded700fe 100644
--- a/packages/compiler/src/frontend/lowering/builtins/module-bindings.ts
+++ b/packages/compiler/src/frontend/lowering/builtins/module-bindings.ts
@@ -185,6 +185,32 @@ export function builtinMemberRequireDecl(
   return spec !== null && canonicalBuiltinModule(spec) !== null;
 }
 
+/** The member's own symbol for a variable bound to a builtin module's
+ * member (`const SD = require("string_decoder").StringDecoder`,
+ * builtinMemberRequireDecl) when the member is a class, so the variable
+ * resolves as a named import of the class does. The symbol itself
+ * otherwise. */
+export function builtinMemberBindingTarget(
+  lowerer: Lowerer,
+  symbol: ts.Symbol | null,
+): ts.Symbol | null {
+  const decl = symbol?.valueDeclaration;
+  if (
+    decl === undefined ||
+    !ts.isVariableDeclaration(decl) ||
+    !builtinMemberRequireDecl(decl.name, decl.initializer)
+  )
+    return symbol;
+  const init = decl.initializer as ts.PropertyAccessExpression;
+  let member = lowerer.checker.getSymbolAtLocation(init.name);
+  if (member && member.flags & ts.SymbolFlags.Alias)
+    member = lowerer.checker.getAliasedSymbol(member);
+  // Only a class member: `require("events").EventEmitter` is a static
+  // property of the exported class, which builtinEmitterInfoOf follows
+  // from the variable itself.
+  return member?.declarations.some(ts.isClassDeclaration) ? member : symbol;
+}
+
 /** The supported builtin module when `decl` destructures a builtin
  * NAMESPACE binding (`const crypto = require('crypto'); const {
  * createSign, sign: mySign } = crypto;` — test/common/crypto.js's
diff --git a/packages/compiler/src/frontend/lowering/builtins/readline.ts b/packages/compiler/src/frontend/lowering/builtins/readline.ts
index aca209017..c2b6984ce 100644
--- a/packages/compiler/src/frontend/lowering/builtins/readline.ts
+++ b/packages/compiler/src/frontend/lowering/builtins/readline.ts
@@ -302,6 +302,15 @@ function interfaceClassModule(lowerer: Lowerer, node: ts.Expression): string | n
     : null;
 }
 
+/** `new readline.Interface(...)` and `new readlinePromises.Interface(...)`:
+ * the constructor createInterface calls, with the same arguments. Null for
+ * other constructors. */
+export function lowerReadlineInterfaceNew(lowerer: Lowerer, expr: ts.NewExpression): IrExpr | null {
+  const module = interfaceClassModule(lowerer, expr.expression);
+  if (module === null) return null;
+  return lowerReadlineCreateCall(lowerer, expr, locOf(expr), module === "readline/promises");
+}
+
 /** `rl instanceof readline.Interface` and `rl instanceof
  * readlinePromises.Interface` over a readline Interface: an interface is
  * an instance of the class of the module that created it (Node's
@@ -1059,10 +1068,12 @@ function lowerOutput(lowerer: Lowerer, node: ts.Expression): IrExpr | "stdout" |
  * goes through rl.terminal with the completer. */
 export function lowerReadlineCreateCall(
   lowerer: Lowerer,
-  expr: ts.CallExpression,
+  expr: ts.CallExpression | ts.NewExpression,
   loc: SrcLoc,
   promises: boolean,
 ): IrExpr {
+  const argNodes = expr.arguments?.nodes ?? [];
+  const what = ts.isNewExpression(expr) ? "new Interface" : "createInterface";
   let input: IrExpr | null = null;
   let output: IrExpr | "stdout" | "none" = "none";
   let crlf: IrExpr = numLit(0, loc);
@@ -1112,39 +1123,38 @@ export function lowerReadlineCreateCall(
     });
     terminal = { kind: "varRef", localId: local.id, type: BOOL, loc };
   };
-  const optsNode = expr.arguments.nodes[0];
+  const optsNode = argNodes[0];
   if (optsNode === undefined) {
     lowerer.noLowering(
-      "createInterface without arguments",
+      `${what} without arguments`,
       expr,
       "pass { input: process.stdin, output: process.stdout } or createInterface(input, output)",
     );
   }
   if (!ts.isObjectLiteralExpression(optsNode)) {
     // createInterface(input, output?, completer?, terminal?)
-    if (expr.arguments.nodes.length > 4) {
+    if (argNodes.length > 4) {
       lowerer.noLowering(
-        `createInterface with ${expr.arguments.nodes.length} arguments`,
-        expr.arguments.nodes[4]!,
+        `${what} with ${argNodes.length} arguments`,
+        argNodes[4]!,
         "pass createInterface(input, output, completer, terminal) or one options object",
       );
     }
     input = lowerInput(lowerer, optsNode);
-    if (expr.arguments.nodes[1] !== undefined)
-      output = lowerOutput(lowerer, expr.arguments.nodes[1]);
-    const completerNode = expr.arguments.nodes[2];
+    if (argNodes[1] !== undefined) output = lowerOutput(lowerer, argNodes[1]);
+    const completerNode = argNodes[2];
     if (
       completerNode !== undefined &&
       !(ts.isIdentifier(completerNode) && completerNode.text === "undefined")
     ) {
       setCompleter(completerNode);
     }
-    if (expr.arguments.nodes[3] !== undefined) setTerminal(expr.arguments.nodes[3]);
+    if (argNodes[3] !== undefined) setTerminal(argNodes[3]);
   } else {
-    if (expr.arguments.nodes.length > 1) {
+    if (argNodes.length > 1) {
       lowerer.noLowering(
-        "createInterface with more than one argument",
-        expr.arguments.nodes[1]!,
+        `${what} with more than one argument`,
+        argNodes[1]!,
         "pass every option in the one options object",
       );
     }
diff --git a/packages/compiler/src/frontend/lowering/expressions/identifiers.ts b/packages/compiler/src/frontend/lowering/expressions/identifiers.ts
index f7ba7a115..d1d94c2ca 100644
--- a/packages/compiler/src/frontend/lowering/expressions/identifiers.ts
+++ b/packages/compiler/src/frontend/lowering/expressions/identifiers.ts
@@ -32,6 +32,7 @@ import {
 } from "../cjs-inline-require.js";
 import { captureContextArguments } from "../function-context.js";
 import { argumentsOwner, typedArgumentsCandidate } from "../typed-arguments.js";
+import { argumentCountValue } from "../argument-counts.js";
 import {
   isCjsExportTableLiteral,
   isCjsJsFile,
@@ -448,7 +449,17 @@ export function lowerIdentifierExpr(lowerer: Lowerer, expr: ts.Identifier, loc:
     // semantics); the rest marker carries the trailing dyn-array ABI.
     const funcType: IrType = funcTypeFromParamShapes(sig.params, sig.returnType);
     lowerer.requireExactArityValue(expr, expr, sig.params, funcType);
-    return { kind: "closure", fnName: sig.name, captures: [], type: funcType, loc };
+    const closure = {
+      kind: "closure" as const,
+      fnName: sig.name,
+      captures: [],
+      type: funcType,
+      loc,
+    };
+    const decl = lowerer.resolveValueSymbol(expr)?.valueDeclaration;
+    return decl && ts.isFunctionDeclaration(decl) && funcType.kind === "func"
+      ? argumentCountValue(lowerer, decl, expr, { ...closure, type: funcType })
+      : closure;
   }
   {
     // A generic function as a VALUE: monomorphized by flow — the
diff --git a/packages/compiler/src/frontend/lowering/expressions/property-access.ts b/packages/compiler/src/frontend/lowering/expressions/property-access.ts
index 874c0c6cc..98aef218f 100644
--- a/packages/compiler/src/frontend/lowering/expressions/property-access.ts
+++ b/packages/compiler/src/frontend/lowering/expressions/property-access.ts
@@ -7,6 +7,7 @@ import { lowerHistogramProperty } from "../builtins/performance.js";
 import { staticFunctionNameRead } from "../static-function-names.js";
 import { typedArgumentsArity } from "../typed-arguments.js";
 import { isThisParameter } from "../call-signatures.js";
+import { argumentCountSlot, contextualCallCount, usedAsValue } from "../argument-counts.js";
 import {
   checkedPrivateValue,
   localPrivateBrandClass,
@@ -690,7 +691,31 @@ export function lowerPropertyAccessExpr(
         const declared = (fn as ts.FunctionLikeDeclaration).parameters.nodes.filter(
           (p) => !isThisParameter(p),
         ).length;
-        const count = Math.max(declared, contextualArgumentCount(lowerer, fn) ?? 0);
+        const contextual = contextualArgumentCount(lowerer, fn);
+        if (contextual === "varies") {
+          lowerer.unsupported(
+            "SC1090",
+            expr,
+            "'arguments.length' in function expressions passed where the expected function type has optional or rest parameters (the count varies per call)",
+          );
+        }
+        // A top-level declaration also used as a value reads the count its
+        // caller stored (argument-counts.ts).
+        if (ts.isFunctionDeclaration(fn) && lowerer.ctx.argumentCount !== undefined)
+          return varRef(lowerer.ctx.argumentCount.id, F64, loc);
+        if (
+          (ts.isFunctionDeclaration(fn) || ts.isMethodDeclaration(fn)) &&
+          usedAsValue(lowerer, fn)
+        ) {
+          lowerer.unsupported(
+            "SC1090",
+            expr,
+            ts.isFunctionDeclaration(fn) && argumentCountSlot(lowerer, fn) !== null
+              ? "'arguments.length' in an arrow function inside a function that is also used as a value"
+              : "'arguments.length' in methods, nested, async and generator functions used as values (a caller of the value can pass more arguments than the function declares)",
+          );
+        }
+        const count = Math.max(declared, contextual ?? 0);
         // Calls through the expression's own name pass the declared count,
         // calls through the wider slot pass more: no constant answers both.
         if (count > declared && referencesOwnName(lowerer, fn)) {
@@ -4562,26 +4587,9 @@ function referencesOwnName(lowerer: Lowerer, fn: ts.Node): boolean {
 }
 
 /** The argument count every caller passes a function expression whose
- * value flows into one fixed-arity function type, an array method's
- * callback or an annotated slot: that type's parameter count. Null when
- * there is no such type or its arity varies. */
-function contextualArgumentCount(lowerer: Lowerer, fn: ts.Node): number | null {
-  if (!ts.isFunctionExpression(fn)) return null;
-  const contextual = lowerer.checker.getContextualType(fn);
-  if (contextual === undefined) return null;
-  const signatures = lowerer.checker.getCallSignatures(
-    lowerer.checker.getNonNullableType(contextual),
-  );
-  if (signatures.length !== 1) return null;
-  const declaration = signatures[0]!.declaration;
-  if (declaration === undefined) return null;
-  let count = 0;
-  for (const parameter of (declaration.parameters?.nodes ?? []) as ts.ParameterDeclaration[]) {
-    if (isThisParameter(parameter)) continue;
-    if (parameter.questionToken || parameter.initializer || parameter.dotDotDotToken) return null;
-    count++;
-  }
-  return count;
+ * value flows into a function type (argument-counts.ts contextualCallCount). */
+function contextualArgumentCount(lowerer: Lowerer, fn: ts.Node): number | "varies" | null {
+  return ts.isFunctionExpression(fn) ? contextualCallCount(lowerer, fn) : null;
 }
 
 /** The property names the program assigns on the global object, through
diff --git a/packages/compiler/src/frontend/lowering/function-context.ts b/packages/compiler/src/frontend/lowering/function-context.ts
index 92042b7d7..702574b63 100644
--- a/packages/compiler/src/frontend/lowering/function-context.ts
+++ b/packages/compiler/src/frontend/lowering/function-context.ts
@@ -46,6 +46,9 @@ export interface FnCtx {
    * rest param): the synthetic trailing dyn-array param `arguments`
    * reads resolve to. */
   argumentsLocal?: IrLocal | null;
+  /** The count `arguments.length` reads in a top-level function declaration
+   * that is also used as a value (argument-counts.ts). */
+  argumentCount?: IrLocal;
   /** Arrows inherit the nearest ordinary function's arguments binding. */
   inheritsArguments?: boolean;
   /** A function lowered on demand on top of an unrelated caller's frames
diff --git a/packages/compiler/src/frontend/lowering/generic-functions.ts b/packages/compiler/src/frontend/lowering/generic-functions.ts
index 7896da578..0f9e41723 100644
--- a/packages/compiler/src/frontend/lowering/generic-functions.ts
+++ b/packages/compiler/src/frontend/lowering/generic-functions.ts
@@ -2082,6 +2082,10 @@ function storedImplicitArgumentType(lowerer: Lowerer, arg: ts.Expression): IrTyp
     isJsSourceFile(ts.getSourceFileOfNode(arg)) &&
     (ts.isArrowFunction(arg) || ts.isFunctionExpression(arg))
   ) {
+    // A closure lowered here before (another call site's instance key, or
+    // a repeated demand) keeps its value for the argument's own lowering.
+    const known = lowerer.speculatedClosure(arg);
+    if (known !== null) return known.type;
     const saved = lowerer.diagSink;
     const diagnostics: typeof lowerer.diags = [];
     lowerer.diagSink = diagnostics;
@@ -2092,8 +2096,17 @@ function storedImplicitArgumentType(lowerer: Lowerer, arg: ts.Expression): IrTyp
       lowerer.diagSink = saved;
     }
     // Direct inferred helpers can receive native callbacks even when their
-    // result is an opaque handle that cannot cross checked storage.
-    if (diagnostics.length === 0 && value?.type.kind === "func") return value.type;
+    // result is an opaque handle that cannot cross checked storage. The
+    // argument's own lowering then takes this value instead of lowering
+    // the closure a second time.
+    if (diagnostics.length === 0 && value?.type.kind === "func") {
+      lowerer.speculatedClosures.set(arg, {
+        ctx: lowerer.ctx,
+        bindings: lowerer.typeParamBindings,
+        value,
+      });
+      return value.type;
+    }
   }
   if (arg.kind === ts.SyntaxKind.ThisKeyword) return lowerer.resolveThis()?.type ?? null;
   if (ts.isAwaitExpression(arg)) {
diff --git a/packages/compiler/src/frontend/lowering/js-collection-types.ts b/packages/compiler/src/frontend/lowering/js-collection-types.ts
new file mode 100644
index 000000000..a4c182523
--- /dev/null
+++ b/packages/compiler/src/frontend/lowering/js-collection-types.ts
@@ -0,0 +1,533 @@
+/* Types of untyped JavaScript collections that their uses prove.
+ *
+ * In a JavaScript file, `const m = new Map()`, `new Set()` and `[]` type as
+ * Map<any, any>, Set<any> and any[] to the checker, so every use lowers to
+ * the checked-dynamic collection operations. When every reference of such a
+ * `const` is a direct use of the collection, the value never leaves the
+ * binding. If its keys, values or elements are then each one primitive type
+ * wherever the program writes and looks them up, the binding lowers as the
+ * typed Map<K, V>, Set<K> or T[], as a JSDoc @type would make it:
+ *
+ * - Map: `m.get(k)`, `m.has(k)`, `m.delete(k)`, `m.clear()`, `m.size`, and
+ *   `m.set(k, v)` as a statement (its result is the map).
+ * - Set: `s.add(k)` as a statement, `s.has(k)`, `s.delete(k)`, `s.clear()`,
+ *   `s.size`.
+ * - Array: `a.push(...)`, `a.pop()`, `a.shift()`, `a.at(i)`, `a.includes(v)`,
+ *   `a.indexOf(v)`, `a.join(separator)`, `a.length`.
+ *
+ * Any other reference (passing, returning, storing, exporting, iterating,
+ * spreading, inspecting the collection, an optional call), a direct `eval`
+ * in the file, or any doubt about a type keeps the binding untyped.
+ *
+ * The types come from carrier declarations in scriptc-overrides.d.ts, which
+ * only the lowering program includes. */
+
+import * as ts from "../ts7/adapter.js";
+import { isJsSourceFile } from "../program.js";
+import { overridesDtsPath } from "../dts-paths.js";
+import type { Lowerer } from "./lowerer.js";
+
+type Primitive = "string" | "number" | "boolean";
+const PRIMITIVES: readonly Primitive[] = ["string", "number", "boolean"];
+
+type Collection = "map" | "set" | "array";
+
+interface Candidate {
+  readonly kind: Collection;
+  readonly declaration: ts.VariableDeclaration;
+  /** Every identifier that names the binding, its declaration's included. */
+  readonly names: ts.Identifier[];
+  /** Map keys or Set members written by set/add. */
+  readonly keyWrites: ts.Expression[];
+  /** Map values written by set. Array elements written by push. */
+  readonly valueWrites: ts.Expression[];
+  /** Arguments compared with keys or elements: get/has/delete, includes/indexOf. */
+  readonly lookups: ts.Expression[];
+  /** Array indexes passed to at(). */
+  readonly indexes: ts.Expression[];
+  /** Separators passed to join(). */
+  readonly separators: ts.Expression[];
+  key: Primitive | undefined;
+  value: Primitive | undefined;
+}
+
+const MAP_LOOKUPS = new Set(["get", "has", "delete"]);
+const SET_LOOKUPS = new Set(["has", "delete"]);
+const ARRAY_READS = new Set(["pop", "shift"]);
+const ARRAY_SEARCHES = new Set(["includes", "indexOf"]);
+/** The expression lowering refuses deeper nesting (SC1090). */
+const MAX_DEPTH = 200;
+
+/** The inferred type of the binding a `new Map()` or `new Set()` initializes,
+ * which the construction adopts. */
+export function initializedCollectionType(
+  lowerer: Lowerer,
+  expr: ts.NewExpression,
+): ts.Type | null {
+  const declaration = expr.parent;
+  return declaration !== undefined &&
+    ts.isVariableDeclaration(declaration) &&
+    declaration.initializer === expr &&
+    ts.isIdentifier(declaration.name)
+    ? inferredCollectionType(lowerer, declaration.name)
+    : null;
+}
+
+/** The inferred collection type of an identifier that names a proven
+ * binding, or null. Computed for every JavaScript file of the program on
+ * first use. */
+export function inferredCollectionType(lowerer: Lowerer, node: ts.Identifier): ts.Type | null {
+  const types = (lowerer.inferredCollectionTypes ??= inferProgram(lowerer));
+  return types.size === 0 ? null : (types.get(node) ?? null);
+}
+
+function inferProgram(lowerer: Lowerer): Map<ts.Node, ts.Type> {
+  const out = new Map<ts.Node, ts.Type>();
+  const carriers = carrierTypes(lowerer);
+  if (carriers === null) return out;
+  for (const file of lowerer.program.getSourceFiles()) {
+    if (file.isDeclarationFile || !isJsSourceFile(file)) continue;
+    for (const candidate of inferFile(lowerer, file)) {
+      const name =
+        candidate.kind === "map"
+          ? `map_${candidate.key}_${candidate.value}`
+          : candidate.kind === "set"
+            ? `set_${candidate.key}`
+            : `array_${candidate.value}`;
+      const type = carriers.get(name);
+      if (type === undefined) continue;
+      for (const id of candidate.names) out.set(id, type);
+    }
+  }
+  return out;
+}
+
+/** The carrier declarations' types, by name (`map_string_number`,
+ * `set_number`, `array_string`, ...), or null when the lowering program does
+ * not include them. */
+function carrierTypes(lowerer: Lowerer): Map<string, ts.Type> | null {
+  const file = lowerer.program.getSourceFile(overridesDtsPath());
+  if (file === undefined) return null;
+  const out = new Map<string, ts.Type>();
+  for (const statement of file.statements.nodes) {
+    if (
+      !ts.isModuleDeclaration(statement) ||
+      !ts.isIdentifier(statement.name) ||
+      statement.name.text !== "__scriptcInferredCollections" ||
+      statement.body === undefined ||
+      !ts.isModuleBlock(statement.body)
+    )
+      continue;
+    for (const member of statement.body.statements.nodes) {
+      if (!ts.isVariableStatement(member)) continue;
+      for (const declaration of member.declarationList.declarations.nodes)
+        if (ts.isIdentifier(declaration.name))
+          out.set(declaration.name.text, lowerer.checker.getTypeAtLocation(declaration.name));
+    }
+  }
+  return out.size > 0 ? out : null;
+}
+
+function inferFile(lowerer: Lowerer, file: ts.SourceFile): Candidate[] {
+  const candidates = new Map<ts.Symbol, Candidate>();
+  let directEval = false;
+  const declarations = (node: ts.Node): void => {
+    if (
+      ts.isCallExpression(node) &&
+      ts.isIdentifier(node.expression) &&
+      node.expression.text === "eval"
+    )
+      directEval = true;
+    if (ts.isVariableDeclaration(node)) {
+      const kind = candidateKind(lowerer, node);
+      const symbol = kind !== null ? lowerer.checker.getSymbolAtLocation(node.name) : undefined;
+      if (kind !== null && symbol !== undefined)
+        candidates.set(symbol, {
+          kind,
+          declaration: node,
+          names: [node.name as ts.Identifier],
+          keyWrites: [],
+          valueWrites: [],
+          lookups: [],
+          indexes: [],
+          separators: [],
+          key: undefined,
+          value: undefined,
+        });
+    }
+  };
+  ts.walkPreorder(file, declarations);
+  if (directEval || candidates.size === 0) return [];
+
+  const names = new Set(
+    [...candidates.values()].map((c) => (c.declaration.name as ts.Identifier).text),
+  );
+  const refused = new Set<ts.Symbol>();
+  const references = (node: ts.Node): void => {
+    if (ts.isIdentifier(node) && names.has(node.text)) {
+      const symbol = referencedSymbol(lowerer, node);
+      const candidate = symbol !== undefined ? candidates.get(symbol) : undefined;
+      if (symbol !== undefined && candidate !== undefined && node !== candidate.declaration.name) {
+        if (recordUse(candidate, node)) candidate.names.push(node);
+        else refused.add(symbol);
+      }
+    }
+  };
+  ts.walkPreorder(file, references);
+  for (const symbol of refused) candidates.delete(symbol);
+  const proven = [...candidates.values()].filter(
+    (c) => c.valueWrites.length > 0 || c.keyWrites.length > 0,
+  );
+  solve(lowerer, proven);
+  return proven.filter((c) =>
+    c.kind === "map" ? c.key && c.value : c.kind === "set" ? c.key : c.value,
+  );
+}
+
+/** The collection a `const` declaration starts empty, when the checker
+ * types it as an untyped Map, Set or array. */
+function candidateKind(lowerer: Lowerer, declaration: ts.VariableDeclaration): Collection | null {
+  if (!ts.isIdentifier(declaration.name) || declaration.initializer === undefined) return null;
+  const list = declaration.parent;
+  if (!ts.isVariableDeclarationList(list) || (list.flags & ts.NodeFlags.Const) === 0) return null;
+  const statement = list.parent;
+  if (
+    ts.isVariableStatement(statement) &&
+    ts.getModifiers(statement)?.some((m) => m.kind === ts.SyntaxKind.ExportKeyword)
+  )
+    return null;
+  const init = declaration.initializer;
+  let kind: Collection | null = null;
+  if (ts.isArrayLiteralExpression(init) && init.elements.nodes.length === 0) kind = "array";
+  else if (
+    ts.isNewExpression(init) &&
+    init.typeArguments === undefined &&
+    (init.arguments?.nodes.length ?? 0) === 0
+  ) {
+    if (lowerer.isStdlibGlobal(init.expression, "Map")) kind = "map";
+    else if (lowerer.isStdlibGlobal(init.expression, "Set")) kind = "set";
+  }
+  if (kind === null) return null;
+  // A JSDoc type, or anything else the checker already knows, stands.
+  const type = lowerer.checker.getTypeAtLocation(declaration.name);
+  const args =
+    kind === "array"
+      ? lowerer.checker.isArrayType(type)
+        ? lowerer.checker.getTypeArguments(type as ts.TypeReference)
+        : []
+      : lowerer.checker.getTypeArguments(type as ts.TypeReference);
+  const arity = kind === "map" ? 2 : 1;
+  if (args.length !== arity || !args.every((a) => (a.flags & ts.TypeFlags.Any) !== 0)) return null;
+  return kind;
+}
+
+function referencedSymbol(lowerer: Lowerer, id: ts.Identifier): ts.Symbol | undefined {
+  const parent = id.parent;
+  if (parent !== undefined && ts.isShorthandPropertyAssignment(parent) && parent.name === id)
+    return lowerer.checker.getShorthandAssignmentValueSymbol(parent);
+  const symbol = lowerer.checker.getSymbolAtLocation(id);
+  // `export { m }` names the export, an alias of the local binding.
+  if (
+    symbol !== undefined &&
+    parent !== undefined &&
+    ts.isExportSpecifier(parent) &&
+    parent.propertyName === undefined &&
+    (symbol.flags & ts.SymbolFlags.Alias) !== 0
+  )
+    return lowerer.checker.getAliasedSymbol(symbol);
+  return symbol;
+}
+
+/** Records a reference's use, or answers false when the reference is not
+ * one of the collection uses this inference proves. */
+function recordUse(candidate: Candidate, id: ts.Identifier): boolean {
+  const access = id.parent;
+  if (
+    access === undefined ||
+    !ts.isPropertyAccessExpression(access) ||
+    access.expression !== id ||
+    access.questionDotToken !== undefined
+  )
+    return false;
+  const name = access.name.text;
+  const call = access.parent;
+  if (call !== undefined && ts.isCallExpression(call) && call.expression === access) {
+    if (call.questionDotToken !== undefined) return false;
+    const args = call.arguments.nodes;
+    if (args.some(ts.isSpreadElement)) return false;
+    switch (candidate.kind) {
+      case "map":
+        if (MAP_LOOKUPS.has(name) && args.length === 1)
+          return (candidate.lookups.push(args[0]!), true);
+        if (name === "clear") return args.length === 0;
+        if (name === "set" && args.length === 2 && isStatement(call)) {
+          candidate.keyWrites.push(args[0]!);
+          candidate.valueWrites.push(args[1]!);
+          return true;
+        }
+        return false;
+      case "set":
+        if (SET_LOOKUPS.has(name) && args.length === 1)
+          return (candidate.lookups.push(args[0]!), true);
+        if (name === "clear") return args.length === 0;
+        if (name === "add" && args.length === 1 && isStatement(call))
+          return (candidate.keyWrites.push(args[0]!), true);
+        return false;
+      case "array":
+        if (name === "push") return (candidate.valueWrites.push(...args), true);
+        if (ARRAY_READS.has(name)) return args.length === 0;
+        if (name === "at" && args.length === 1) return (candidate.indexes.push(args[0]!), true);
+        if (ARRAY_SEARCHES.has(name) && args.length === 1)
+          return (candidate.lookups.push(args[0]!), true);
+        if (name === "join" && args.length <= 1) return (candidate.separators.push(...args), true);
+        return false;
+    }
+  }
+  return (candidate.kind === "array" ? name === "length" : name === "size") && isPlainRead(access);
+}
+
+function isStatement(call: ts.CallExpression): boolean {
+  let node: ts.Node = call;
+  while (node.parent !== undefined && ts.isParenthesizedExpression(node.parent)) node = node.parent;
+  return node.parent !== undefined && ts.isExpressionStatement(node.parent);
+}
+
+/** A read that neither writes nor deletes the property. */
+function isPlainRead(access: ts.PropertyAccessExpression): boolean {
+  let node: ts.Node = access;
+  while (node.parent !== undefined && ts.isParenthesizedExpression(node.parent)) node = node.parent;
+  const parent = node.parent;
+  if (parent === undefined) return false;
+  if (
+    ts.isBinaryExpression(parent) &&
+    parent.left === node &&
+    isAssignment(parent.operatorToken.kind)
+  )
+    return false;
+  if (
+    (ts.isPrefixUnaryExpression(parent) || ts.isPostfixUnaryExpression(parent)) &&
+    (parent.operator === ts.SyntaxKind.PlusPlusToken ||
+      parent.operator === ts.SyntaxKind.MinusMinusToken)
+  )
+    return false;
+  return !ts.isDeleteExpression(parent);
+}
+
+function isAssignment(kind: ts.SyntaxKind): boolean {
+  return kind >= ts.SyntaxKind.FirstAssignment && kind <= ts.SyntaxKind.LastAssignment;
+}
+
+/** Decides each candidate's key and value types: a fixed point in which a
+ * decided candidate's reads type the expressions built from them. */
+function solve(lowerer: Lowerer, candidates: Candidate[]): void {
+  const bySymbol = new Map<ts.Symbol, Candidate>();
+  for (const c of candidates) {
+    const symbol = lowerer.checker.getSymbolAtLocation(c.declaration.name);
+    if (symbol) bySymbol.set(symbol, c);
+  }
+  for (let changed = true; changed;) {
+    changed = false;
+    for (const c of candidates) {
+      if (c.kind !== "array" && c.key === undefined) {
+        const key = uniform(lowerer, bySymbol, c.keyWrites);
+        if (key !== null) {
+          c.key = key;
+          changed = true;
+        }
+      }
+      if (c.kind !== "set" && c.value === undefined) {
+        // Each candidate type is assumed for this collection's own reads.
+        const accepted = PRIMITIVES.filter((assumed) => {
+          c.value = assumed;
+          const found = uniform(lowerer, bySymbol, c.valueWrites);
+          c.value = undefined;
+          return found === assumed;
+        });
+        if (accepted.length === 1) {
+          c.value = accepted[0];
+          changed = true;
+        }
+      }
+    }
+  }
+  for (const c of candidates) {
+    const element = c.kind === "array" ? c.value : c.key;
+    // Lookups compare with the stored type; an index is a number.
+    const lookupsAgree =
+      element !== undefined &&
+      c.lookups.every((e) => primitiveOf(lowerer, bySymbol, e, new Set()) === element) &&
+      c.indexes.every((e) => primitiveOf(lowerer, bySymbol, e, new Set()) === "number") &&
+      c.separators.every((e) => primitiveOf(lowerer, bySymbol, e, new Set()) === "string");
+    if (!lookupsAgree) {
+      c.key = undefined;
+      c.value = undefined;
+    }
+  }
+}
+
+/** The one primitive type of every expression, or null. */
+function uniform(
+  lowerer: Lowerer,
+  bySymbol: Map<ts.Symbol, Candidate>,
+  exprs: readonly ts.Expression[],
+): Primitive | null {
+  let found: Primitive | null = null;
+  for (const e of exprs) {
+    const p = primitiveOf(lowerer, bySymbol, e, new Set());
+    if (p === null || (found !== null && p !== found)) return null;
+    found = p;
+  }
+  return found;
+}
+
+function checkerPrimitive(type: ts.Type): Primitive | null {
+  const flags = type.flags;
+  if (flags & (ts.TypeFlags.String | ts.TypeFlags.StringLiteral)) return "string";
+  if (flags & (ts.TypeFlags.Number | ts.TypeFlags.NumberLiteral)) return "number";
+  if (flags & (ts.TypeFlags.Boolean | ts.TypeFlags.BooleanLiteral)) return "boolean";
+  return null;
+}
+
+/** The primitive type `e` always evaluates to, or null. The checker answers
+ * where it knows; where it types `e` as `any`, the value's structure does:
+ * a read of a decided candidate, arithmetic and comparisons over such
+ * values, and constants defined from them. An expression nested deeper than
+ * the lowering's nesting limit answers null. */
+function primitiveOf(
+  lowerer: Lowerer,
+  bySymbol: Map<ts.Symbol, Candidate>,
+  e: ts.Expression,
+  visiting: Set<ts.Node>,
+  depth = 0,
+): Primitive | null {
+  if (depth > MAX_DEPTH) return null;
+  const type = lowerer.checker.getTypeAtLocation(e);
+  if ((type.flags & (ts.TypeFlags.Any | ts.TypeFlags.Unknown)) === 0) return checkerPrimitive(type);
+  const of = (x: ts.Expression): Primitive | null =>
+    primitiveOf(lowerer, bySymbol, x, visiting, depth + 1);
+  if (ts.isParenthesizedExpression(e)) return of(e.expression);
+  if (ts.isPrefixUnaryExpression(e)) {
+    if (e.operator === ts.SyntaxKind.ExclamationToken) return "boolean";
+    return e.operator !== ts.SyntaxKind.PlusPlusToken &&
+      e.operator !== ts.SyntaxKind.MinusMinusToken &&
+      of(e.operand) === "number"
+      ? "number"
+      : null;
+  }
+  if (ts.isConditionalExpression(e)) {
+    const yes = of(e.whenTrue);
+    return yes !== null && yes === of(e.whenFalse) ? yes : null;
+  }
+  if (ts.isBinaryExpression(e)) {
+    const op = e.operatorToken.kind;
+    switch (op) {
+      case ts.SyntaxKind.LessThanToken:
+      case ts.SyntaxKind.GreaterThanToken:
+      case ts.SyntaxKind.LessThanEqualsToken:
+      case ts.SyntaxKind.GreaterThanEqualsToken:
+      case ts.SyntaxKind.EqualsEqualsEqualsToken:
+      case ts.SyntaxKind.ExclamationEqualsEqualsToken:
+      case ts.SyntaxKind.EqualsEqualsToken:
+      case ts.SyntaxKind.ExclamationEqualsToken:
+        return "boolean";
+      case ts.SyntaxKind.QuestionQuestionToken:
+      case ts.SyntaxKind.BarBarToken: {
+        // `m.get(k) ?? d` and `a.pop() || d`: the stored type or a default of it.
+        const stored = storedRead(lowerer, bySymbol, e.left);
+        const fallback = of(e.right);
+        return stored !== null && stored === fallback ? stored : null;
+      }
+      case ts.SyntaxKind.PlusToken: {
+        const left = of(e.left);
+        const right = of(e.right);
+        if (left === "number" && right === "number") return "number";
+        return left !== null && right !== null && (left === "string" || right === "string")
+          ? "string"
+          : null;
+      }
+      case ts.SyntaxKind.MinusToken:
+      case ts.SyntaxKind.AsteriskToken:
+      case ts.SyntaxKind.SlashToken:
+      case ts.SyntaxKind.PercentToken:
+      case ts.SyntaxKind.AsteriskAsteriskToken:
+      case ts.SyntaxKind.BarToken:
+      case ts.SyntaxKind.AmpersandToken:
+      case ts.SyntaxKind.CaretToken:
+      case ts.SyntaxKind.LessThanLessThanToken:
+      case ts.SyntaxKind.GreaterThanGreaterThanToken:
+      case ts.SyntaxKind.GreaterThanGreaterThanGreaterThanToken:
+        return of(e.left) === "number" && of(e.right) === "number" ? "number" : null;
+      default:
+        return null;
+    }
+  }
+  if (ts.isCallExpression(e) || ts.isPropertyAccessExpression(e))
+    return candidateRead(lowerer, bySymbol, e);
+  // A constant defined from such an expression.
+  if (ts.isIdentifier(e) && !visiting.has(e)) {
+    const symbol = lowerer.checker.getSymbolAtLocation(e);
+    const declaration = symbol?.valueDeclaration;
+    if (
+      declaration !== undefined &&
+      ts.isVariableDeclaration(declaration) &&
+      declaration.initializer !== undefined &&
+      ts.isVariableDeclarationList(declaration.parent) &&
+      (declaration.parent.flags & ts.NodeFlags.Const) !== 0
+    ) {
+      visiting.add(e);
+      const p = primitiveOf(lowerer, bySymbol, declaration.initializer, visiting, depth + 1);
+      visiting.delete(e);
+      return p;
+    }
+  }
+  return null;
+}
+
+/** The candidate a use reads, when `e` is `c.member` or `c.member(...)` of a
+ * candidate `c`. */
+function candidateOf(
+  lowerer: Lowerer,
+  bySymbol: Map<ts.Symbol, Candidate>,
+  e: ts.Expression,
+): { candidate: Candidate; member: string } | null {
+  const access = ts.isCallExpression(e) ? e.expression : e;
+  if (!ts.isPropertyAccessExpression(access) || !ts.isIdentifier(access.expression)) return null;
+  const symbol = lowerer.checker.getSymbolAtLocation(access.expression);
+  const candidate = symbol !== undefined ? bySymbol.get(symbol) : undefined;
+  return candidate !== undefined ? { candidate, member: access.name.text } : null;
+}
+
+/** The type of a candidate's read that may answer undefined: `m.get(k)`,
+ * `a.pop()`, `a.shift()`, `a.at(i)`. */
+function storedRead(
+  lowerer: Lowerer,
+  bySymbol: Map<ts.Symbol, Candidate>,
+  e: ts.Expression,
+): Primitive | null {
+  while (ts.isParenthesizedExpression(e)) e = e.expression;
+  if (!ts.isCallExpression(e)) return null;
+  const read = candidateOf(lowerer, bySymbol, e);
+  if (read === null) return null;
+  const { candidate, member } = read;
+  if (candidate.kind === "map" && member === "get") return candidate.value ?? null;
+  if (candidate.kind === "array" && (ARRAY_READS.has(member) || member === "at"))
+    return candidate.value ?? null;
+  return null;
+}
+
+/** The type of a candidate's read that always answers a value: size,
+ * length, has, includes, indexOf, push. */
+function candidateRead(
+  lowerer: Lowerer,
+  bySymbol: Map<ts.Symbol, Candidate>,
+  e: ts.Expression,
+): Primitive | null {
+  const read = candidateOf(lowerer, bySymbol, e);
+  if (read === null) return null;
+  const { candidate, member } = read;
+  if (!ts.isCallExpression(e)) return member === "size" || member === "length" ? "number" : null;
+  if (member === "has" || member === "includes" || member === "delete") return "boolean";
+  if (member === "indexOf" || (candidate.kind === "array" && member === "push")) return "number";
+  if (candidate.kind === "array" && member === "join") return "string";
+  return null;
+}
diff --git a/packages/compiler/src/frontend/lowering/lower-classes.ts b/packages/compiler/src/frontend/lowering/lower-classes.ts
index 0a2ac6106..bdd335361 100644
--- a/packages/compiler/src/frontend/lowering/lower-classes.ts
+++ b/packages/compiler/src/frontend/lowering/lower-classes.ts
@@ -18,8 +18,11 @@ import {
   superIntoResultSlot,
 } from "./constructor-returns.js";
 import { checkedIterableSpread } from "./checked-iterable-spread.js";
-import { lowerReadlineUiNew } from "./builtins/readline.js";
-import { builtinMemberRequireDecl } from "./builtins/module-bindings.js";
+import { lowerReadlineInterfaceNew, lowerReadlineUiNew } from "./builtins/readline.js";
+import {
+  builtinMemberBindingTarget,
+  builtinMemberRequireDecl,
+} from "./builtins/module-bindings.js";
 import { checkedClassUnionAssertion } from "./class-assertions.js";
 import { collectClassMethodMutations } from "./class-method-mutations.js";
 import { InternalCompilerError } from "../../errors.js";
@@ -113,6 +116,7 @@ import { PoisonError, dynFallbackType, jsFuncNameOf, newFnCtx, own } from "./low
 import { isAssertClassNew } from "./assert-instances.js";
 import { isCjsRequireGlobal } from "./lower-node-module.js";
 import { lowerArrayConstructor } from "./containers/array-construction.js";
+import { initializedCollectionType } from "./js-collection-types.js";
 import { lowerMapSeedNew, lowerSetSeedNew } from "./containers/collection-methods.js";
 import { lowerObjectConstructor } from "./containers/indexed-objects.js";
 import { knownBufEncoding, lowerBufferConstruct } from "./containers/bytes.js";
@@ -9823,7 +9827,7 @@ export function lowerNew(lowerer: Lowerer, expr: ts.NewExpression): IrExpr {
   }
   const worker = lowerWorkerNew(lowerer, expr);
   if (worker) return worker;
-  const readlineUi = lowerReadlineUiNew(lowerer, expr);
+  const readlineUi = lowerReadlineUiNew(lowerer, expr) ?? lowerReadlineInterfaceNew(lowerer, expr);
   if (readlineUi) return readlineUi;
   const channel = lowerMessageChannelNew(lowerer, expr) ?? lowerBroadcastChannelNew(lowerer, expr);
   if (channel) return channel;
@@ -10123,6 +10127,102 @@ export function lowerNew(lowerer: Lowerer, expr: ts.NewExpression): IrExpr {
     const agent = lowerHttpAgentNew(lowerer, expr);
     if (agent) return agent;
   }
+  // `new crypto.X509Certificate(data)` — the Dirent-style data record:
+  // the certificate's lowered members (fingerprint — the SHA-1 of the
+  // DER, uppercase colon-separated — plus the validFrom/validTo
+  // validity window in Node's ASN1_TIME_print shape) compute AT
+  // CONSTRUCTION, when Node parses too, so unparseable input throws
+  // Node's exact PEM error here (ERR_OSSL_PEM_NO_START_LINE) and the
+  // handle never exists. Both import forms (`crypto.X509Certificate`
+  // through the namespace, named `X509Certificate`); Buffer input only
+  // — the readFileSync idiom. mapType interns the matching record, so
+  // locals and the composed member reads all flow. The data argument
+  // feeds THREE field computations, so construction goes through an
+  // interned helper whose parameter evaluates it exactly once.
+  // It runs before the standard-library constructor fence below, which
+  // also sees `const X = require("crypto").X509Certificate` as the
+  // builtin class.
+  {
+    const callee = expr.expression;
+    const isX509 =
+      (ts.isPropertyAccessExpression(callee) &&
+        callee.name.text === "X509Certificate" &&
+        lowerer.builtinNamespaceModuleOf(callee.expression) === "crypto") ||
+      (ts.isIdentifier(callee) &&
+        (() => {
+          const bi = lowerer.builtinImportOf(callee);
+          return bi?.module === "crypto" && bi.member === "X509Certificate";
+        })());
+    if (isX509) {
+      const args: readonly ts.Expression[] = expr.arguments?.nodes ?? [];
+      if (args.length !== 1) {
+        lowerer.noLowering(
+          "X509Certificate with this argument shape",
+          expr,
+          "the supported form is new X509Certificate(readFileSync(path))",
+        );
+      }
+      const data = lowerer.lowerExpr(args[0]!);
+      const isBytes = data.type.kind === "bytes" && data.type.elem === "u8";
+      if (!isBytes && data.type.kind !== "string") {
+        lowerer.noLowering(
+          `X509Certificate over '${lowerer.fmt(data.type)}' data`,
+          args[0]!,
+          "pass the certificate Buffer or PEM string (an fs.readFileSync result)",
+        );
+      }
+      const t = lowerer.mapTypeOf(lowerer.typeOf(expr));
+      if (t?.kind !== "record") lowerer.badType(expr, lowerer.typeOf(expr));
+      const key = `x509.record:${isBytes ? "bytes" : "str"}`;
+      let helper = lowerer.valueHelpers.get(key);
+      if (!helper) {
+        helper = keyedHelperName(lowerer, "%x509.record", key);
+        lowerer.valueHelpers.set(key, helper);
+        const dataT = data.type;
+        const dRef: IrExpr = { kind: "varRef", localId: "d.0", type: dataT, loc };
+        const field = (
+          name: string,
+          fn:
+            | "crypto.x509Fingerprint"
+            | "crypto.x509FingerprintStr"
+            | "crypto.x509ValidFrom"
+            | "crypto.x509ValidFromStr"
+            | "crypto.x509ValidTo"
+            | "crypto.x509ValidToStr",
+        ): { name: string; value: IrExpr } => ({
+          name,
+          value: { kind: "libCall", fn, args: [dRef], type: STRING, loc },
+        });
+        lowerer.liftedFns.push({
+          name: helper,
+          params: [{ localId: "d.0", name: "d", type: dataT }],
+          returnType: t,
+          locals: [{ id: "d.0", name: "d", type: dataT, mutable: false }],
+          body: [
+            {
+              kind: "return",
+              value: {
+                kind: "recordLit",
+                fields: [
+                  field(
+                    "fingerprint",
+                    isBytes ? "crypto.x509Fingerprint" : "crypto.x509FingerprintStr",
+                  ),
+                  field("validFrom", isBytes ? "crypto.x509ValidFrom" : "crypto.x509ValidFromStr"),
+                  field("validTo", isBytes ? "crypto.x509ValidTo" : "crypto.x509ValidToStr"),
+                ],
+                type: t,
+                loc,
+              },
+              loc,
+            },
+          ],
+          loc,
+        });
+      }
+      return { kind: "call", callee: helper, args: [data], type: t, loc };
+    }
+  }
   if (
     ts.isIdentifier(expr.expression) ||
     (ts.isPropertyAccessExpression(expr.expression) &&
@@ -10136,7 +10236,12 @@ export function lowerNew(lowerer: Lowerer, expr: ts.NewExpression): IrExpr {
     // (a no-op for every non-import= binding).
     if (ts.isIdentifier(expr.expression)) fenceEarlyAliasUse(lowerer, expr.expression, expr);
     const identifier = ts.isIdentifier(expr.expression) ? expr.expression : expr.expression.name;
-    const symbol = ts.isIdentifier(identifier) ? lowerer.resolveValueSymbol(identifier) : null;
+    // `const SD = require("string_decoder").StringDecoder; new SD()`
+    // constructs the member, as a named import of it does.
+    const symbol = builtinMemberBindingTarget(
+      lowerer,
+      ts.isIdentifier(identifier) ? lowerer.resolveValueSymbol(identifier) : null,
+    );
     if (ts.isIdentifier(identifier) && lowerer.isStdlibGlobal(identifier, "Array")) {
       return lowerArrayConstructor(lowerer, expr, expr.arguments?.nodes ?? []);
     }
@@ -10595,7 +10700,11 @@ export function lowerNew(lowerer: Lowerer, expr: ts.NewExpression): IrExpr {
       // expression). The CONTEXTUAL type carries the annotation — adopt
       // it when it is a supported map. TS type arguments keep winning:
       // their expression type already maps.
-      if (mapped?.kind !== "map" || contextualSeed) {
+      const inferred = initializedCollectionType(lowerer, expr);
+      if (inferred) {
+        tsType = inferred;
+        mapped = lowerer.mapTypeOf(inferred);
+      } else if (mapped?.kind !== "map" || contextualSeed) {
         const ctx = lowerer.checker.getContextualType(expr);
         const ctxMapped = ctx ? lowerer.mapTypeOf(ctx) : null;
         if (ctx && ctxMapped?.kind === "map") {
@@ -10735,7 +10844,8 @@ export function lowerNew(lowerer: Lowerer, expr: ts.NewExpression): IrExpr {
     // `new Set<T>()`: construct + bulk add from supported values,
     // including live collection cursors. Duplicates collapse in insertion order.
     if (ts.isIdentifier(identifier) && lowerer.isStdlibGlobal(identifier, "Set")) {
-      const tsType = lowerer.typeOf(expr);
+      const inferred = initializedCollectionType(lowerer, expr);
+      const tsType = inferred ?? lowerer.typeOf(expr);
       let mapped = lowerer.mapTypeOf(tsType);
       const fieldType = assignedThisFieldType(lowerer, expr);
       if (mapped?.kind !== "set" && fieldType?.kind === "set") mapped = fieldType;
@@ -10926,99 +11036,6 @@ export function lowerNew(lowerer: Lowerer, expr: ts.NewExpression): IrExpr {
       );
     }
   }
-  // `new crypto.X509Certificate(data)` — the Dirent-style data record:
-  // the certificate's lowered members (fingerprint — the SHA-1 of the
-  // DER, uppercase colon-separated — plus the validFrom/validTo
-  // validity window in Node's ASN1_TIME_print shape) compute AT
-  // CONSTRUCTION, when Node parses too, so unparseable input throws
-  // Node's exact PEM error here (ERR_OSSL_PEM_NO_START_LINE) and the
-  // handle never exists. Both import forms (`crypto.X509Certificate`
-  // through the namespace, named `X509Certificate`); Buffer input only
-  // — the readFileSync idiom. mapType interns the matching record, so
-  // locals and the composed member reads all flow. The data argument
-  // feeds THREE field computations, so construction goes through an
-  // interned helper whose parameter evaluates it exactly once.
-  {
-    const callee = expr.expression;
-    const isX509 =
-      (ts.isPropertyAccessExpression(callee) &&
-        callee.name.text === "X509Certificate" &&
-        lowerer.builtinNamespaceModuleOf(callee.expression) === "crypto") ||
-      (ts.isIdentifier(callee) &&
-        (() => {
-          const bi = lowerer.builtinImportOf(callee);
-          return bi?.module === "crypto" && bi.member === "X509Certificate";
-        })());
-    if (isX509) {
-      const args: readonly ts.Expression[] = expr.arguments?.nodes ?? [];
-      if (args.length !== 1) {
-        lowerer.noLowering(
-          "X509Certificate with this argument shape",
-          expr,
-          "the supported form is new X509Certificate(readFileSync(path))",
-        );
-      }
-      const data = lowerer.lowerExpr(args[0]!);
-      const isBytes = data.type.kind === "bytes" && data.type.elem === "u8";
-      if (!isBytes && data.type.kind !== "string") {
-        lowerer.noLowering(
-          `X509Certificate over '${lowerer.fmt(data.type)}' data`,
-          args[0]!,
-          "pass the certificate Buffer or PEM string (an fs.readFileSync result)",
-        );
-      }
-      const t = lowerer.mapTypeOf(lowerer.typeOf(expr));
-      if (t?.kind !== "record") lowerer.badType(expr, lowerer.typeOf(expr));
-      const key = `x509.record:${isBytes ? "bytes" : "str"}`;
-      let helper = lowerer.valueHelpers.get(key);
-      if (!helper) {
-        helper = keyedHelperName(lowerer, "%x509.record", key);
-        lowerer.valueHelpers.set(key, helper);
-        const dataT = data.type;
-        const dRef: IrExpr = { kind: "varRef", localId: "d.0", type: dataT, loc };
-        const field = (
-          name: string,
-          fn:
-            | "crypto.x509Fingerprint"
-            | "crypto.x509FingerprintStr"
-            | "crypto.x509ValidFrom"
-            | "crypto.x509ValidFromStr"
-            | "crypto.x509ValidTo"
-            | "crypto.x509ValidToStr",
-        ): { name: string; value: IrExpr } => ({
-          name,
-          value: { kind: "libCall", fn, args: [dRef], type: STRING, loc },
-        });
-        lowerer.liftedFns.push({
-          name: helper,
-          params: [{ localId: "d.0", name: "d", type: dataT }],
-          returnType: t,
-          locals: [{ id: "d.0", name: "d", type: dataT, mutable: false }],
-          body: [
-            {
-              kind: "return",
-              value: {
-                kind: "recordLit",
-                fields: [
-                  field(
-                    "fingerprint",
-                    isBytes ? "crypto.x509Fingerprint" : "crypto.x509FingerprintStr",
-                  ),
-                  field("validFrom", isBytes ? "crypto.x509ValidFrom" : "crypto.x509ValidFromStr"),
-                  field("validTo", isBytes ? "crypto.x509ValidTo" : "crypto.x509ValidToStr"),
-                ],
-                type: t,
-                loc,
-              },
-              loc,
-            },
-          ],
-          loc,
-        });
-      }
-      return { kind: "call", callee: helper, args: [data], type: t, loc };
-    }
-  }
   // `new X(...)` through a class VALUE (a classval-typed binding, array
   // element, map read, param): the newValue dispatch through the class
   // object's construct thunk. Arguments complete against the STATIC
diff --git a/packages/compiler/src/frontend/lowering/lower-functions.ts b/packages/compiler/src/frontend/lowering/lower-functions.ts
index c1cca9c64..bd9b5f55d 100644
--- a/packages/compiler/src/frontend/lowering/lower-functions.ts
+++ b/packages/compiler/src/frontend/lowering/lower-functions.ts
@@ -1,5 +1,6 @@
 import type { ScrDiagnostic } from "../../diagnostics/diagnostic.js";
 import { declareTypedArguments } from "./typed-arguments.js";
+import { declareArgumentCount } from "./argument-counts.js";
 import { dynUndefinedExpr, varRef } from "../../ir/build.js";
 import { everyStmtList, transformStmtList } from "../../ir/traverse.js";
 import { bindingNeverReassigned } from "./binding-analysis.js";
@@ -534,12 +535,18 @@ export function lowerFunction(lowerer: Lowerer, decl: ts.FunctionDeclaration): I
   const diagsBefore = lowerer.diags.length;
   lowerer.fnStack.push(ctx);
   try {
+    // The count a call through a value stored is taken (and its slot
+    // cleared) before the parameters bind: a destructuring default that
+    // throws must not leave the slot set for the next direct call.
+    const argumentCount: IrStmt[] = [];
+    declareArgumentCount(lowerer, decl, argumentCount);
     const receiver =
       decl.parameters.nodes.some(isThisParameter) || isJsSourceFile(ts.getSourceFileOfNode(decl))
         ? declareCallReceiver(lowerer, decl, sig.generator !== undefined)
         : null;
     const { params, prologue } = lowerer.declareParams(decl.parameters.nodes, sig.params);
     if (receiver) prologue.unshift(receiver);
+    prologue.unshift(...argumentCount);
     declareFunctionArguments(
       lowerer,
       decl,
diff --git a/packages/compiler/src/frontend/lowering/lower-stream.ts b/packages/compiler/src/frontend/lowering/lower-stream.ts
index 04c2fa043..d67cf257e 100644
--- a/packages/compiler/src/frontend/lowering/lower-stream.ts
+++ b/packages/compiler/src/frontend/lowering/lower-stream.ts
@@ -2837,6 +2837,107 @@ export function streamClassAliasDecl(
 const PIPELINE_STAGE_EXPECTED =
   "of type function or an instance of Blob, ReadableStream, WritableStream, Stream, Iterable, AsyncIterable, or Promise or { readable, writable } pair";
 
+/** A checked-dynamic pipeline stage (an untyped source or destination):
+ * a boxed native stream unwraps at run time; other values (functions,
+ * pairs) throw the not-supported Error there. */
+function unboxDynStage(lowerer: Lowerer, v: IrExpr, node: ts.Expression): IrExpr {
+  noteUntypedStreamFlow(lowerer, null);
+  return {
+    kind: "libCall",
+    fn: "stream.unboxDyn",
+    args: [
+      v,
+      strLit(
+        lowerer.dynNodeStreams
+          ? "pipeline stages other than native streams (JavaScript stream objects, iterables, generators, plain functions, and { readable, writable } pairs) are not supported yet"
+          : "pipeline stages other than native streams (iterables, generators, plain functions, and { readable, writable } pairs) are not supported yet",
+        locOf(node),
+      ),
+    ],
+    type: { kind: "object", className: "%Duplex" },
+    loc: locOf(node),
+  };
+}
+
+/** A pipeline whose first stage is checked-dynamic, lowered with that
+ * stage already evaluated: `expr` is the stream it unboxes to, or, when
+ * `iterable` is set, the value to pump as an iterable source. */
+interface DynPipelineSource {
+  expr: IrExpr;
+  iterable: boolean;
+}
+
+/** pipeline() whose first stage is a stream argument (not a function
+ * stage, an iterable of a static type, or a cipher value), lowered here.
+ * When the value is checked-dynamic, Node keeps a stream there and pipes
+ * it, and keeps an iterable (a string, an array, a generator, any value
+ * with an iterator method) and pumps it into the next stage. The value is
+ * evaluated once, and stream.dynIterableSource picks the lowering at run
+ * time. Null when the first stage is not such an argument. */
+function lowerDynSourcePipeline(
+  lowerer: Lowerer,
+  node: ts.Expression,
+  loc: SrcLoc,
+  build: (source: DynPipelineSource) => IrExpr,
+): IrExpr | null {
+  if (pipelineStageFunction(lowerer, node) !== null) return null;
+  if (iterableSourceType(lowerer, node) !== null || isValueStream(lowerer, node)) return null;
+  let value = lowerer.lowerExpr(node);
+  // A Set, a Map or a generator object is iterated through its
+  // checked-dynamic value.
+  if (BOXED_SOURCE_KINDS.has(value.type.kind) && lowerer.dynConvertible(value.type)) {
+    value = { kind: "dynFrom", value, type: DYN, loc: value.loc };
+  }
+  if (value.type.kind !== "dyn") {
+    let stream: IrExpr;
+    try {
+      stream = streamArgOf(lowerer, node, value, "pipeline");
+    } catch (e) {
+      if (e instanceof StreamArgTypeThrow) return e.expr;
+      throw e;
+    }
+    return build({ expr: stream, iterable: false });
+  }
+  const local = lowerer.declareHiddenLocal("%pipelineSource", DYN);
+  const ref = varRef(local.id, DYN, loc);
+  const stream = build({ expr: unboxDynStage(lowerer, ref, node), iterable: false });
+  const iterable = build({ expr: ref, iterable: true });
+  const type = stream.type;
+  return {
+    kind: "seqExpr",
+    stmts: [{ kind: "varDecl", localId: local.id, init: value, loc }],
+    result: {
+      kind: "ternary",
+      cond: { kind: "libCall", fn: "stream.dynIterableSource", args: [ref], type: BOOL, loc },
+      then: lowerer.coerceToExpected(iterable, type),
+      else_: stream,
+      type,
+      loc,
+    },
+    type,
+    loc,
+  };
+}
+
+/** The static types of a pipeline source that lowerDynSourcePipeline boxes
+ * as checked-dynamic values. An array or a string reaches it when the
+ * checker types the argument as any while its value keeps a static type. */
+const BOXED_SOURCE_KINDS: ReadonlySet<IrType["kind"]> = new Set([
+  "set",
+  "map",
+  "generator",
+  "array",
+  "string",
+]);
+
+/** lowerPipelineStages' source arguments for a checked-dynamic first stage. */
+function stagesSource(
+  src: DynPipelineSource | null,
+): [ReadonlyMap<number, IrExpr> | undefined, IrExpr | undefined] {
+  if (src === null) return [undefined, undefined];
+  return src.iterable ? [undefined, src.expr] : [new Map([[0, src.expr]]), undefined];
+}
+
 /** A stream-rooted argument, lowered — or the pointed fence (pipeline's
  * iterable/generator/function stages have no lowering yet). `what` picks
  * the validation ladder: finished() rejects EVERY non-stream through
@@ -2848,7 +2949,16 @@ function lowerStreamArg(
   node: ts.Expression,
   what: "finished" | "pipeline" | "addAbortSignal" | "compose",
 ): IrExpr {
-  const lowered = lowerer.lowerExpr(node);
+  return streamArgOf(lowerer, node, lowerer.lowerExpr(node), what);
+}
+
+/** lowerStreamArg over the argument's lowered value. */
+function streamArgOf(
+  lowerer: Lowerer,
+  node: ts.Expression,
+  lowered: IrExpr,
+  what: "finished" | "pipeline" | "addAbortSignal" | "compose",
+): IrExpr {
   // A Hash, Hmac, Cipheriv or Decipheriv stage is its Transform; pipeline
   // and compose stages keep the value alive until the stream settles.
   const v = isValueStream(lowerer, node)
@@ -2857,24 +2967,7 @@ function lowerStreamArg(
   // A checked-dynamic pipeline stage (an untyped source or destination):
   // a boxed native stream unwraps at run time; other values (iterables,
   // functions) throw the not-supported Error there.
-  if (v.type.kind === "dyn" && what === "pipeline") {
-    noteUntypedStreamFlow(lowerer, null);
-    return {
-      kind: "libCall",
-      fn: "stream.unboxDyn",
-      args: [
-        v,
-        strLit(
-          lowerer.dynNodeStreams
-            ? "pipeline stages other than native streams (JavaScript stream objects, iterables, generators, plain functions, and { readable, writable } pairs) are not supported yet"
-            : "pipeline stages other than native streams (iterables, generators, plain functions, and { readable, writable } pairs) are not supported yet",
-          locOf(node),
-        ),
-      ],
-      type: { kind: "object", className: "%Duplex" },
-      loc: locOf(node),
-    };
-  }
+  if (v.type.kind === "dyn" && what === "pipeline") return unboxDynStage(lowerer, v, node);
   const info = v.type.kind === "object" ? lowerer.classes.get(v.type.className) : undefined;
   if (!streamSidesOf(lowerer, info)) {
     // A provably-non-stream value in a JS source (the invalid-input
@@ -3210,57 +3303,82 @@ export function lowerStreamModuleCall(
           ? pipelineOptions(lowerer, lastArg)
           : null;
       const stageArgs = opts ? args.slice(0, -1) : args;
-      if (hasPipelineFunctionStage(lowerer, stageArgs)) {
-        const staged = lowerPipelineStages(lowerer, stageArgs, loc);
-        const type = staged.destPromise ?? { kind: "promise", inner: VOID };
-        // A promise destination's value settles the result (same type).
-        if (!opts) {
-          return staged.wrap({
-            kind: "libCall",
-            fn: "sp.pipelineStages",
-            args: staged.args,
-            type,
-            loc,
-          });
-        }
-        // the options: end, and the signal (flags bit 6)
-        const flagsArg = staged.args[3]!;
-        if (flagsArg.kind !== "numLit")
-          throw new InternalCompilerError("pipeline flags not a literal");
-        const args = [...staged.args, boolLit(opts.end, loc)];
-        if (opts.signal) {
-          args[3] = numLit(flagsArg.value | 64, loc);
-          args.push(opts.signal);
-        }
-        return staged.wrap({ kind: "libCall", fn: "sp.pipelineStagesOpts", args, type, loc });
-      }
-      let streams: IrExpr[];
-      try {
-        streams = stageArgs.map((a) => lowerStreamArg(lowerer, a, "pipeline"));
-      } catch (e) {
-        if (e instanceof StreamArgTypeThrow) return e.expr;
-        throw e;
-      }
-      if (opts) {
-        const head = [numLit(streams.length, loc), ...streams, boolLit(opts.end, loc)];
-        return {
-          kind: "libCall",
-          fn: opts.signal ? "sp.pipelineOptsSignal" : "sp.pipelineOpts",
-          args: opts.signal ? [...head, opts.signal] : head,
-          type: { kind: "promise", inner: VOID },
-          loc,
-        };
-      }
-      return {
-        kind: "libCall",
-        fn: "sp.pipeline",
-        args: [numLit(streams.length, loc), ...streams],
-        type: { kind: "promise", inner: VOID },
-        loc,
-      };
+      const build = (src: DynPipelineSource | null): IrExpr =>
+        lowerPromisePipeline(lowerer, stageArgs, opts, src, loc);
+      return lowerDynSourcePipeline(lowerer, stageArgs[0]!, loc, build) ?? build(null);
     }
     return null;
   }
+  return lowerStreamModuleCallbackCall(lowerer, call, bi, loc);
+}
+
+/** stream/promises pipeline(...stages[, options]), with the first stage
+ * pre-lowered when it is checked-dynamic (lowerDynSourcePipeline). */
+function lowerPromisePipeline(
+  lowerer: Lowerer,
+  stageArgs: readonly ts.Expression[],
+  opts: ReturnType<typeof pipelineOptions> | null,
+  src: DynPipelineSource | null,
+  loc: SrcLoc,
+): IrExpr {
+  if (src?.iterable === true || hasPipelineFunctionStage(lowerer, stageArgs)) {
+    const staged = lowerPipelineStages(lowerer, stageArgs, loc, ...stagesSource(src));
+    const type = staged.destPromise ?? { kind: "promise", inner: VOID };
+    // A promise destination's value settles the result (same type).
+    if (!opts) {
+      return staged.wrap({
+        kind: "libCall",
+        fn: "sp.pipelineStages",
+        args: staged.args,
+        type,
+        loc,
+      });
+    }
+    // the options: end, and the signal (flags bit 6)
+    const flagsArg = staged.args[3]!;
+    if (flagsArg.kind !== "numLit") throw new InternalCompilerError("pipeline flags not a literal");
+    const args = [...staged.args, boolLit(opts.end, loc)];
+    if (opts.signal) {
+      args[3] = numLit(flagsArg.value | 64, loc);
+      args.push(opts.signal);
+    }
+    return staged.wrap({ kind: "libCall", fn: "sp.pipelineStagesOpts", args, type, loc });
+  }
+  let streams: IrExpr[];
+  try {
+    streams = stageArgs.map((a, i) =>
+      i === 0 && src !== null ? src.expr : lowerStreamArg(lowerer, a, "pipeline"),
+    );
+  } catch (e) {
+    if (e instanceof StreamArgTypeThrow) return e.expr;
+    throw e;
+  }
+  if (opts) {
+    const head = [numLit(streams.length, loc), ...streams, boolLit(opts.end, loc)];
+    return {
+      kind: "libCall",
+      fn: opts.signal ? "sp.pipelineOptsSignal" : "sp.pipelineOpts",
+      args: opts.signal ? [...head, opts.signal] : head,
+      type: { kind: "promise", inner: VOID },
+      loc,
+    };
+  }
+  return {
+    kind: "libCall",
+    fn: "sp.pipeline",
+    args: [numLit(streams.length, loc), ...streams],
+    type: { kind: "promise", inner: VOID },
+    loc,
+  };
+}
+
+/** The node:stream and stream/consumers members of lowerStreamModuleCall. */
+function lowerStreamModuleCallbackCall(
+  lowerer: Lowerer,
+  call: ts.CallExpression,
+  bi: { module: string; member: string },
+  loc: SrcLoc,
+): IrExpr | null {
   if (bi.module === "stream/consumers") {
     const args = call.arguments.nodes;
     // text/json/buffer/arrayBuffer(stream) → a pending promise the
@@ -3411,67 +3529,9 @@ export function lowerStreamModuleCall(
         "the supported form is pipeline(source, ...streams, callback)",
       );
     }
-    if (hasPipelineFunctionStage(lowerer, args.slice(0, -1))) {
-      const staged = lowerPipelineStages(lowerer, args.slice(0, -1), loc);
-      const cbNode = args[args.length - 1]!;
-      const cbParams =
-        ts.isArrowFunction(cbNode) || ts.isFunctionExpression(cbNode)
-          ? cbNode.parameters.nodes.length
-          : 0;
-      if (staged.dest === 2 || (staged.dest === 1 && cbParams > 1)) {
-        // callback(err, value): the destination's value (undefined for
-        // an async generator) reaches the callback through the
-        // checked-dynamic path.
-        const cb = lowerer.lowerExprExpecting(args[args.length - 1]!, DYN);
-        // SCR_PIPE_TYPED_CB: a typed callback whose error parameter cannot
-        // hold undefined takes null for no error; others take undefined.
-        const stagedArgs = [...staged.args];
-        const flagsArg = stagedArgs[3]!;
-        if (flagsArg.kind !== "numLit")
-          throw new InternalCompilerError("pipeline flags not a literal");
-        if (
-          !isJsSourceFile(ts.getSourceFileOfNode(call)) &&
-          !callbackErrorAdmitsUndefined(lowerer, cb)
-        )
-          stagedArgs[3] = numLit(flagsArg.value | 128, loc);
-        return staged.wrap({
-          kind: "libCall",
-          fn: "stream.pipelineStagesDyn",
-          args: [...stagedArgs, cb],
-          type: staged.last,
-          loc,
-        });
-      }
-      const { cb, dyn } = lowerEosCallback(
-        lowerer,
-        "pipeline",
-        args[args.length - 1]!,
-        staged.last,
-      );
-      return staged.wrap({
-        kind: "libCall",
-        fn: dyn ? "stream.pipelineStagesDyn" : "stream.pipelineStages",
-        args: [...staged.args, cb],
-        type: staged.last,
-        loc,
-      });
-    }
-    let streams: IrExpr[];
-    try {
-      streams = args.slice(0, -1).map((a) => lowerStreamArg(lowerer, a, "pipeline"));
-    } catch (e) {
-      if (e instanceof StreamArgTypeThrow) return e.expr;
-      throw e;
-    }
-    const last = streams[streams.length - 1]!;
-    const { cb, dyn } = lowerEosCallback(lowerer, "pipeline", args[args.length - 1]!, last.type);
-    return {
-      kind: "libCall",
-      fn: dyn ? "stream.pipelineDyn" : "stream.pipeline",
-      args: [numLit(streams.length, loc), ...streams, cb],
-      type: last.type,
-      loc,
-    };
+    const build = (src: DynPipelineSource | null): IrExpr =>
+      lowerCallbackPipeline(lowerer, call, args, src, loc);
+    return lowerDynSourcePipeline(lowerer, args[0]!, loc, build) ?? build(null);
   }
 
   if (bi.member === "compose") {
@@ -3484,6 +3544,75 @@ export function lowerStreamModuleCall(
   return null;
 }
 
+/** pipeline(...stages, callback), with the first stage pre-lowered when it
+ * is checked-dynamic (lowerDynSourcePipeline). */
+function lowerCallbackPipeline(
+  lowerer: Lowerer,
+  call: ts.CallExpression,
+  args: readonly ts.Expression[],
+  src: DynPipelineSource | null,
+  loc: SrcLoc,
+): IrExpr {
+  if (src?.iterable === true || hasPipelineFunctionStage(lowerer, args.slice(0, -1))) {
+    const staged = lowerPipelineStages(lowerer, args.slice(0, -1), loc, ...stagesSource(src));
+    const cbNode = args[args.length - 1]!;
+    const cbParams =
+      ts.isArrowFunction(cbNode) || ts.isFunctionExpression(cbNode)
+        ? cbNode.parameters.nodes.length
+        : 0;
+    if (staged.dest === 2 || (staged.dest === 1 && cbParams > 1)) {
+      // callback(err, value): the destination's value (undefined for
+      // an async generator) reaches the callback through the
+      // checked-dynamic path.
+      const cb = lowerer.lowerExprExpecting(args[args.length - 1]!, DYN);
+      // SCR_PIPE_TYPED_CB: a typed callback whose error parameter cannot
+      // hold undefined takes null for no error; others take undefined.
+      const stagedArgs = [...staged.args];
+      const flagsArg = stagedArgs[3]!;
+      if (flagsArg.kind !== "numLit")
+        throw new InternalCompilerError("pipeline flags not a literal");
+      if (
+        !isJsSourceFile(ts.getSourceFileOfNode(call)) &&
+        !callbackErrorAdmitsUndefined(lowerer, cb)
+      )
+        stagedArgs[3] = numLit(flagsArg.value | 128, loc);
+      return staged.wrap({
+        kind: "libCall",
+        fn: "stream.pipelineStagesDyn",
+        args: [...stagedArgs, cb],
+        type: staged.last,
+        loc,
+      });
+    }
+    const { cb, dyn } = lowerEosCallback(lowerer, "pipeline", args[args.length - 1]!, staged.last);
+    return staged.wrap({
+      kind: "libCall",
+      fn: dyn ? "stream.pipelineStagesDyn" : "stream.pipelineStages",
+      args: [...staged.args, cb],
+      type: staged.last,
+      loc,
+    });
+  }
+  let streams: IrExpr[];
+  try {
+    streams = args
+      .slice(0, -1)
+      .map((a, i) => (i === 0 && src !== null ? src.expr : lowerStreamArg(lowerer, a, "pipeline")));
+  } catch (e) {
+    if (e instanceof StreamArgTypeThrow) return e.expr;
+    throw e;
+  }
+  const last = streams[streams.length - 1]!;
+  const { cb, dyn } = lowerEosCallback(lowerer, "pipeline", args[args.length - 1]!, last.type);
+  return {
+    kind: "libCall",
+    fn: dyn ? "stream.pipelineDyn" : "stream.pipeline",
+    args: [numLit(streams.length, loc), ...streams, cb],
+    type: last.type,
+    loc,
+  };
+}
+
 /** A pipeline() function stage: an inline function or arrow function, or
  * an identifier bound to a function declaration or to a `const`
  * initialized with one, which nothing reassigns. */
@@ -3788,6 +3917,302 @@ function lowerWebStreamSource(
   };
 }
 
+/** A checked-dynamic pipeline source that is an iterable at run time
+ * (stream.dynIterableSource), as an async generator closure that iterates
+ * it as Node's pipeline does with `for await`: the value's async iterator,
+ * or its sync iterator stepped like the typed iterable sources. Each value
+ * is yielded; leaving early closes the iterator through its return(). */
+function lowerDynIterableSource(
+  lowerer: Lowerer,
+  value: IrExpr,
+  loc: SrcLoc,
+): { closure: IrExpr; out: SyncGenType } | null {
+  const out: SyncGenType = {
+    kind: "generator",
+    async: true,
+    yieldT: DYN,
+    retT: VOID,
+    nextT: UNDEFINED_T,
+  };
+  const genResultT = genResultRecord(DYN, VOID, lowerer.shapes, lowerer.unions);
+  if (!genResultT) return null;
+  const key = "dynIterableSource";
+  let maker = lowerer.valueHelpers.get(key);
+  if (!maker) {
+    maker = `%stream.dynIterableSource.${lowerer.valueHelpers.size}`;
+    const genName = `${maker}.gen`;
+    const frame = new LiftedFrame(loc);
+    const srcId = frame.local("src", DYN, false);
+    const itId = frame.local("it", DYN, false);
+    const syncId = frame.local("sync", BOOL, false);
+    const nextId = frame.local("next", DYN, false);
+    const stepId = frame.local("step", DYN, true);
+    const valueId = frame.local("value", DYN, true);
+    const activeId = frame.local("active", BOOL, true);
+    const closeId = frame.local("close", DYN, false);
+    const it = (): IrExpr => varRef(itId, DYN, loc);
+    const sync = (): IrExpr => varRef(syncId, BOOL, loc);
+    const step = (): IrExpr => varRef(stepId, DYN, loc);
+    const value = (): IrExpr => varRef(valueId, DYN, loc);
+    // for await over a sync iterator awaits each value, the final one
+    // included (CreateAsyncFromSyncIterator): an object (a promise or a
+    // thenable) passes on its result
+    const awaitsValue = (): IrExpr => ({
+      kind: "logical",
+      op: "&&",
+      left: sync(),
+      right: {
+        kind: "logical",
+        op: "||",
+        left: { kind: "dynTest", test: "promise", value: value(), type: BOOL, loc },
+        right: {
+          kind: "logical",
+          op: "&&",
+          left: {
+            kind: "dynTest",
+            test: "nullish",
+            negated: true,
+            value: value(),
+            type: BOOL,
+            loc,
+          },
+          right: { kind: "dynTest", test: "object", value: value(), type: BOOL, loc },
+          type: BOOL,
+          loc,
+        },
+        type: BOOL,
+        loc,
+      },
+      type: BOOL,
+      loc,
+    });
+    const get = (obj: IrExpr, name: string): IrExpr => ({
+      kind: "dynKeyGet",
+      value: obj,
+      key: strLit(name, loc),
+      type: DYN,
+      loc,
+    });
+    const awaitDyn = (v: IrExpr): IrExpr => ({
+      kind: "libCall",
+      fn: "async.awaitDyn",
+      args: [v],
+      type: DYN,
+      loc,
+    });
+    const call = (callee: IrExpr, calleeName: string): IrExpr => ({
+      kind: "dynCall",
+      callee,
+      receiver: it(),
+      calleeName,
+      args: [],
+      type: DYN,
+      loc,
+    });
+    const setActive = (on: boolean): IrStmt => ({
+      kind: "assign",
+      localId: activeId,
+      value: boolLit(on, loc),
+      loc,
+    });
+    const nextCall = (): IrExpr => call(varRef(nextId, DYN, loc), "iterator method");
+    const body: IrStmt[] = [
+      {
+        kind: "varDecl",
+        localId: itId,
+        init: {
+          kind: "libCall",
+          fn: "dyn.iteratorResult",
+          args: [
+            {
+              kind: "libCall",
+              fn: "dyn.asyncIteratorOpen",
+              args: [varRef(srcId, DYN, loc), strLit("", loc)],
+              type: DYN,
+              loc,
+            },
+          ],
+          type: DYN,
+          loc,
+        },
+        loc,
+      },
+      {
+        kind: "varDecl",
+        localId: syncId,
+        init: { kind: "libCall", fn: "dyn.asyncIteratorWasSync", args: [], type: BOOL, loc },
+        loc,
+      },
+      { kind: "varDecl", localId: nextId, init: get(it(), "next"), loc },
+      { kind: "varDecl", localId: stepId, init: dynUndefinedExpr(loc), loc },
+      { kind: "varDecl", localId: valueId, init: dynUndefinedExpr(loc), loc },
+      { kind: "varDecl", localId: activeId, init: boolLit(false, loc), loc },
+      {
+        kind: "tryCatch",
+        tryBody: [
+          {
+            kind: "while",
+            cond: boolLit(true, loc),
+            body: [
+              setActive(false),
+              {
+                kind: "assign",
+                localId: stepId,
+                value: {
+                  kind: "libCall",
+                  fn: "dyn.iteratorResult",
+                  args: [
+                    {
+                      kind: "ternary",
+                      cond: sync(),
+                      then: nextCall(),
+                      else_: awaitDyn(nextCall()),
+                      type: DYN,
+                      loc,
+                    },
+                  ],
+                  type: DYN,
+                  loc,
+                },
+                loc,
+              },
+              {
+                kind: "if",
+                cond: {
+                  kind: "dynTest",
+                  test: "truthy",
+                  value: get(step(), "done"),
+                  type: BOOL,
+                  loc,
+                },
+                // a sync iterator's final value is awaited too: a rejection
+                // rejects the pump, and the finished iterator is not closed.
+                // An async iterator's final value is not read.
+                then: [
+                  {
+                    kind: "if",
+                    cond: sync(),
+                    then: [
+                      { kind: "assign", localId: valueId, value: get(step(), "value"), loc },
+                      {
+                        kind: "if",
+                        cond: awaitsValue(),
+                        then: [{ kind: "exprStmt", expr: awaitDyn(value()), loc }],
+                        else_: null,
+                        loc,
+                      },
+                    ],
+                    else_: null,
+                    loc,
+                  },
+                  { kind: "break", loc },
+                ],
+                else_: null,
+                loc,
+              },
+              setActive(true),
+              { kind: "assign", localId: valueId, value: get(step(), "value"), loc },
+              // a rejection here closes the iterator
+              {
+                kind: "if",
+                cond: awaitsValue(),
+                then: [{ kind: "assign", localId: valueId, value: awaitDyn(value()), loc }],
+                else_: null,
+                loc,
+              },
+              {
+                kind: "exprStmt",
+                expr: { kind: "yieldExpr", value: value(), type: VOID, loc },
+                loc,
+              },
+            ],
+            loc,
+          },
+        ],
+        catchBody: null,
+        catchLocalId: null,
+        finallyBody: [
+          {
+            kind: "if",
+            cond: varRef(activeId, BOOL, loc),
+            then: [
+              { kind: "varDecl", localId: closeId, init: get(it(), "return"), loc },
+              {
+                kind: "if",
+                cond: {
+                  kind: "unary",
+                  op: "!",
+                  operand: {
+                    kind: "dynTest",
+                    test: "nullish",
+                    value: varRef(closeId, DYN, loc),
+                    type: BOOL,
+                    loc,
+                  },
+                  type: BOOL,
+                  loc,
+                },
+                then: [
+                  {
+                    kind: "exprStmt",
+                    expr: awaitDyn(call(varRef(closeId, DYN, loc), "iterator return")),
+                    loc,
+                  },
+                ],
+                else_: null,
+                loc,
+              },
+            ],
+            else_: null,
+            loc,
+          },
+        ],
+        loc,
+      },
+      { kind: "return", value: null, loc },
+    ];
+    const srcLocal = frame.locals.find((l) => l.id === srcId)!;
+    srcLocal.boxed = true;
+    lowerer.valueHelpers.set(key, maker);
+    lowerer.liftedFns.push({
+      name: genName,
+      params: [],
+      returnType: VOID,
+      async: true,
+      captures: [{ localId: srcId, name: "src", type: DYN }],
+      locals: frame.locals,
+      body,
+      generator: { yieldT: DYN, nextT: UNDEFINED_T, resultType: genResultT },
+      loc,
+    });
+    const makerSrc: IrLocal = { id: "src.0", name: "src", type: DYN, mutable: false, boxed: true };
+    lowerer.liftedFns.push({
+      name: maker,
+      params: [{ localId: makerSrc.id, name: makerSrc.name, type: DYN }],
+      returnType: funcOf([], out),
+      locals: [makerSrc],
+      body: [
+        {
+          kind: "return",
+          value: {
+            kind: "closure",
+            fnName: genName,
+            captures: [makerSrc.id],
+            type: funcOf([], out),
+            loc,
+          },
+          loc,
+        },
+      ],
+      loc,
+    });
+  }
+  return {
+    closure: { kind: "call", callee: maker, args: [value], type: funcOf([], out), loc },
+    out,
+  };
+}
+
 function lowerIterableSource(
   lowerer: Lowerer,
   node: ts.Expression,
@@ -3951,6 +4376,7 @@ function lowerPipelineStages(
   stageArgs: readonly ts.Expression[],
   loc: SrcLoc,
   preLowered?: ReadonlyMap<number, IrExpr>,
+  dynSource?: IrExpr,
 ): PipelineStagesLowering {
   if (stageArgs.length > 52) {
     lowerer.noLowering(
@@ -3975,6 +4401,15 @@ function lowerPipelineStages(
   const argOrder: { list: IrExpr[]; index: number }[] = [];
   for (let j = 0; j < stageArgs.length; j++) {
     const arg = stageArgs[j]!;
+    if (j === 0 && dynSource !== undefined) {
+      const source = lowerDynIterableSource(lowerer, dynSource, loc);
+      if (source === null) lowerer.noLowering("pipeline over this source", arg);
+      positions += 1;
+      argOrder.push({ list: stages, index: stages.length });
+      stages.push(source!.closure);
+      srcT = source!.out;
+      continue;
+    }
     // compose's head and tail functions arrive as Duplex.from streams
     const fn = preLowered?.has(j) ? null : pipelineStageFunction(lowerer, arg);
     const syncSourceFn =
diff --git a/packages/compiler/src/frontend/lowering/lowerer.ts b/packages/compiler/src/frontend/lowering/lowerer.ts
index 6fd6d863a..62b7d55c5 100644
--- a/packages/compiler/src/frontend/lowering/lowerer.ts
+++ b/packages/compiler/src/frontend/lowering/lowerer.ts
@@ -156,6 +156,7 @@ import {
   trackClassFieldCreation,
 } from "./class-dynamic-dispatch.js";
 import { guardClassAttributeWrites } from "./class-attributes.js";
+import { inferredCollectionType } from "./js-collection-types.js";
 import { guardFrozenWrites, markSloppyRecordWrites } from "./frozen-records.js";
 import { applyHandleProperties } from "./handle-properties.js";
 import { installBufferValue } from "./containers/bytes.js";
@@ -2377,6 +2378,9 @@ export class Lowerer {
    * every name destructured from a present source optional. */
   readonly runtimeOptionalPatternTypes = new Map<ts.Node, IrType>();
   readonly runtimeOptionalReduceTypes = new Map<ts.CallExpression, IrType>();
+  /** Each function declaration's `arguments.length` count slot, or null
+   * (argument-counts.ts argumentCountSlot). */
+  readonly argumentCountSlots = new Map<ts.FunctionDeclaration, string | null>();
   /** Non-null assertions whose value is forwarded into a slot promoted to
    * hold undefined. `x!` has no runtime effect, so these keep the stored
    * undefined state instead of trapping before any dereference. */
@@ -2853,6 +2857,9 @@ export class Lowerer {
   /** Parameters read as `any` in the error-path copy of an error-first
    * callback. */
   readonly anySymbols = new Set<ts.Symbol>();
+  /** The collection types that uses prove for untyped JavaScript bindings,
+   * by every identifier naming one (js-collection-types.ts). */
+  inferredCollectionTypes: Map<ts.Node, ts.Type> | undefined = undefined;
   /** overload-choice's answers per call, binding and function. */
   readonly dynamicOverloadAnswers = new Map<ts.Node, boolean>();
   /** Error parameters of an error-first callback with a known value in
@@ -3174,6 +3181,14 @@ export class Lowerer {
    * (builtins/fs-paths.ts): each lowers to the hidden local that holds its
    * value, which is null until the path's lowering has saved it. */
   readonly stagedArguments = new Map<ts.Expression, { localId: string; type: IrType } | null>();
+  /** Closure arguments an implicit-any call already lowered to learn their
+   * type (storedImplicitArgumentType), by node: the argument's own lowering
+   * in the same function and with the same type bindings takes that value
+   * once instead of lowering the closure again. */
+  readonly speculatedClosures = new Map<
+    ts.Expression,
+    { ctx: FnCtx; bindings: Map<ts.Symbol, IrType> | null; value: IrExpr }
+  >();
   /** What each fs path argument of a URL-path call lowers to (the plans
    * of builtins/fs-paths.ts). */
   readonly fsPathPlans = new Map<ts.Expression, FsPathPlan>();
@@ -9075,6 +9090,10 @@ export class Lowerer {
   }
 
   typeOf(node: ts.Node): ts.Type {
+    if (ts.isIdentifier(node)) {
+      const inferred = inferredCollectionType(this, node);
+      if (inferred) return inferred;
+    }
     if (this.anySymbols.size > 0 && this.readsAnySymbol(node)) return this.checker.getAnyType();
     // A checked-dynamic argument that another overload could take at run
     // time leaves the resolved overload's return a guess, and JS bindings
@@ -12938,12 +12957,30 @@ export class Lowerer {
   /* ── expressions ──────────────────────────────────────────────────── */
 
   lowerExpr(expr: ts.Expression): IrExpr {
+    if (this.speculatedClosures.size > 0) {
+      const speculated = this.speculatedClosure(expr);
+      if (speculated !== null) {
+        this.speculatedClosures.delete(expr);
+        return speculated;
+      }
+    }
     const streamValue = this.valueStreamReceivers.get(expr);
     if (streamValue)
       return valueStreamOf(varRef(streamValue.localId, streamValue.type, locOf(expr)));
     return this.stagedArgument(expr) ?? lowerExpr(this, expr);
   }
 
+  /** The value storedImplicitArgumentType lowered for `expr` in the active
+   * function and type bindings (speculatedClosures), or null. */
+  speculatedClosure(expr: ts.Expression): IrExpr | null {
+    const entry = this.speculatedClosures.get(expr);
+    return entry !== undefined &&
+      entry.ctx === this.ctx &&
+      entry.bindings === this.typeParamBindings
+      ? entry.value
+      : null;
+  }
+
   /** The saved value of a staged fs argument (stagedArguments, keyed
    * without type-only wrappers), or null for every other expression. */
   private stagedArgument(node: ts.Expression): IrExpr | null {
diff --git a/packages/compiler/src/frontend/tsc-codes.ts b/packages/compiler/src/frontend/tsc-codes.ts
index e8ffc29fb..a2831f1d1 100644
--- a/packages/compiler/src/frontend/tsc-codes.ts
+++ b/packages/compiler/src/frontend/tsc-codes.ts
@@ -128,6 +128,12 @@ export const JS_RELAXED_TSC_CODES: ReadonlySet<number> = new Set([
   // JavaScript subclasses may widen fields or add method parameters.
   // Class lowering checks native storage and override ABIs at each use.
   2415,
+  // a private or protected constructor (`new readline.Interface(...)`,
+  // whose declaration is protected): JavaScript has no constructor
+  // accessibility, and Node constructs the class. Each `new` meets the
+  // lowerer's per-site constructor fences.
+  2673,
+  2674,
   // unresolvable modules: Node throws WHEN the require executes — the
   // binding types as any and reached uses fence instead of passing unchecked
   2307,
diff --git a/packages/compiler/src/index.ts b/packages/compiler/src/index.ts
index 4fb64968b..8291528fa 100644
--- a/packages/compiler/src/index.ts
+++ b/packages/compiler/src/index.ts
@@ -94,6 +94,7 @@ import { createNativeLinkPlan } from "./backend/link-plan.js";
 import {
   nativeProfileKey,
   nativeProfileRefusal,
+  type NativeProfileMatch,
   type NativeProfileOptions,
 } from "./backend/native-profile.js";
 import {
@@ -659,6 +660,7 @@ interface NativeProgramObject {
   partitionPaths: string[];
   artifactPath: string;
   dependencies: NativeArtifactDependency[];
+  profileMatch?: NativeProfileMatch;
 }
 
 async function emitNativeProgramObject(
@@ -694,6 +696,7 @@ async function emitNativeProgramObject(
       partitionPaths: artifact.outputPaths.slice(1),
       artifactPath,
       dependencies: artifact.dependencies,
+      ...(artifact.profileMatch === undefined ? {} : { profileMatch: artifact.profileMatch }),
     };
   } catch (error) {
     await removeNativeProgramObject(linkPath, nativePartitionPaths(linkPath, partitions));
@@ -1852,6 +1855,7 @@ async function compileTracked(
   }
   await pruneBuildCache(cacheRoot);
   timing("complete");
+  const profileMatch = nativeProgramObject?.profileMatch;
   return {
     ok: true,
     artifact: {
@@ -1859,6 +1863,7 @@ async function compileTracked(
       path: opts.outPath,
       translationUnitPath: llvmPath,
       backend,
+      ...(profileMatch === undefined ? {} : { profileMatch }),
     },
     binaryPath: opts.outPath,
     llvmPath,
diff --git a/packages/compiler/src/ir/builtin-effects.ts b/packages/compiler/src/ir/builtin-effects.ts
index 7317116db..0c82bbad0 100644
--- a/packages/compiler/src/ir/builtin-effects.ts
+++ b/packages/compiler/src/ir/builtin-effects.ts
@@ -91,6 +91,7 @@ export const MAY_THROW_LIB_FNS: ReadonlySet<IrLibFn> = new Set([
   "procStream.write",
   "child.stdioStream",
   "stream.unboxDyn",
+  "stream.dynIterableSource",
   "stream.handleStream",
   "stream.pipeDyn",
   "stream.unpipeDyn",
diff --git a/packages/compiler/src/ir/builtin-signatures.ts b/packages/compiler/src/ir/builtin-signatures.ts
index 1109f7f95..b2bb52395 100644
--- a/packages/compiler/src/ir/builtin-signatures.ts
+++ b/packages/compiler/src/ir/builtin-signatures.ts
@@ -2788,6 +2788,10 @@ export const LIB_FN_SIGS = defineLibFnSignatures({
     argTypes: [DYN, STRING],
     result: { kind: "object", className: "%Duplex" },
   },
+  /** Whether pipeline() pumps a checked-dynamic first stage as an
+   * iterable (a string, or a value with an iterator method that is not a
+   * stream with pipe() and on()) instead of piping it as a stream. */
+  "stream.dynIterableSource": { argTypes: [DYN], result: BOOL },
   /** The Transform a Cipheriv or Decipheriv reads and writes as a stream
    * (+1, created on first use and owned by the cipher). */
   "stream.handleStream": {
@@ -4930,6 +4934,9 @@ export const LIB_FN_SIGS = defineLibFnSignatures({
    * and crypto callback adapters pass NULL after an error. Borrowed arg;
    * never throws. */
   "bytes.isAbsent": { argTypes: [BYTES_U8], result: BOOL },
+  /** Whether two typed arrays share no byte: their element ranges in
+   * memory do not overlap. Borrowed args of any element kind; never throws. */
+  "bytes.disjoint": { argTypes: [null, null], result: BOOL },
   /** The absent Buffer result itself (NULL), where an error-path callback
    * body passes its undefined result on into a Buffer slot. Owned (+1);
    * never throws. */
diff --git a/packages/compiler/src/ir/row-pairs.test.ts b/packages/compiler/src/ir/row-pairs.test.ts
new file mode 100644
index 000000000..f9db5b89d
--- /dev/null
+++ b/packages/compiler/src/ir/row-pairs.test.ts
@@ -0,0 +1,147 @@
+import { expect, test } from "vitest";
+import { BOOL, F64, VOID, type IrExpr, type IrFunction, type IrModule, type IrStmt } from "./ir.js";
+import { pairRowSums } from "./row-pairs.js";
+
+const loc = { file: "row-pairs.ts", start: 0, end: 0 };
+const BYTES = { kind: "bytes", elem: "f64" } as const;
+const num = (value: number): IrExpr => ({ loc, kind: "numLit", value, type: F64 });
+const ref = (localId: string): IrExpr => ({ loc, kind: "varRef", localId, type: F64 });
+const bytes = (localId: string): IrExpr & { kind: "varRef" } => ({
+  loc,
+  kind: "varRef",
+  localId,
+  type: BYTES,
+});
+const bin = (op: "+" | "/" | "<", left: IrExpr, right: IrExpr): IrExpr => ({
+  loc,
+  kind: "bin",
+  op,
+  left,
+  right,
+  type: op === "<" ? BOOL : F64,
+});
+const step = (id: string): IrStmt => ({
+  loc,
+  kind: "assign",
+  localId: id,
+  value: bin("+", ref(id), num(1)),
+});
+const counted = (
+  id: string,
+  start: IrExpr,
+  end: IrExpr,
+  body: IrStmt[],
+): IrStmt & { kind: "for" } => ({
+  loc,
+  kind: "for",
+  init: { loc, kind: "varDecl", localId: id, init: start },
+  cond: bin("<", ref(id), end),
+  update: step(id),
+  body,
+});
+const get = (array: string, index: IrExpr): IrExpr => ({
+  loc,
+  kind: "bytesIntrinsic",
+  method: "get",
+  receiver: bytes(array),
+  args: [index],
+  type: F64,
+  invalidNaN: true,
+});
+
+/** for (i = start; i < end; i++) { t = 0; for (j = 0; j < n; j++) t += u[j] / (i + j + 1); v[i] = t } */
+function rowSum(
+  term: IrExpr = bin("/", get("u", ref("j")), bin("+", bin("+", ref("i"), ref("j")), num(1))),
+) {
+  return counted("i", ref("start"), ref("end"), [
+    { loc, kind: "varDecl", localId: "t", init: num(0) },
+    counted("j", num(0), ref("n"), [
+      { loc, kind: "assign", localId: "t", value: bin("+", ref("t"), term) },
+    ]),
+    { loc, kind: "bytesSet", arr: bytes("v"), index: ref("i"), value: ref("t") },
+  ]);
+}
+
+const local = (id: string, type: IrExpr["type"] = F64) => ({ id, name: id, type, mutable: true });
+function mod(body: IrStmt[]): IrModule {
+  const fn: IrFunction = {
+    loc,
+    name: "rows",
+    params: [],
+    locals: [
+      ...["i", "j", "t", "start", "end", "n"].map((id) => local(id)),
+      local("u", BYTES),
+      local("v", BYTES),
+    ],
+    returnType: VOID,
+    body,
+  };
+  return { irVersion: 17, sourceFile: "row-pairs.ts", entry: "rows", functions: [fn] };
+}
+
+test("a row-sum nest runs two rows per pass behind its bounds and overlap test", () => {
+  const result = pairRowSums(mod([rowSum()]));
+  const top = result.functions[0]!.body[0] as IrStmt & { kind: "block" };
+  expect(top.kind).toBe("block");
+  const [go, from, branch, rest] = top.body as [
+    IrStmt & { kind: "varDecl" },
+    IrStmt & { kind: "varDecl" },
+    IrStmt & { kind: "if" },
+    IrStmt & { kind: "for" },
+  ];
+  // The test: int32 bounds, start <= end, and the stored array disjoint
+  // from the array the term reads.
+  expect(JSON.stringify(go.init)).toContain('"fn":"bytes.disjoint"');
+  expect(branch.else_).toBeNull();
+  const pairs = branch.then[0] as IrStmt & { kind: "for" };
+  expect(pairs.body.filter((s) => s.kind === "bytesSet")).toHaveLength(2);
+  const inner = pairs.body[2] as IrStmt & { kind: "for" };
+  // One read of u[j] for both rows, then the two sums.
+  expect(inner.body).toHaveLength(3);
+  expect(inner.body[0]).toMatchObject({ kind: "varDecl", init: { kind: "bytesIntrinsic" } });
+  // The second row's term reads `row + 1`.
+  const row = pairs.init?.kind === "varDecl" ? pairs.init.localId : "";
+  expect((inner.body[2] as IrStmt & { kind: "assign" }).value).toMatchObject({
+    op: "+",
+    right: {
+      op: "/",
+      left: { kind: "varRef" },
+      right: {
+        op: "+",
+        left: {
+          op: "+",
+          left: { op: "+", left: { kind: "varRef", localId: row }, right: { value: 1 } },
+        },
+      },
+    },
+  });
+  // The loop as written runs from `end - ((end - start) & 1)` after the
+  // pairs, or from the start when the test failed: the nest's one other copy.
+  expect(from.init).toMatchObject({
+    kind: "ternary",
+    then: { op: "-" },
+    else_: { localId: "start" },
+  });
+  expect(rest.init).toMatchObject({
+    kind: "varDecl",
+    localId: "i",
+    init: { localId: from.localId },
+  });
+  expect(top.body).toHaveLength(4);
+  expect(
+    result.functions[0]!.locals.map((l) => l.id).filter((id) => id.startsWith("%pair.")),
+  ).toHaveLength(7);
+});
+
+test("nests whose term calls, writes or reads the sum keep their loops", () => {
+  const calls: IrExpr = { loc, kind: "call", callee: "f", args: [ref("j")], type: F64 };
+  const readsSum = bin("+", ref("t"), get("u", ref("j")));
+  for (const term of [calls, readsSum]) {
+    const m = mod([rowSum(term)]);
+    expect(pairRowSums(m)).toBe(m);
+  }
+  const stored = rowSum();
+  stored.body[2] = { loc, kind: "bytesSet", arr: bytes("v"), index: ref("j"), value: ref("t") };
+  const m = mod([stored]);
+  expect(pairRowSums(m)).toBe(m);
+});
diff --git a/packages/compiler/src/ir/row-pairs.ts b/packages/compiler/src/ir/row-pairs.ts
new file mode 100644
index 000000000..2e189f42e
--- /dev/null
+++ b/packages/compiler/src/ir/row-pairs.ts
@@ -0,0 +1,447 @@
+import {
+  BOOL,
+  F64,
+  type IrExpr,
+  type IrFunction,
+  type IrLocal,
+  type IrModule,
+  type IrStmt,
+} from "./ir.js";
+import { everyExpr, everyExprChild, mapExprChildren, transformStmtList } from "./traverse.js";
+
+/** Inner-loop expressions above this many nodes keep the loop as written. */
+const MAX_TERM_NODES = 64;
+
+/** The number operators a term may use: they run no other code and cannot
+ * throw. */
+const PURE_BINARY = new Set<string>([
+  "+",
+  "-",
+  "*",
+  "/",
+  "%",
+  "**",
+  "&",
+  "|",
+  "^",
+  "<<",
+  ">>",
+  ">>>",
+]);
+
+/** A loop nest that sums one term per row:
+ *
+ *   for (let i = start; i < end; i++) {
+ *     let t = <literal>;
+ *     for (let j = from; j < to; j++) t = t + <term>;
+ *     out[i] = t;
+ *   }
+ */
+interface RowSum {
+  row: string;
+  start: IrExpr;
+  end: IrExpr;
+  sum: string;
+  init: IrExpr & { kind: "numLit" };
+  inner: IrStmt & { kind: "for" };
+  column: string;
+  term: IrExpr;
+  out: IrExpr & { kind: "varRef" };
+  /** The typed arrays the term reads. */
+  reads: (IrExpr & { kind: "varRef" })[];
+}
+
+function isRef(e: IrExpr | null | undefined, id: string): boolean {
+  return e?.kind === "varRef" && e.localId === id;
+}
+
+/** `id = id + 1`, or `id++` as a statement. */
+function unitStep(update: IrStmt | null, id: string): boolean {
+  if (update?.kind === "exprStmt")
+    return update.expr.kind === "incDec" && update.expr.localId === id && update.expr.op === "+";
+  return (
+    update?.kind === "assign" &&
+    update.localId === id &&
+    update.value.kind === "bin" &&
+    update.value.op === "+" &&
+    isRef(update.value.left, id) &&
+    update.value.right.kind === "numLit" &&
+    update.value.right.value === 1
+  );
+}
+
+/** A plain number local: unboxed and outside the temporal dead zone. */
+function plainNumber(locals: ReadonlyMap<string, IrLocal>, id: string): boolean {
+  const local = locals.get(id);
+  return local !== undefined && local.type.kind === "f64" && !local.boxed && !local.tdz;
+}
+
+function readsLocal(e: IrExpr, id: string): boolean {
+  return !everyExpr(e, { stmt: () => true, expr: (x) => !isRef(x, id) });
+}
+
+/** The row-sum shape of a `for` statement, or null. Bounds and receivers
+ * must be literals or locals the nest never writes. */
+function matchRowSum(
+  s: IrStmt & { kind: "for" },
+  locals: ReadonlyMap<string, IrLocal>,
+): RowSum | null {
+  if (s.labels?.length || s.init?.kind !== "varDecl" || !s.init.init) return null;
+  const row = s.init.localId;
+  if (!plainNumber(locals, row) || !unitStep(s.update, row)) return null;
+  const cond = s.cond;
+  if (cond?.kind !== "bin" || cond.op !== "<" || !isRef(cond.left, row)) return null;
+  const start = s.init.init,
+    end = cond.right;
+  if (s.body.length !== 3) return null;
+  const [decl, inner, store] = s.body as [IrStmt, IrStmt, IrStmt];
+  if (decl.kind !== "varDecl" || decl.init?.kind !== "numLit") return null;
+  const sum = decl.localId;
+  if (!plainNumber(locals, sum)) return null;
+  if (inner.kind !== "for" || inner.labels?.length) return null;
+  if (inner.init?.kind !== "varDecl" || !inner.init.init) return null;
+  const column = inner.init.localId;
+  if (!plainNumber(locals, column) || !unitStep(inner.update, column)) return null;
+  const innerCond = inner.cond;
+  if (innerCond?.kind !== "bin" || innerCond.op !== "<" || !isRef(innerCond.left, column))
+    return null;
+  if (inner.body.length !== 1) return null;
+  const add = inner.body[0]!;
+  if (
+    add.kind !== "assign" ||
+    add.localId !== sum ||
+    add.value.kind !== "bin" ||
+    add.value.op !== "+" ||
+    !isRef(add.value.left, sum)
+  )
+    return null;
+  const term = add.value.right;
+  if (
+    store.kind !== "bytesSet" ||
+    store.arr.kind !== "varRef" ||
+    store.arr.type.kind !== "bytes" ||
+    !isRef(store.index, row) ||
+    !isRef(store.value, sum)
+  )
+    return null;
+  const out = store.arr;
+  // Values the nest reads but never writes: the row counter, the column
+  // counter and the sum are its only writes.
+  const written = new Set([row, sum, column]);
+  // A typed array's length is fixed, so a stable receiver's length is
+  // stable too.
+  const stable = (e: IrExpr): boolean =>
+    e.kind === "numLit" ||
+    (e.kind === "varRef" &&
+      !written.has(e.localId) &&
+      locals.get(e.localId) !== undefined &&
+      !locals.get(e.localId)!.boxed &&
+      !locals.get(e.localId)!.tdz) ||
+    (e.kind === "bytesIntrinsic" &&
+      e.method === "length" &&
+      e.args.length === 0 &&
+      e.receiver.kind === "varRef" &&
+      e.receiver.type.kind === "bytes" &&
+      stable(e.receiver));
+  if (!stable(start) || !stable(end) || !stable(out)) return null;
+  if (!stable(inner.init.init) || !stable(innerCond.right)) return null;
+  const reads: (IrExpr & { kind: "varRef" })[] = [];
+  let nodes = 0;
+  const pure = (e: IrExpr): boolean => {
+    if (++nodes > MAX_TERM_NODES) return false;
+    switch (e.kind) {
+      case "numLit":
+        return e.type.kind === "f64";
+      case "varRef":
+        return e.type.kind === "f64" && (e.localId === row || e.localId === column || stable(e));
+      case "bin":
+        return e.type.kind === "f64" && PURE_BINARY.has(e.op) && pure(e.left) && pure(e.right);
+      case "unary":
+        return e.type.kind === "f64" && (e.op === "-" || e.op === "~") && pure(e.operand);
+      case "bytesIntrinsic":
+        if (stable(e)) return true;
+        if (
+          e.method !== "get" ||
+          e.args.length !== 1 ||
+          e.receiver.kind !== "varRef" ||
+          e.receiver.type.kind !== "bytes" ||
+          !stable(e.receiver)
+        )
+          return false;
+        reads.push(e.receiver);
+        return pure(e.args[0]!);
+      default:
+        return false;
+    }
+  };
+  if (!pure(term) || readsLocal(term, sum)) return null;
+  return { row, start, end, sum, init: decl.init, inner, column, term, out, reads };
+}
+
+/** A fresh copy of `e` with `substitute` applied to its local reads and
+ * `replace` to the nodes it answers for. */
+function copy(
+  e: IrExpr,
+  substitute: (ref: IrExpr & { kind: "varRef" }) => IrExpr,
+  replace: (node: IrExpr) => IrExpr | null = () => null,
+): IrExpr {
+  const replaced = replace(e);
+  if (replaced !== null) return replaced;
+  if (e.kind === "varRef") return substitute(e);
+  return mapExprChildren(
+    { ...e },
+    (child) => copy(child, substitute, replace),
+    (stmt) => stmt,
+  );
+}
+
+const bin = (
+  op: "+" | "-" | "&" | "|",
+  left: IrExpr,
+  right: IrExpr,
+  loc: IrExpr["loc"],
+): IrExpr => ({
+  kind: "bin",
+  op,
+  left,
+  right,
+  type: F64,
+  loc,
+});
+
+/** Runs the row sums of `match` two rows per pass when the test holds and
+ * as written otherwise. */
+function pairRows(
+  s: IrStmt & { kind: "for" },
+  match: RowSum,
+  fresh: (name: string) => string,
+  done: Set<IrStmt>,
+): IrStmt {
+  const loc = s.loc;
+  const ref = (id: string): IrExpr => ({ kind: "varRef", localId: id, type: F64, loc });
+  const num = (value: number): IrExpr => ({ kind: "numLit", value, type: F64, loc });
+  const again = (e: IrExpr): IrExpr => copy(e, (r) => ({ ...r }));
+  const row = fresh("row"),
+    sum = fresh("sum"),
+    next = fresh("next"),
+    column = fresh("column");
+  const atColumn = (r: IrExpr & { kind: "varRef" }): IrExpr =>
+    r.localId === match.column ? { ...r, localId: column } : { ...r };
+  // Element reads that do not depend on the row are read once per column
+  // and shared by both rows.
+  const shared = new Map<IrExpr, string>();
+  const elements: IrStmt[] = [];
+  const collect = (e: IrExpr): boolean => {
+    if (e.kind === "bytesIntrinsic" && e.method === "get" && !readsLocal(e, match.row)) {
+      const id = fresh("element");
+      shared.set(e, id);
+      elements.push({ kind: "varDecl", localId: id, init: copy(e, atColumn), loc });
+      return true;
+    }
+    return everyExprChild(e, collect, () => true);
+  };
+  collect(match.term);
+  // The second row's term reads `row + 1`, so its counter sums stay visible
+  // to the counted-loop analysis.
+  const termAt = (rowExpr: () => IrExpr): IrExpr =>
+    copy(
+      match.term,
+      (r) => (r.localId === match.row ? rowExpr() : atColumn(r)),
+      (node) => {
+        const id = shared.get(node);
+        return id === undefined ? null : { kind: "varRef", localId: id, type: node.type, loc };
+      },
+    );
+  const innerCond = match.inner.cond as IrExpr & { kind: "bin" };
+  const pairs: IrStmt = {
+    kind: "for",
+    init: { kind: "varDecl", localId: row, init: again(match.start), loc },
+    cond: {
+      kind: "bin",
+      op: "<",
+      left: ref(row),
+      right: bin("-", again(match.end), num(1), loc),
+      type: BOOL,
+      loc,
+    },
+    update: { kind: "assign", localId: row, value: bin("+", ref(row), num(2), loc), loc },
+    body: [
+      { kind: "varDecl", localId: sum, init: { ...match.init }, loc },
+      { kind: "varDecl", localId: next, init: { ...match.init }, loc },
+      {
+        kind: "for",
+        init: {
+          kind: "varDecl",
+          localId: column,
+          init: again((match.inner.init as IrStmt & { kind: "varDecl" }).init!),
+          loc,
+        },
+        cond: { ...innerCond, left: ref(column), right: again(innerCond.right) },
+        update: { kind: "assign", localId: column, value: bin("+", ref(column), num(1), loc), loc },
+        body: [
+          ...elements,
+          {
+            kind: "assign",
+            localId: sum,
+            value: bin(
+              "+",
+              ref(sum),
+              termAt(() => ref(row)),
+              loc,
+            ),
+            loc,
+          },
+          {
+            kind: "assign",
+            localId: next,
+            value: bin(
+              "+",
+              ref(next),
+              termAt(() => bin("+", ref(row), num(1), loc)),
+              loc,
+            ),
+            loc,
+          },
+        ],
+        loc,
+      },
+      { kind: "bytesSet", arr: { ...match.out }, index: ref(row), value: ref(sum), loc },
+      {
+        kind: "bytesSet",
+        arr: { ...match.out },
+        index: bin("+", ref(row), num(1), loc),
+        value: ref(next),
+        loc,
+      },
+    ],
+    loc,
+  };
+  // Both bounds are int32, so `row < end - 1` is `row + 1 < end`, and the
+  // nest has at least two rows. The overlap tests run only then, so they never
+  // read an array the program assigns only when there are rows. Rows are
+  // independent when the written array shares no byte with an array the
+  // term reads; an array that is not there answers false.
+  const int32 = (e: IrExpr): IrExpr => ({
+    kind: "bin",
+    op: "===",
+    left: bin("|", again(e), num(0), loc),
+    right: again(e),
+    type: BOOL,
+    loc,
+  });
+  const tests: IrExpr[] = [
+    int32(match.start),
+    int32(match.end),
+    {
+      kind: "bin",
+      op: "<",
+      left: bin("+", again(match.start), num(1), loc),
+      right: again(match.end),
+      type: BOOL,
+      loc,
+    },
+    ...match.reads.map((read): IrExpr => ({
+      kind: "libCall",
+      fn: "bytes.disjoint",
+      args: [{ ...match.out }, { ...read }],
+      type: BOOL,
+      loc,
+    })),
+  ];
+  const test = tests.reduce((left, right) => ({
+    kind: "logical",
+    op: "&&",
+    left,
+    right,
+    type: BOOL,
+    loc,
+  }));
+  // The loop as written then runs the rows the pairs left: the last one when
+  // the count is odd (from `end - ((end - start) & 1)`), or all of them when
+  // the test failed. It is the nest's only other copy.
+  const go = fresh("go"),
+    from = fresh("from");
+  const paired: IrExpr = { kind: "bin", op: "===", left: ref(go), right: num(1), type: BOOL, loc };
+  const rest: IrStmt = {
+    ...s,
+    init: { kind: "varDecl", localId: match.row, init: ref(from), loc },
+  };
+  done.add(rest);
+  return {
+    kind: "block",
+    body: [
+      {
+        kind: "varDecl",
+        localId: go,
+        init: { kind: "ternary", cond: test, then: num(1), else_: num(0), type: F64, loc },
+        loc,
+      },
+      {
+        kind: "varDecl",
+        localId: from,
+        init: {
+          kind: "ternary",
+          cond: paired,
+          then: bin(
+            "-",
+            again(match.end),
+            bin("&", bin("-", again(match.end), again(match.start), loc), num(1), loc),
+            loc,
+          ),
+          else_: again(match.start),
+          type: F64,
+          loc,
+        },
+        loc,
+      },
+      { kind: "if", cond: { ...paired }, then: [pairs], else_: null, loc },
+      rest,
+    ],
+    loc,
+  };
+}
+
+/** Runs loop nests that sum one term per row (spectral-norm's `au`) two rows
+ * per pass.
+ *
+ * Each row's sum keeps its order of additions, so every sum has the bits
+ * the nest computes as written: the second row's term is the same
+ * expression of `row + 1`. The term only reads numbers, the counters and
+ * typed-array elements, so it runs no other code and cannot throw. The
+ * nest runs paired when both row bounds are int32 (so `row < end - 1` is
+ * `row + 1 < end`), there are at least two rows, and the array the rows
+ * store into shares no byte with any array the term reads, so no row reads
+ * another row's result. An odd row count runs its last row as written.
+ * Otherwise the nest runs as written. Two independent sums take turns in
+ * the inner loop, so neither waits on the other's addition. */
+export function pairRowSums(mod: IrModule): IrModule {
+  let changed = false;
+  const functions = mod.functions.map((fn): IrFunction => {
+    if (fn.async || fn.generator) return fn;
+    const locals = new Map(fn.locals.map((local) => [local.id, local]));
+    const added: IrLocal[] = [];
+    const ids = new Set(fn.locals.map((local) => local.id));
+    const fresh = (name: string): string => {
+      let id = `%pair.${name}.${ids.size}`;
+      while (ids.has(id)) id += "_";
+      ids.add(id);
+      added.push({ id, name: `%pair.${name}`, type: F64, mutable: true });
+      return id;
+    };
+    const paired = new Set<IrStmt>();
+    const body = transformStmtList(fn.body, {
+      expr: (e) => e,
+      stmt: (s) => {
+        if (s.kind !== "for" || paired.has(s)) return s;
+        const match = matchRowSum(s, locals);
+        if (match === null) return s;
+        paired.add(s);
+        return pairRows(s, match, fresh, paired);
+      },
+    });
+    if (added.length === 0) return fn;
+    changed = true;
+    return { ...fn, locals: [...fn.locals, ...added], body };
+  });
+  return changed ? { ...mod, functions } : mod;
+}
diff --git a/packages/compiler/src/ir/runtime-features.ts b/packages/compiler/src/ir/runtime-features.ts
index df7d6241a..50890107e 100644
--- a/packages/compiler/src/ir/runtime-features.ts
+++ b/packages/compiler/src/ir/runtime-features.ts
@@ -266,6 +266,7 @@ const STREAM_LIB_FNS: ReadonlySet<IrLibFn> = new Set([
   "stream.prop",
   "stream.stdioWritable",
   "stream.unboxDyn",
+  "stream.dynIterableSource",
   "stream.handleStream",
   "stream.unpipeDyn",
   "stream.workerStdio",
diff --git a/packages/compiler/src/ir/scalar-records.test.ts b/packages/compiler/src/ir/scalar-records.test.ts
index 7b355ee26..2392562a3 100644
--- a/packages/compiler/src/ir/scalar-records.test.ts
+++ b/packages/compiler/src/ir/scalar-records.test.ts
@@ -442,3 +442,31 @@ test("keeps the call when an argument holds a counted temporary of its own", ()
   (decl.init as IrExpr & { kind: "call" }).args[1] = length;
   expect(literalTransformed(mod)).toBe(mod);
 });
+
+test("renames a producer's parameter where its literal writes it", () => {
+  const mod = literalFixture([element("head", "0", STRING), element("tail", "1", STRING)]);
+  const split = mod.functions.find((fn) => fn.name === "split")!;
+  // Lowering declares every parameter mutable.
+  for (const l of split.locals) l.mutable = true;
+  const ret = split.body[0] as IrStmt & { kind: "return" };
+  const literal = ret.value as IrExpr & { kind: "recordLit" };
+  // `at++ > (at = 0)`
+  literal.fields[2]!.value = {
+    kind: "bin",
+    op: ">",
+    left: { kind: "incDec", op: "+", prefix: false, localId: "at", type: F64, loc },
+    right: { kind: "assignExpr", localId: "at", value: num(0), type: F64, loc },
+    type: BOOL,
+    loc,
+  };
+  const caller = main(literalTransformed(mod));
+  const [text, at] = caller.body
+    .slice(0, 2)
+    .map((s) => (s as IrStmt & { kind: "varDecl" }).localId);
+  expect(at).not.toBe("at");
+  const writes = [...nodes(caller.body, "incDec"), ...nodes(caller.body, "assignExpr")];
+  expect(writes.map((w) => w["localId"])).toEqual([at, at]);
+  // Only the written parameter's binding is mutable.
+  expect(caller.locals.find((l) => l.id === at)?.mutable).toBe(true);
+  expect(caller.locals.find((l) => l.id === text)?.mutable).toBe(false);
+});
diff --git a/packages/compiler/src/ir/scalar-records.ts b/packages/compiler/src/ir/scalar-records.ts
index f75e522bf..5d57cb3ec 100644
--- a/packages/compiler/src/ir/scalar-records.ts
+++ b/packages/compiler/src/ir/scalar-records.ts
@@ -371,6 +371,8 @@ interface LiteralProducer {
   shape: IrRecordShape;
   literal: IrExpr & { kind: "recordLit" };
   size: number;
+  /** The parameters the literal writes (`a++`, `a = b`). */
+  written: ReadonlySet<string>;
 }
 
 /** Whether `value` holds no counted temporary of its own: every counted
@@ -443,17 +445,22 @@ function literalProducer(
   )
     return null;
   let size = 0;
+  const written = new Set<string>();
   const small = everyExpr(literal, {
-    expr: (node) =>
-      ++size <= MAX_CALLEE_NODES &&
-      node.kind !== "closure" &&
-      node.kind !== "selfRef" &&
-      node.kind !== "awaitExpr" &&
-      node.kind !== "awaitUnionExpr" &&
-      node.kind !== "yieldExpr",
+    expr: (node) => {
+      if (node.kind === "incDec" || node.kind === "assignExpr") written.add(node.localId);
+      return (
+        ++size <= MAX_CALLEE_NODES &&
+        node.kind !== "closure" &&
+        node.kind !== "selfRef" &&
+        node.kind !== "awaitExpr" &&
+        node.kind !== "awaitUnionExpr" &&
+        node.kind !== "yieldExpr"
+      );
+    },
     stmt: () => false,
   });
-  return small ? { fn, shape, literal, size } : null;
+  return small ? { fn, shape, literal, size, written } : null;
 }
 
 /** Rewrite every statement list under `body`, innermost first. */
@@ -628,7 +635,7 @@ export function scalarizeLiteralResults(mod: IrModule): IrModule {
             id: renamed.get(l.id)!,
             name: l.name,
             type: l.type,
-            mutable: false,
+            mutable: p.written.has(l.id),
           };
           if (l.source !== undefined) copy.source = l.source;
           added.push(copy);
@@ -641,10 +648,12 @@ export function scalarizeLiteralResults(mod: IrModule): IrModule {
             loc: decl.loc,
           });
         });
+        // Reads and writes of a parameter (`a++`, `a = b`) both name it.
         const rename = (value: IrExpr): IrExpr =>
           transformExpr(value, {
             expr: (node) =>
-              node.kind === "varRef" && renamed.has(node.localId)
+              (node.kind === "varRef" || node.kind === "incDec" || node.kind === "assignExpr") &&
+              renamed.has(node.localId)
                 ? { ...node, localId: renamed.get(node.localId)! }
                 : node,
             stmt: (node) => node,
diff --git a/packages/compiler/test/js-collection-types.test.ts b/packages/compiler/test/js-collection-types.test.ts
new file mode 100644
index 000000000..6a6f65798
--- /dev/null
+++ b/packages/compiler/test/js-collection-types.test.ts
@@ -0,0 +1,64 @@
+import { mkdtempSync, readFileSync, rmSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+import { expect, test } from "vitest";
+import { compile, deserializeModule, validateModule } from "../src/index.js";
+
+// Corpus 7985 exercises every refusal: only the collections whose uses
+// prove their types lower as typed collections; the rest stay untyped.
+test("untyped JavaScript collections take the types their uses prove, and only those", async () => {
+  const dir = mkdtempSync(join(tmpdir(), "scriptc-js-collection-types-"));
+  try {
+    const entry = join(
+      import.meta.dirname,
+      "../../../tests/corpus/7985-untyped-collection-types.mjs",
+    );
+    const output = join(dir, "main.ir.json");
+    const lowered = await compile(entry, { outputKind: "ir", outDir: dir, outPath: output });
+    if (!lowered.ok) throw new Error(JSON.stringify(lowered.diagnostics));
+    const module = deserializeModule(readFileSync(output, "utf8"));
+    expect(validateModule(module)).toEqual([]);
+    const types = new Map(module.globals.map((g) => [g.name, JSON.stringify(g.type)]));
+    const map = (key: string, value: string) =>
+      JSON.stringify({ kind: "map", key: { kind: key }, value: { kind: value } });
+    const set = (elem: string) => JSON.stringify({ kind: "set", elem: { kind: elem } });
+    const array = (elem: string) => JSON.stringify({ kind: "array", elem: { kind: elem } });
+    // Untyped collections hold checked-dynamic keys, values and elements.
+    const typed = (t: string) => !t.includes('"dyn"');
+    expect(Object.fromEntries([...types].filter(([, t]) => typed(t)))).toEqual({
+      counts: map("string", "f64"),
+      seen: set("f64"),
+      stack: array("f64"),
+      words: array("string"),
+      byNumber: map("f64", "string"),
+      nums: array("f64"),
+      flags: set("bool"),
+      cache: map("string", "f64"),
+      "%loaded": '{"kind":"bool"}',
+    });
+    for (const untyped of [
+      "strings",
+      "numbers",
+      "mixedKeys",
+      "mixedValues",
+      "mixedElements",
+      "passed",
+      "stored",
+      "iterated",
+      "eachSet",
+      "shown",
+      "chained",
+      "optional",
+      "indexed",
+      "truncated",
+      "spread",
+      "unread",
+      "exported",
+      "listed",
+      "renamed",
+    ])
+      expect(typed(types.get(untyped)!), untyped).toBe(false);
+  } finally {
+    rmSync(dir, { recursive: true, force: true });
+  }
+});
diff --git a/packages/compiler/test/native-codegen-integration.test.ts b/packages/compiler/test/native-codegen-integration.test.ts
index cecebb49c..1be1b7a3f 100644
--- a/packages/compiler/test/native-codegen-integration.test.ts
+++ b/packages/compiler/test/native-codegen-integration.test.ts
@@ -378,7 +378,7 @@ done:
     expect(await symbols(helperObject, ["-u"])).toEqual(await symbols(clangObject, ["-u"]));
     expect(await symbols(helperObject, ["-gU"])).toEqual(await symbols(clangObject, ["-gU"]));
     expect(await symbols(helperObject, ["-gU"])).toEqual(["0000000000000000 T _main"]);
-    expect(await symbols(helperObject, ["-u"])).toContain("_scr_runtime_abi_v37");
+    expect(await symbols(helperObject, ["-u"])).toContain("_scr_runtime_abi_v42");
 
     const clangExe = join(dir, "clang-program");
     const linkDriver = join(dir, "clang-driver.c");
@@ -712,7 +712,7 @@ done:
       .split("\n");
     expect(undefinedSymbols).toContain("_sf_scale");
     expect(undefinedSymbols).toContain("_sf_callback_mix");
-    expect(undefinedSymbols).toContain("_scr_runtime_abi_v37");
+    expect(undefinedSymbols).toContain("_scr_runtime_abi_v42");
   });
 
   test("helper and clang object paths retain the same outbound FFI C ABI", async () => {
diff --git a/packages/compiler/test/native-profile.test.ts b/packages/compiler/test/native-profile.test.ts
index 53d077017..9d04739d0 100644
--- a/packages/compiler/test/native-profile.test.ts
+++ b/packages/compiler/test/native-profile.test.ts
@@ -110,6 +110,65 @@ function expectedExit(argc: number, edit = 0): number {
   return Number(total % 251n);
 }
 
+const INLINED = 6;
+
+/** A program whose helpers are too large for the inliner that runs before the
+ * profile annotation and each called twice, so they reach the annotation,
+ * and which the profile-guided inliner then inlines into their hot call
+ * sites in `main` and deletes. */
+function inlinedProgram(): string {
+  const lines = ["; profiled program, helpers inlined after annotation"];
+  for (let index = 0; index < INLINED; index++) {
+    lines.push(
+      `define internal i64 @g${index}(i64 %n) {`,
+      "entry:",
+      "  %x0 = add i64 %n, " + index,
+    );
+    for (let step = 1; step <= 40; step++)
+      lines.push(`  %x${step} = ${step % 2 === 0 ? "xor" : "mul"} i64 %x${step - 1}, ${step + 3}`);
+    lines.push("  ret i64 %x40", "}");
+  }
+  lines.push(
+    "define i32 @main(i32 %argc, ptr %argv) {",
+    "entry:",
+    "  %wide = sext i32 %argc to i64",
+    "  %limit = mul i64 %wide, 1000",
+    "  br label %loop",
+    "loop:",
+    "  %i = phi i64 [ 0, %entry ], [ %next, %loop ]",
+    "  %total = phi i64 [ 0, %entry ], [ %grown, %loop ]",
+    ...Array.from({ length: INLINED }, (_, index) => [
+      `  %a${index} = call i64 @g${index}(i64 %i)`,
+      `  %b${index} = call i64 @g${index}(i64 %a${index})`,
+      `  %t${index} = add i64 ${index === 0 ? "%total" : `%t${index - 1}`}, %b${index}`,
+    ]).flat(),
+    `  %grown = add i64 %t${INLINED - 1}, 0`,
+    "  %next = add i64 %i, 1",
+    "  %again = icmp ult i64 %next, %limit",
+    "  br i1 %again, label %loop, label %exit",
+    "exit:",
+    "  %low = urem i64 %grown, 251",
+    "  %code = trunc i64 %low to i32",
+    "  ret i32 %code",
+    "}",
+  );
+  return lines.join("\n") + "\n";
+}
+
+function inlinedExit(argc: number): number {
+  const mask = (1n << 64n) - 1n;
+  const g = (index: number, n: bigint) => {
+    let x = (n + BigInt(index)) & mask;
+    for (let step = 1; step <= 40; step++)
+      x = step % 2 === 0 ? x ^ BigInt(step + 3) : (x * BigInt(step + 3)) & mask;
+    return x;
+  };
+  let total = 0n;
+  for (let i = 0n; i < BigInt(argc * 1000); i++)
+    for (let index = 0; index < INLINED; index++) total = (total + g(index, g(index, i))) & mask;
+  return Number(total % 251n);
+}
+
 type EmitOptions = {
   outputs: number;
   name: string;
@@ -119,12 +178,17 @@ type EmitOptions = {
 };
 
 async function emit(dir: string, source: string, options: EmitOptions): Promise<string[]> {
+  return (await emitReporting(dir, source, options)).outputs;
+}
+
+/** `emit`, also returning the helper's stdout (its profile match report). */
+async function emitReporting(dir: string, source: string, options: EmitOptions) {
   const input = join(dir, `${options.name}.ll`);
   await writeFile(input, source);
   const outputs = Array.from({ length: options.outputs }, (_, index) =>
     join(dir, `${options.name}.${index}.o`),
   );
-  await execFileAsync(helper!, [
+  const { stdout } = await execFileAsync(helper!, [
     "emit",
     "--input",
     input,
@@ -145,7 +209,7 @@ async function emit(dir: string, source: string, options: EmitOptions): Promise<
     ...(options.use === undefined ? [] : ["--profile-use", options.use]),
     ...(options.cache === undefined ? [] : ["--cache-dir", options.cache]),
   ]);
-  return outputs;
+  return { outputs, stdout };
 }
 
 async function link(dir: string, objects: string[], name: string, instrumented = false) {
@@ -181,12 +245,18 @@ async function merge(output: string, inputs: string[]) {
 
 /** An instrumented build of `source` run with 1 and 3 arguments, its raw
  * profiles merged into one indexed profile. */
-async function collect(dir: string, source: string, outputs: number, name: string) {
+async function collect(
+  dir: string,
+  source: string,
+  outputs: number,
+  name: string,
+  expected: (argc: number) => number = expectedExit,
+) {
   const raw = join(dir, `${name}-raw`);
   const objects = await emit(dir, source, { outputs, name, generate: join(raw, "%p.profraw") });
   const executable = await link(dir, objects, name, true);
-  expect(await run(executable, 1)).toBe(expectedExit(1));
-  expect(await run(executable, 3)).toBe(expectedExit(3));
+  expect(await run(executable, 1)).toBe(expected(1));
+  expect(await run(executable, 3)).toBe(expected(3));
   const profiles = (await readdir(raw)).map((file) => join(raw, file));
   expect(profiles).toHaveLength(2);
   const profile = join(dir, `${name}.profdata`);
@@ -286,6 +356,26 @@ describe.runIf(helper !== null)("LLVM profile-guided optimization", () => {
       expect(await run(await link(dir, objects, "edited"), 2)).toBe(expectedExit(2, 7));
     });
 
+    test("the match report counts annotated functions that inlining removes", async () => {
+      const dir = await mkdtemp(join(tmpdir(), "scriptc-profile-match-"));
+      dirs.push(dir);
+      const { profile } = await collect(dir, inlinedProgram(), 1, "train", inlinedExit);
+      const { outputs, stdout } = await emitReporting(dir, inlinedProgram(), {
+        outputs: 1,
+        name: "guided",
+        use: profile,
+      });
+      expect(await run(await link(dir, outputs, "guided"), 2)).toBe(inlinedExit(2));
+      // Every g<index> and main received counts, though optimization then
+      // inlined the helpers into main and deleted them.
+      expect(JSON.parse(stdout.trim().split("\n").at(-1)!)).toEqual({
+        ok: true,
+        profile: { annotated: INLINED + 1, mismatched: 0, missing: 0, unknown_partitions: 0 },
+      });
+      const symbols = await execFileAsync("nm", [outputs[0]!]);
+      expect(symbols.stdout).not.toMatch(/\bg\d\b/);
+    });
+
     test("the profile keys the partition cache", async () => {
       const dir = await mkdtemp(join(tmpdir(), "scriptc-profile-cache-"));
       dirs.push(dir);
diff --git a/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/7381-function-value-arguments-length.ts.json b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/7381-function-value-arguments-length.ts.json
new file mode 100644
index 000000000..b9e5eacd1
--- /dev/null
+++ b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/7381-function-value-arguments-length.ts.json
@@ -0,0 +1,6 @@
+{
+ "order": [
+  "<repo>/tests/corpus/7381-function-value-arguments-length.ts"
+ ],
+ "diags": []
+}
diff --git a/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/7503-pipeline-untyped-sources.mjs.json b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/7503-pipeline-untyped-sources.mjs.json
new file mode 100644
index 000000000..4d93af132
--- /dev/null
+++ b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/7503-pipeline-untyped-sources.mjs.json
@@ -0,0 +1,6 @@
+{
+ "order": [
+  "<repo>/tests/corpus/7503-pipeline-untyped-sources.mjs"
+ ],
+ "diags": []
+}
diff --git a/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/7504-pipeline-untyped-source-errors.mjs.json b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/7504-pipeline-untyped-source-errors.mjs.json
new file mode 100644
index 000000000..19402586b
--- /dev/null
+++ b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/7504-pipeline-untyped-source-errors.mjs.json
@@ -0,0 +1,6 @@
+{
+ "order": [
+  "<repo>/tests/corpus/7504-pipeline-untyped-source-errors.mjs"
+ ],
+ "diags": []
+}
diff --git a/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/7505-pipeline-untyped-sources.cjs.json b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/7505-pipeline-untyped-sources.cjs.json
new file mode 100644
index 000000000..29d85351d
--- /dev/null
+++ b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/7505-pipeline-untyped-sources.cjs.json
@@ -0,0 +1,6 @@
+{
+ "order": [
+  "<repo>/tests/corpus/7505-pipeline-untyped-sources.cjs"
+ ],
+ "diags": []
+}
diff --git a/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/7506-pipeline-any-source.ts.json b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/7506-pipeline-any-source.ts.json
new file mode 100644
index 000000000..942a2b3e8
--- /dev/null
+++ b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/7506-pipeline-any-source.ts.json
@@ -0,0 +1,6 @@
+{
+ "order": [
+  "<repo>/tests/corpus/7506-pipeline-any-source.ts"
+ ],
+ "diags": []
+}
diff --git a/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/7507-stream-write-error-drain.mjs.json b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/7507-stream-write-error-drain.mjs.json
new file mode 100644
index 000000000..6fa55d09d
--- /dev/null
+++ b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/7507-stream-write-error-drain.mjs.json
@@ -0,0 +1,6 @@
+{
+ "order": [
+  "<repo>/tests/corpus/7507-stream-write-error-drain.mjs"
+ ],
+ "diags": []
+}
diff --git a/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/7985-untyped-collection-types.mjs.json b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/7985-untyped-collection-types.mjs.json
new file mode 100644
index 000000000..a807063cc
--- /dev/null
+++ b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/7985-untyped-collection-types.mjs.json
@@ -0,0 +1,6 @@
+{
+ "order": [
+  "<repo>/tests/corpus/7985-untyped-collection-types.mjs"
+ ],
+ "diags": []
+}
diff --git a/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/8116-destructured-results.ts.json b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/8116-destructured-results.ts.json
new file mode 100644
index 000000000..f19871791
--- /dev/null
+++ b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/8116-destructured-results.ts.json
@@ -0,0 +1,6 @@
+{
+ "order": [
+  "<repo>/tests/corpus/8116-destructured-results.ts"
+ ],
+ "diags": []
+}
diff --git a/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/8117-scalar-slice-bounds.ts.json b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/8117-scalar-slice-bounds.ts.json
new file mode 100644
index 000000000..1e12a8cd2
--- /dev/null
+++ b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/8117-scalar-slice-bounds.ts.json
@@ -0,0 +1,6 @@
+{
+ "order": [
+  "<repo>/tests/corpus/8117-scalar-slice-bounds.ts"
+ ],
+ "diags": []
+}
diff --git a/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/8194-row-pair-sums.ts.json b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/8194-row-pair-sums.ts.json
new file mode 100644
index 000000000..afb53a915
--- /dev/null
+++ b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/8194-row-pair-sums.ts.json
@@ -0,0 +1,6 @@
+{
+ "order": [
+  "<repo>/tests/corpus/8194-row-pair-sums.ts"
+ ],
+ "diags": []
+}
diff --git a/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/8223-argument-count-destructured-parameter.ts.json b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/8223-argument-count-destructured-parameter.ts.json
new file mode 100644
index 000000000..6cae202d3
--- /dev/null
+++ b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/8223-argument-count-destructured-parameter.ts.json
@@ -0,0 +1,6 @@
+{
+ "order": [
+  "<repo>/tests/corpus/8223-argument-count-destructured-parameter.ts"
+ ],
+ "diags": []
+}
diff --git a/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/8230-readline-interface-constructor.cjs.json b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/8230-readline-interface-constructor.cjs.json
new file mode 100644
index 000000000..05573974c
--- /dev/null
+++ b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/8230-readline-interface-constructor.cjs.json
@@ -0,0 +1,6 @@
+{
+ "order": [
+  "<repo>/tests/corpus/8230-readline-interface-constructor.cjs"
+ ],
+ "diags": []
+}
diff --git a/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/8231-builtin-member-binding-new.cjs.json b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/8231-builtin-member-binding-new.cjs.json
new file mode 100644
index 000000000..1f54f4886
--- /dev/null
+++ b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/8231-builtin-member-binding-new.cjs.json
@@ -0,0 +1,6 @@
+{
+ "order": [
+  "<repo>/tests/corpus/8231-builtin-member-binding-new.cjs"
+ ],
+ "diags": []
+}
diff --git a/packages/compiler/test/ts7/baselines/order-parity/tests/diagnostics/function-value-arguments-length.ts.json b/packages/compiler/test/ts7/baselines/order-parity/tests/diagnostics/function-value-arguments-length.ts.json
new file mode 100644
index 000000000..6aa894918
--- /dev/null
+++ b/packages/compiler/test/ts7/baselines/order-parity/tests/diagnostics/function-value-arguments-length.ts.json
@@ -0,0 +1,6 @@
+{
+ "order": [
+  "<repo>/tests/diagnostics/function-value-arguments-length.ts"
+ ],
+ "diags": []
+}
diff --git a/packages/runtime-darwin-arm64/scripts/build.mjs b/packages/runtime-darwin-arm64/scripts/build.mjs
index b6b855c05..56f5ca559 100644
--- a/packages/runtime-darwin-arm64/scripts/build.mjs
+++ b/packages/runtime-darwin-arm64/scripts/build.mjs
@@ -366,7 +366,7 @@ async function build() {
       package: packageManifest.name,
       version: packageManifest.version,
       target: RUNTIME_PACK_MATRIX.target,
-      runtime_abi: { version: 37, marker: "scr_runtime_abi_v37" },
+      runtime_abi: { version: 42, marker: "scr_runtime_abi_v42" },
       compiler: {
         command: compiler,
         identity: compilerVersion,
diff --git a/packages/runtime-pack-common/scripts/build.mjs b/packages/runtime-pack-common/scripts/build.mjs
index 479d8aad4..c571a25c5 100644
--- a/packages/runtime-pack-common/scripts/build.mjs
+++ b/packages/runtime-pack-common/scripts/build.mjs
@@ -469,7 +469,7 @@ async function build() {
       package: packageManifest.name,
       version: packageManifest.version,
       target: matrix.target,
-      runtime_abi: { version: 37, marker: "scr_runtime_abi_v37" },
+      runtime_abi: { version: 42, marker: "scr_runtime_abi_v42" },
       compiler: {
         command: compiler,
         identity: compilerVersion,
diff --git a/packages/runtime/src/scr_console.c b/packages/runtime/src/scr_console.c
index 086340014..6c15f932c 100644
--- a/packages/runtime/src/scr_console.c
+++ b/packages/runtime/src/scr_console.c
@@ -148,7 +148,7 @@ static void scr_enable_utf8_console_output(void) {
 }
 #endif
 
-void scr_runtime_abi_v37(void) {}
+void scr_runtime_abi_v42(void) {}
 
 void scr_init(void) {
 #ifdef SCR_WORKERS
diff --git a/packages/runtime/src/scr_core.h b/packages/runtime/src/scr_core.h
index 3707b8d50..5415ccdc6 100644
--- a/packages/runtime/src/scr_core.h
+++ b/packages/runtime/src/scr_core.h
@@ -81,7 +81,7 @@ void scr_init(void);
 /* Program objects emitted by the bundled LLVM helper reference this symbol.
  * Its versioned spelling makes a mismatched manual runtime link fail before
  * the program can start. */
-void scr_runtime_abi_v37(void);
+void scr_runtime_abi_v42(void);
 void scr_runtime_workers_v8(void);
 
 /* ── the trap funnel (scr_console.c; scr_library.c under -DSCR_LIB) ──────
diff --git a/packages/runtime/src/scr_stream.c b/packages/runtime/src/scr_stream.c
index 552fef146..03255c306 100644
--- a/packages/runtime/src/scr_stream.c
+++ b/packages/runtime/src/scr_stream.c
@@ -6872,10 +6872,12 @@ static bool scr_stream_write_chunk_enc(ScrStream *s, ScrBytes *chunk /*borrowed*
     scr_stream_do_write(s, scr_stream_wchunk_retain(st, chunk), cb, enc);
   }
   /* Node computes the answer AFTER the (possibly synchronous) write:
-   * a sync completion has already drained its bytes from length. */
-  bool ret = st->w.length < st->w.hwm && !st->destroyed && !st->errored;
-  if (!ret) st->w.need_drain = true;
-  return ret;
+   * a sync completion has already drained its bytes from length. Only
+   * the length sets needDrain; an errored or destroyed stream answers
+   * false without waiting for 'drain'. */
+  bool below = st->w.length < st->w.hwm;
+  if (!below) st->w.need_drain = true;
+  return below && !st->destroyed && !st->errored;
 }
 
 static bool scr_stream_write_chunk(ScrStream *s, ScrBytes *chunk /*borrowed*/, ScrClosure *cb /*moves*/) {
@@ -8514,3 +8516,31 @@ ScrStream *scr_stream_unbox_dyn(const ScrDyn *value, const ScrStr *message) {
   scr_throw_error_msg_code(SCR_ERR_ERROR, message->data, message->len, "SC2020");
   return NULL;
 }
+
+/* pipeline.js's first stage: a value with pipe() and on() methods is a
+ * readable node stream, which the stage ladder tests before isIterable;
+ * a string, or a value with a Symbol.asyncIterator or Symbol.iterator
+ * method, is an iterable the pipeline pumps into the next stage. */
+bool scr_stream_dyn_iterable_source(const ScrDyn *value) {
+  ScrStream *stream = scr_stream_of_handle(value);
+  if (!stream) stream = scr_stream_boxed(value);
+  if (stream) {
+    scr_stream_release(stream);
+    return false;
+  }
+  if (value->kind == SCR_DYN_STR) return true;
+  if (value->kind == SCR_DYN_OBJ) {
+    ScrDyn *pipe = scr_dyn_obj_read(value, "pipe", 4);
+    if (!pipe) return false;
+    bool streamish = pipe->kind == SCR_DYN_FUNC;
+    scr_dyn_release(pipe);
+    if (streamish) {
+      ScrDyn *on = scr_dyn_obj_read(value, "on", 2);
+      if (!on) return false;
+      streamish = on->kind == SCR_DYN_FUNC;
+      scr_dyn_release(on);
+      if (streamish) return false;
+    }
+  }
+  return scr_dyn_iterable_kind(value) > 0;
+}
diff --git a/packages/runtime/src/scr_stream.h b/packages/runtime/src/scr_stream.h
index a4798c629..7acad0eeb 100644
--- a/packages/runtime/src/scr_stream.h
+++ b/packages/runtime/src/scr_stream.h
@@ -145,6 +145,10 @@ ScrStream *scr_stream_pipe_general(ScrStream *src, ScrStream *dst, bool end);
 /* A boxed native stream out of a checked-dynamic pipeline stage (+1), or
  * NULL with `message` thrown as the not-supported Error. */
 ScrStream *scr_stream_unbox_dyn(const struct ScrDyn *value, const ScrStr *message);
+/* Whether pipeline() keeps a checked-dynamic first stage as an iterable
+ * to pump rather than as a stream (false with an exception pending when
+ * reading the value's members throws). */
+bool scr_stream_dyn_iterable_source(const struct ScrDyn *value);
 /* Handles that are Transforms (ScrDynHandleOps.transform: Cipheriv and
  * Decipheriv). scr_stream_of_handle: the handle's Transform (+1), built on
  * first use, or NULL when the value is not such a handle.
diff --git a/scripts/build-native-cli.mts b/scripts/build-native-cli.mts
index e539cea1d..30e0ae39b 100644
--- a/scripts/build-native-cli.mts
+++ b/scripts/build-native-cli.mts
@@ -20,6 +20,12 @@ const sanitize = process.env["SCRIPTC_SAN"] === "1";
 // SCRIPTC_NATIVE_PROFILE_USE names; an empty SCRIPTC_NATIVE_PROFILE_USE
 // builds without one.
 const profileGenerate = process.env["SCRIPTC_NATIVE_PROFILE_GENERATE"] || undefined;
+// Below this share of matching functions, the build warns that the profile
+// has lost most of its gain. A profile refreshed on the same commit matched
+// 92.7% of the native compiler's profiled functions, one refreshed 1.5 days
+// and about 200 commits earlier 87.2%, and that one kept about half of the
+// fresh profile's gain on a cold tsc-ts build.
+const PROFILE_MATCH_WARNING = 0.9;
 const committedProfile = join(root, "packages/compiler/native/cli.profdata.gz");
 const requestedProfile = process.env["SCRIPTC_NATIVE_PROFILE_USE"];
 const useProfile =
@@ -127,6 +133,25 @@ if (!result.ok)
       )
       .join("\n"),
   );
+// How well the profile still describes the compiler: a function whose
+// control flow changed since the profile was collected is optimized as
+// without one. The share falls as compiler code changes, and with it the
+// profile's gain (refresh it with scripts/refresh-cli-profile.mjs).
+const match = useProfile && "artifact" in result ? result.artifact.profileMatch : undefined;
+if (match !== undefined) {
+  const profiled = match.annotated + match.mismatched;
+  const share = profiled === 0 ? 0 : match.annotated / profiled;
+  console.log(
+    `native compiler profile: ${match.annotated} of ${profiled} profiled functions match (${(100 * share).toFixed(1)}%); ${match.missing} functions have no profile` +
+      (match.unknownPartitions > 0
+        ? `; ${match.unknownPartitions} partitions reused without counts`
+        : ""),
+  );
+  if (match.unknownPartitions === 0 && share < PROFILE_MATCH_WARNING)
+    console.warn(
+      `warning: the native compiler profile matches ${(100 * share).toFixed(1)}% of the functions it describes; run scripts/refresh-cli-profile.mjs`,
+    );
+}
 const compilerVersion = (
   JSON.parse(readFileSync(join(root, "packages/compiler/package.json"), "utf8")) as {
     version: string;
diff --git a/tests/corpus/7381-function-value-arguments-length.ts b/tests/corpus/7381-function-value-arguments-length.ts
new file mode 100644
index 000000000..059ff1487
--- /dev/null
+++ b/tests/corpus/7381-function-value-arguments-length.ts
@@ -0,0 +1,27 @@
+// `arguments.length` in a function declaration counts the arguments its
+// caller passes: a direct call passes the declared parameters, and a call
+// through the function as a value passes the parameters of the function
+// type it was passed as (3 for map and filter, 4 for reduce, 2 for sort).
+function scaled(value: number): number {
+  return value * 10 + arguments.length;
+}
+function total(acc: number, value: number): number {
+  return acc + value + arguments.length;
+}
+function compare(a: number, b: number): number {
+  return arguments.length === 2 ? a - b : 0;
+}
+console.log([1, 2].map(scaled).join(), scaled(3));
+console.log([5, 6].filter(scaled).length, [1, 2].reduce(total, 0), total(1, 2));
+console.log([3, 1, 2].sort(compare).join());
+
+function apply(f: (a: number, b: number, c: number) => number): number {
+  return f(1, 2, 3);
+}
+console.log(apply(scaled));
+
+// A direct call inside a call through the value sees its own count.
+function depth(n: number): number {
+  return n <= 0 ? arguments.length : depth(n - 1) + arguments.length;
+}
+console.log([2].map(depth).join(), depth(2));
diff --git a/tests/corpus/7503-pipeline-untyped-sources.mjs b/tests/corpus/7503-pipeline-untyped-sources.mjs
new file mode 100644
index 000000000..e676fd607
--- /dev/null
+++ b/tests/corpus/7503-pipeline-untyped-sources.mjs
@@ -0,0 +1,102 @@
+// pipeline() over untyped iterable sources: arrays, strings, Sets, generator
+// objects, async generator objects, class iterables and object values,
+// into Writables, a Transform chain, a function stage and the promise form.
+// Each source is pumped as Node pumps an iterable, on the same ticks.
+import { pipeline, Writable, Transform } from "node:stream";
+import { pipeline as pipelineP } from "node:stream/promises";
+
+const log = [];
+function show(chunk) {
+  return typeof chunk === "string" ? chunk : Buffer.isBuffer(chunk) ? chunk.toString() : JSON.stringify(chunk);
+}
+function objectSink(label) {
+  return new Writable({
+    objectMode: true,
+    write(chunk, _enc, cb) {
+      log.push(`${label} write ${show(chunk)}`);
+      cb();
+    },
+    final(cb) { log.push(`${label} final`); cb(); },
+  });
+}
+function sink(label, objectMode = false) {
+  if (objectMode) return objectSink(label);
+  return new Writable({
+    write(chunk, _enc, cb) {
+      log.push(`${label} write ${typeof chunk === "string" ? chunk : Buffer.isBuffer(chunk) ? chunk.toString() : JSON.stringify(chunk)}`);
+      cb();
+    },
+    final(cb) { log.push(`${label} final`); cb(); },
+  });
+}
+function* gen() {
+  log.push("gen start");
+  yield "g1";
+  log.push("gen mid");
+  yield "g2";
+  log.push("gen end");
+}
+async function* agen() {
+  yield "a1";
+  await null;
+  yield "a2";
+}
+const sources = {
+  array: () => ["x", "y"],
+  string: () => "hey",
+  set: () => new Set(["s1", "s2"]),
+  gen: () => gen(),
+  agen: () => agen(),
+  iterable: () => ({ *[Symbol.iterator]() { yield "i1"; yield "i2"; } }),
+  objects: () => [{ n: 1 }, { n: 2 }],
+};
+const names = Object.keys(sources);
+async function run(name) {
+  const src = sources[name]();
+  const objectMode = name === "objects";
+  await new Promise((resolve) => {
+    queueMicrotask(() => log.push(`${name} microtask`));
+    pipeline(src, sink(name, objectMode), (err) => {
+      log.push(`${name} done ${err ? err.code ?? err.message : err}`);
+      resolve(undefined);
+    });
+    log.push(`${name} after call`);
+  });
+}
+for (const n of names) await run(n);
+
+// a transform chain and a function stage after an untyped source
+const src2 = ["p", "q"];
+await new Promise((resolve) =>
+  pipeline(
+    src2,
+    new Transform({ transform(c, _e, cb) { cb(null, c.toString().toUpperCase()); } }),
+    sink("transform"),
+    (err) => { log.push(`transform done ${err}`); resolve(undefined); },
+  ),
+);
+const src3 = new Set(["f1", "f2"]);
+await new Promise((resolve) =>
+  pipeline(
+    src3,
+    async function* (source) { for await (const c of source) yield c + "!"; },
+    sink("fnstage"),
+    (err) => { log.push(`fnstage done ${err}`); resolve(undefined); },
+  ),
+);
+// promises form
+const src4 = ["m", "n"];
+await pipelineP(src4, sink("promises"));
+log.push("promises resolved");
+// an error thrown by the source
+function* bad() { yield "ok"; throw new Error("source failed"); }
+const src5 = bad();
+await new Promise((resolve) =>
+  pipeline(src5, sink("bad"), (err) => { log.push(`bad done ${err && err.message}`); resolve(undefined); }),
+);
+// a source of non-string values into a byte-mode writable
+const src6 = [1, 2];
+await new Promise((resolve) =>
+  pipeline(src6, sink("numbers"), (err) => { log.push(`numbers done ${err && err.code}`); resolve(undefined); }),
+);
+console.log(log.join("\n"));
diff --git a/tests/corpus/7504-pipeline-untyped-source-errors.mjs b/tests/corpus/7504-pipeline-untyped-source-errors.mjs
new file mode 100644
index 000000000..9dd671606
--- /dev/null
+++ b/tests/corpus/7504-pipeline-untyped-source-errors.mjs
@@ -0,0 +1,171 @@
+// pipeline() over untyped iterable sources: a destination that fails closes
+// the source, backpressure waits for drain, an abort signal stops the pump,
+// async generator and Map sources reach object-mode destinations, and a
+// sync source's promise values, its final value included, are awaited.
+import { pipeline, Writable, PassThrough } from "node:stream";
+import { pipeline as pipelineP } from "node:stream/promises";
+
+function* counted(label, n) {
+  try {
+    for (let i = 0; i < n; i++) {
+      console.log(label, "yield", i);
+      yield `${label}${i}`;
+    }
+  } finally {
+    console.log(label, "finally");
+  }
+}
+function pick(v) {
+  return v;
+}
+
+// the destination fails on its second write: the source's finally runs
+await new Promise((resolve) => {
+  let n = 0;
+  const dst = new Writable({
+    write(chunk, _e, cb) {
+      n++;
+      console.log("fail write", String(chunk));
+      cb(n === 2 ? new Error("write failed") : null);
+    },
+  });
+  pipeline(pick(counted("f", 5)), dst, (err) => {
+    console.log("fail done", err && err.message);
+    resolve(undefined);
+  });
+});
+
+// backpressure: a one-byte highWaterMark and asynchronous writes
+await new Promise((resolve) => {
+  const dst = new Writable({
+    highWaterMark: 1,
+    write(chunk, _e, cb) {
+      console.log("slow write", String(chunk));
+      setTimeout(cb, 1);
+    },
+  });
+  pipeline(pick(counted("b", 3)), dst, (err) => {
+    console.log("slow done", err);
+    resolve(undefined);
+  });
+});
+
+// an abort signal
+const ac = new AbortController();
+try {
+  await pipelineP(
+    pick(counted("s", 10)),
+    new Writable({
+      write(chunk, _e, cb) {
+        console.log("signal write", String(chunk));
+        if (String(chunk) === "s1") ac.abort();
+        cb();
+      },
+    }),
+    { signal: ac.signal },
+  );
+} catch (err) {
+  console.log("signal rejected", err.name);
+}
+
+// an untyped async generator into a PassThrough read with for await
+const out = new PassThrough({ objectMode: true });
+const both = pipelineP(
+  pick((async function* () {
+    yield "u1";
+    yield "u2";
+  })()),
+  out,
+);
+for await (const c of out) console.log("passthrough", c);
+await both;
+console.log("passthrough resolved");
+
+// a Map source of entries into an object-mode sink
+await new Promise((resolve) => {
+  pipeline(
+    pick(new Map([["k", 1]])),
+    new Writable({ objectMode: true, write(c, _e, cb) { console.log("map write", JSON.stringify(c)); cb(); } }),
+    (err) => { console.log("map done", err); resolve(undefined); },
+  );
+});
+
+// sync sources whose values are promises: each value is awaited, as
+// for await over a sync iterator does, and a rejection closes the source
+await pipelineP(
+  pick([Promise.resolve("p1"), "p2", new Promise((r) => setTimeout(() => r("p3"), 1))]),
+  new Writable({ write(c, _e, cb) { console.log("byte write", String(c)); cb(); } }),
+);
+function* promised() {
+  try {
+    yield Promise.resolve({ n: 1 });
+    yield Promise.reject(new Error("value rejected"));
+    yield "never";
+  } finally {
+    console.log("promised finally");
+  }
+}
+try {
+  await pipelineP(
+    pick(promised()),
+    new Writable({ objectMode: true, write(c, _e, cb) { console.log("object write", JSON.stringify(c)); cb(); } }),
+  );
+} catch (err) {
+  console.log("promised rejected", err.message);
+}
+
+// a sync source's final value is awaited too: a rejected one rejects the
+// pipeline, and a resolved one is not written
+function* finalRejects() {
+  yield "f1";
+  return Promise.reject(new Error("final value rejected"));
+}
+function* finalResolves() {
+  yield "r1";
+  return Promise.resolve("not written");
+}
+for (const [label, source] of [["finalRejects", finalRejects], ["finalResolves", finalResolves]]) {
+  try {
+    await pipelineP(
+      pick(source()),
+      new Writable({ objectMode: true, write(c, _e, cb) { console.log(label, "write", c); cb(); } }),
+    );
+    console.log(label, "resolved");
+  } catch (err) {
+    console.log(label, "rejected", err.message);
+  }
+}
+
+// the final result's value is read for a sync iterator only: an async
+// iterator's done result is not read, so a throwing value getter there is
+// never called
+function finalResult(label, i) {
+  if (i === 1) return { value: `${label}1`, done: false };
+  return {
+    done: true,
+    get value() {
+      throw new Error(`${label} final value read`);
+    },
+  };
+}
+function asyncFinal() {
+  let i = 0;
+  return { [Symbol.asyncIterator]() { return { async next() { i++; return finalResult("async", i); } }; } };
+}
+function syncFinal() {
+  let i = 0;
+  return { [Symbol.iterator]() { return { next() { i++; return finalResult("sync", i); } }; } };
+}
+async function runFinal(label, source) {
+  try {
+    await pipelineP(
+      pick(source),
+      new Writable({ objectMode: true, write(c, _e, cb) { console.log(label, "write", c); cb(); } }),
+    );
+    console.log(label, "resolved");
+  } catch (err) {
+    console.log(label, "rejected", err.message);
+  }
+}
+await runFinal("async final", asyncFinal());
+await runFinal("sync final", syncFinal());
diff --git a/tests/corpus/7505-pipeline-untyped-sources.cjs b/tests/corpus/7505-pipeline-untyped-sources.cjs
new file mode 100644
index 000000000..6dd67b4ff
--- /dev/null
+++ b/tests/corpus/7505-pipeline-untyped-sources.cjs
@@ -0,0 +1,40 @@
+// pipeline() over untyped sources in CommonJS: a source that a function
+// answers, which may be an array, a generator, a stream, a string, or a
+// Buffer, whose bytes are numbers that a byte-mode Writable refuses.
+const { pipeline, Readable, Writable } = require("node:stream");
+
+function sink(label) {
+  const parts = [];
+  return new Writable({
+    write(chunk, _enc, cb) {
+      parts.push(String(chunk));
+      cb();
+    },
+    final(cb) {
+      console.log(label, parts.join("|"));
+      cb();
+    },
+  });
+}
+
+function* letters() {
+  yield "a";
+  yield "b";
+}
+
+const sources = [
+  () => ["one", "two"],
+  () => letters(),
+  () => Readable.from(["r1", "r2"]),
+  () => "xyz",
+  () => Buffer.from("buf"),
+];
+
+function run(i) {
+  if (i === sources.length) return;
+  pipeline(sources[i](), sink(`source ${i}`), (err) => {
+    console.log(`source ${i} done`, err === undefined ? "ok" : err.code);
+    run(i + 1);
+  });
+}
+run(0);
diff --git a/tests/corpus/7506-pipeline-any-source.ts b/tests/corpus/7506-pipeline-any-source.ts
new file mode 100644
index 000000000..84eec0cf4
--- /dev/null
+++ b/tests/corpus/7506-pipeline-any-source.ts
@@ -0,0 +1,48 @@
+// pipeline() in TypeScript over sources typed any or unknown, and over a
+// Set, a Map and an async generator object, which are pumped as iterables.
+import { pipeline, Writable } from "node:stream";
+import { pipeline as pipelineP } from "node:stream/promises";
+
+function sink(label: string): Writable {
+  const parts: string[] = [];
+  return new Writable({
+    objectMode: true,
+    write(chunk: unknown, _enc, cb) {
+      parts.push(JSON.stringify(chunk));
+      cb();
+    },
+    final(cb) {
+      console.log(label, parts.join(" "));
+      cb();
+    },
+  });
+}
+
+async function* ticks(): AsyncGenerator<string> {
+  yield "t1";
+  await new Promise((r) => setTimeout(r, 1));
+  yield "t2";
+}
+
+async function main(): Promise<void> {
+  const anySource: any = ["any1", "any2"];
+  await pipelineP(anySource, sink("any"));
+  const unknownSource: unknown = new Set([1, 2, 3]);
+  await pipelineP(unknownSource as any, sink("unknown"));
+  await pipelineP(new Set(["s"]), sink("set"));
+  await pipelineP(
+    new Map([
+      ["a", 1],
+      ["b", 2],
+    ]),
+    sink("map"),
+  );
+  await pipelineP(ticks(), sink("async generator"));
+  await new Promise<void>((resolve) => {
+    pipeline(anySource, sink("callback"), (err?: Error | null) => {
+      console.log("callback done", err);
+      resolve();
+    });
+  });
+}
+void main();
diff --git a/tests/corpus/7507-stream-write-error-drain.mjs b/tests/corpus/7507-stream-write-error-drain.mjs
new file mode 100644
index 000000000..8298bcb42
--- /dev/null
+++ b/tests/corpus/7507-stream-write-error-drain.mjs
@@ -0,0 +1,37 @@
+// A write whose callback fails synchronously: write() answers false and
+// writableNeedDrain stays false, so no 'drain' follows the error, and a
+// pipeline pumping a generator stops at the failed write.
+import { pipeline, Writable } from "node:stream";
+
+const w = new Writable({
+  write(_chunk, _enc, cb) {
+    cb(new Error("bad write"));
+  },
+});
+w.on("error", (e) => console.log("error event", e.message));
+w.on("drain", () => console.log("drain"));
+const r = w.write("a");
+console.log("returned", r, "needDrain", w.writableNeedDrain, "errored", w.errored.message);
+
+function* source() {
+  try {
+    for (let i = 0; i < 4; i++) {
+      console.log("yield", i);
+      yield `c${i}`;
+    }
+  } finally {
+    console.log("source finally");
+  }
+}
+let writes = 0;
+pipeline(
+  source(),
+  new Writable({
+    write(chunk, _enc, cb) {
+      writes++;
+      console.log("write", String(chunk));
+      cb(writes === 2 ? new Error("second write failed") : null);
+    },
+  }),
+  (err) => console.log("pipeline done", err.message),
+);
diff --git a/tests/corpus/7985-untyped-collection-types.mjs b/tests/corpus/7985-untyped-collection-types.mjs
new file mode 100644
index 000000000..de4e581c2
--- /dev/null
+++ b/tests/corpus/7985-untyped-collection-types.mjs
@@ -0,0 +1,125 @@
+// Untyped Maps, Sets and arrays whose uses prove their key, value and
+// element types compile as typed collections; any other use keeps them
+// untyped. Both must answer as Node does, including SameValueZero keys,
+// lookups of another type, and every use that refuses the typing.
+
+// Typed: counters, membership sets and stacks used only through their
+// methods.
+const counts = new Map();
+const seen = new Set();
+const stack = [];
+const words = [];
+for (let i = 0; i < 40; i++) {
+  const key = "k" + (i % 7);
+  counts.set(key, (counts.get(key) ?? 0) + 1);
+  seen.add(i % 5);
+  stack.push(i * 2);
+  if (stack.length > 6) stack.shift();
+  words.push(key);
+}
+console.log(counts.size, counts.get("k3"), counts.get("missing"), counts.has("k6"), counts.delete("k0"), counts.size);
+console.log(seen.size, seen.has(4), seen.has(5), seen.delete(4), seen.has(4));
+console.log(stack.length, stack.at(-1), stack.at(0), stack.pop(), stack.includes(70), stack.indexOf(72), words.join("|").length);
+
+// SameValueZero keys and NaN elements on typed collections.
+const byNumber = new Map();
+byNumber.set(NaN, "nan");
+byNumber.set(-0, "zero");
+byNumber.set(1.5, "x");
+console.log(byNumber.get(NaN), byNumber.get(0), byNumber.get(-0), byNumber.size, byNumber.has(1.5));
+const nums = [];
+nums.push(NaN);
+nums.push(-0);
+console.log(nums.includes(NaN), nums.indexOf(NaN), nums.includes(0), nums.indexOf(0));
+const flags = new Set();
+flags.add(true);
+console.log(flags.has(true), flags.has(false), flags.size);
+
+// Captured by a closure that only uses it.
+const cache = new Map();
+const latest = () => cache.get("c") ?? -1;
+console.log(latest());
+for (let i = 0; i < 3; i++) cache.set("c", i * 10);
+console.log(latest(), cache.size);
+
+// Lookups of another type: the typing refuses, and the answers stay Node's.
+const strings = new Map();
+strings.set("1", "one");
+console.log(strings.get(1), strings.get("1"), strings.has(1));
+const numbers = [];
+numbers.push(1);
+console.log(numbers.includes("1"), numbers.indexOf("1"), numbers.at("0"));
+
+// Mixed key, value and element types.
+const mixedKeys = new Map();
+mixedKeys.set(1, "number");
+mixedKeys.set("1", "string");
+console.log(mixedKeys.size, mixedKeys.get(1), mixedKeys.get("1"));
+const mixedValues = new Map();
+mixedValues.set("a", 1);
+mixedValues.set("b", "two");
+console.log(mixedValues.get("a"), mixedValues.get("b"));
+const mixedElements = [];
+mixedElements.push(1);
+mixedElements.push("two");
+console.log(mixedElements.join(","), mixedElements.length);
+
+// Escapes and other uses that keep the collection untyped.
+const passed = new Map();
+passed.set("x", 1);
+function sizeOf(m) {
+  return m.size;
+}
+console.log(sizeOf(passed));
+function make() {
+  const made = new Set();
+  made.add("y");
+  return made;
+}
+console.log(make().has("y"));
+const stored = [];
+stored.push(3);
+const holder = { list: stored };
+holder.list.push(4);
+console.log(stored.length, holder.list.join("+"));
+const iterated = new Map();
+iterated.set("a", 1);
+iterated.set("b", 2);
+for (const [k, v] of iterated) console.log("entry", k, v);
+const eachSet = new Set();
+eachSet.add(3);
+eachSet.forEach((v) => console.log("member", v));
+const shown = new Map();
+shown.set("s", 1);
+console.log(shown);
+const chained = new Map();
+chained.set("a", 1).set("b", 2);
+console.log(chained.size);
+const optional = new Map();
+optional.set("o", 1);
+console.log(optional?.get("o"), optional.get?.("o"));
+const indexed = [];
+indexed.push(1);
+indexed[3] = 2;
+console.log(indexed.length, indexed.join(","));
+const truncated = [];
+truncated.push(1, 2, 3);
+truncated.length = 1;
+console.log(truncated.length, truncated.at(0));
+const spread = [];
+spread.push(5);
+console.log(Math.max(...spread));
+const unread = new Map();
+console.log(unread.size, unread.get("none"));
+export const exported = new Map();
+exported.set("e", 1);
+console.log(exported.get("e"));
+// Exported through an export list, under its own name or another: an
+// importer sees an untyped Map.
+const listed = new Map();
+listed.set("l", 1);
+console.log(listed.get("l"));
+const renamed = new Map();
+renamed.set("r", 1);
+console.log(renamed.get("r"));
+export { listed, renamed as alias };
diff --git a/tests/corpus/8116-destructured-results.ts b/tests/corpus/8116-destructured-results.ts
new file mode 100644
index 000000000..71c884d49
--- /dev/null
+++ b/tests/corpus/8116-destructured-results.ts
@@ -0,0 +1,146 @@
+// Small functions returning a tuple or record that the caller destructures
+// at once: string slices and numbers in the fields, a parameter that may be
+// undefined at run time (a missing array element passed with `!`), fields
+// the caller does not read, object destructuring in another order than the
+// literal, arguments and fields with side effects, non-ASCII and astral
+// receivers, slices that escape into arrays, closures and returns, bodies
+// with loops and locals, a recursive producer, sites inside switch, try and
+// loops, and an async caller.
+const log: string[] = [];
+function note(label: string, value: number): number {
+  log.push(label);
+  return value;
+}
+
+function split(text: string, at: number): [string, string] {
+  return [text.slice(0, at), text.slice(at)];
+}
+
+function divmod(a: number, b: number): [number, number] {
+  return [Math.floor(a / b), a % b];
+}
+
+interface Name {
+  first: string;
+  last: string;
+}
+
+function name(full: string): Name {
+  const space = full.indexOf(" ");
+  return { first: full.slice(0, space), last: full.slice(space + 1) };
+}
+
+function trimmed(text: string): [number, number] {
+  let start = 0;
+  let end = text.length;
+  while (start < end && text.charCodeAt(start) === 32) start++;
+  while (end > start && text.charCodeAt(end - 1) === 32) end--;
+  return [start, end];
+}
+
+function noted(a: number, b: number): [number, number] {
+  return [note("first field", a), note("second field", b)];
+}
+
+function digits(n: number): [number, string] {
+  if (n < 10) return [1, String(n)];
+  const [count, rest] = digits(Math.floor(n / 10));
+  return [count + 1, rest + String(n % 10)];
+}
+
+const texts = ["alpha", "be", "", "x", "héllo", "a😀b"];
+let total = 0;
+const kept: string[] = [];
+const later: (() => string)[] = [];
+for (let i = 0; i < 40; i++) {
+  const [q, r] = divmod(i * 31 - 200, 7);
+  const [head, tail] = split(texts[i % texts.length]!, r % 4);
+  total += q + r + head.length * 2 + tail.length;
+  if (i % 9 === 0) kept.push(head);
+  if (i % 11 === 0) later.push(() => tail);
+}
+console.log("total", total);
+console.log("kept", kept.join("|"));
+console.log("later", later.map((f) => f()).join("|"));
+
+for (const [a, b] of [
+  [7, 2],
+  [-7, 2],
+  [7.5, 2],
+  [1, 0],
+  [-1, 0],
+  [0, -3],
+]) {
+  const [q, r] = divmod(a!, b!);
+  console.log("divmod", a, b, q, r, Object.is(r, -0));
+}
+const [only] = divmod(17, 5);
+console.log("only", only);
+
+const { last, first } = name("Ada Lovelace");
+console.log("name", first, last);
+const { first: alone } = name("Grace Hopper");
+console.log("alone", alone);
+
+const [left, right] = trimmed("   padded text  ");
+console.log("trimmed", left, right);
+
+const [x, y] = noted(note("first argument", 1), note("second argument", 2));
+console.log("order", x, y, log.join(", "));
+
+const [count, spelled] = digits(90210);
+console.log("digits", count, spelled);
+
+for (const text of ["héllo", "a😀b", "日本語"]) {
+  for (const at of [0, 1, 2, 3, -1, 10]) {
+    const [head, tail] = split(text, at);
+    console.log("unicode", text, at, head, tail, head.length, tail.length);
+  }
+}
+
+function tails(): string[] {
+  const out: string[] = [];
+  for (const text of texts) {
+    const [, tail] = split(text, 1);
+    out.push(tail);
+  }
+  return out;
+}
+console.log("tails", tails().join("|"));
+
+function lastTail(text: string): string {
+  const [, tail] = split(text, 2);
+  return tail;
+}
+console.log("lastTail", lastTail("returned"));
+
+for (const kind of ["one", "two", "other"]) {
+  switch (kind) {
+    case "one": {
+      const [h, t] = split("switch", 3);
+      console.log("switch", kind, h, t);
+      break;
+    }
+    case "two": {
+      const [q, r] = divmod(-9, 4);
+      console.log("switch", kind, q, r);
+      break;
+    }
+    default:
+      console.log("switch", kind);
+  }
+}
+
+try {
+  const [head, tail] = split(texts[99]!, 1);
+  console.log("unreachable", head, tail);
+} catch (error) {
+  console.log("missing", (error as Error).name, (error as Error).message);
+}
+
+async function asyncCaller(text: string): Promise<string> {
+  await Promise.resolve();
+  const [head, tail] = split(text, 3);
+  return `${head}-${tail}`;
+}
+asyncCaller("awaited").then((value) => console.log("async", value));
diff --git a/tests/corpus/8117-scalar-slice-bounds.ts b/tests/corpus/8117-scalar-slice-bounds.ts
new file mode 100644
index 000000000..412221368
--- /dev/null
+++ b/tests/corpus/8117-scalar-slice-bounds.ts
@@ -0,0 +1,25 @@
+// slice() and substring() results read only for their length and code
+// units, so no string is built: ASCII, non-ASCII, astral and empty
+// receivers, with integer, negative, fractional, -0, NaN, infinite and huge
+// bounds, one bound or two, and substring's swapped bounds.
+const receivers = ["", "a", "hello world", "héllo", "a😀b", "日本語テキスト"];
+const bounds = [0, 1, 2, -1, -2, -0, 0.5, -0.5, 3.7, -3.7, NaN, Infinity, -Infinity, 1e20, -1e20];
+
+function sliceFacts(text: string, start: number, end: number): string {
+  const both = text.slice(start, end);
+  const from = text.slice(start);
+  const sub = text.substring(start, end);
+  const subFrom = text.substring(start);
+  return [both.length, both.charCodeAt(0), from.length, from.charCodeAt(from.length - 1), sub.length, sub.charCodeAt(0), subFrom.length].join(",");
+}
+
+let lines = 0;
+for (const text of receivers) {
+  const facts: string[] = [];
+  for (const start of bounds) {
+    for (const end of bounds) facts.push(sliceFacts(text, start, end));
+  }
+  console.log(JSON.stringify(text), facts.length, facts.join(" "));
+  lines++;
+}
+console.log("receivers", lines);
diff --git a/tests/corpus/8194-row-pair-sums.ts b/tests/corpus/8194-row-pair-sums.ts
new file mode 100644
index 000000000..f28242ff5
--- /dev/null
+++ b/tests/corpus/8194-row-pair-sums.ts
@@ -0,0 +1,90 @@
+// Row sums (`for i { t = 0; for j t += term; out[i] = t }`) over typed
+// arrays, run with even and odd row counts, an empty range, fractional,
+// negative-zero and out-of-int32 bounds, and output arrays that are the
+// input array or overlap it in one buffer, so a row may read an earlier
+// row's result.
+function rows(u: Float64Array, v: Float64Array, start: number, end: number, n: number): void {
+  for (let i = start; i < end; i++) {
+    let t = 0;
+    for (let j = 0; j < n; j++) t += u[j]! / ((((i + j) * (i + j + 1)) >>> 1) + i + 1);
+    v[i] = t;
+  }
+}
+
+function weighted(u: Float32Array, w: Int32Array, v: Float64Array, start: number, end: number): void {
+  for (let i = start; i < end; i++) {
+    let t = 0.5;
+    for (let j = 0; j < u.length; j++) t += u[j]! * w[(i + j) % w.length]! - i * 0.25;
+    v[i] = t;
+  }
+}
+
+const show = (label: string, v: Float64Array): void => {
+  const parts: string[] = [];
+  for (const x of v) parts.push(Object.is(x, -0) ? "-0" : x.toPrecision(17));
+  console.log(label, parts.join(" "));
+};
+
+const n = Number(process.argv[2] ?? 7);
+const u = new Float64Array(n).map((_, k) => k + 1);
+for (const [start, end] of [
+  [0, n],
+  [0, n - 1],
+  [1, n],
+  [2, 2],
+  [3, 1],
+  [-0, 3],
+  [0.5, 4],
+  [0, 4.5],
+  [2 ** 31 - 3, 2 ** 31 + 1],
+  [NaN, 3],
+]) {
+  const v = new Float64Array(n + 2);
+  rows(u, v, start!, end!, n);
+  show(`rows ${start} ${end}`, v);
+}
+
+// The output is the input: each row reads the rows stored before it.
+const same = new Float64Array(n).map((_, k) => k + 1);
+rows(same, same, 0, n, n);
+show("same", same);
+
+// Overlapping views of one buffer, offset by one element.
+const buffer = new ArrayBuffer(8 * (n + 1));
+const lo = new Float64Array(buffer, 0, n).map((_, k) => k + 1);
+const hi = new Float64Array(buffer, 8, n);
+rows(lo, hi, 0, n, n);
+show("overlap", new Float64Array(buffer));
+
+// Disjoint views of one buffer.
+const shared = new ArrayBuffer(16 * n);
+const a = new Float64Array(shared, 0, n).map((_, k) => n - k);
+const b = new Float64Array(shared, 8 * n, n);
+rows(a, b, 0, n, n);
+show("disjoint", b);
+
+const f = new Float32Array(n).map((_, k) => k / 3);
+const w = new Int32Array([3, -1, 4, 1, -5]);
+const out = new Float64Array(n);
+weighted(f, w, out, 0, n);
+show("weighted", out);
+weighted(f, w, out, 1, n - 1);
+show("weighted inner", out);
+
+// Arrays a program assigns only when the row range is nonempty: an empty or
+// one-row range never reads them, and the paired copy does not either.
+function lazyRows(n: number, start: number, end: number): number {
+  let u!: Float64Array;
+  let v!: Float64Array;
+  if (end > start) {
+    u = new Float64Array(n).fill(1);
+    v = new Float64Array(end);
+  }
+  for (let i = start; i < end; i++) {
+    let t = 0;
+    for (let j = 0; j < n; j++) t += u[j]! / (i + j + 1);
+    v[i] = t;
+  }
+  return end > start ? v[end - 1]! : -1;
+}
+console.log("lazy", lazyRows(4, 3, 3), lazyRows(4, 2, 3), lazyRows(4, 0, 5), lazyRows(0, 5, 5));
diff --git a/tests/corpus/8223-argument-count-destructured-parameter.ts b/tests/corpus/8223-argument-count-destructured-parameter.ts
new file mode 100644
index 000000000..def8c5ba8
--- /dev/null
+++ b/tests/corpus/8223-argument-count-destructured-parameter.ts
@@ -0,0 +1,14 @@
+// A function with a destructured parameter that reads arguments.length,
+// called through a value (map passes three arguments) and directly. The
+// count is taken before the parameters bind, so each call sees its own.
+
+function probe({ v, w }: { v: number; w: string }): string {
+  return `${arguments.length}:${v}${w}`;
+}
+
+console.log([{ v: 1, w: "a" }, { v: 2, w: "b" }].map(probe));
+console.log(probe({ v: 3, w: "c" }));
+const call = (f: (item: { v: number; w: string }, index: number) => string): string =>
+  f({ v: 4, w: "d" }, 0);
+console.log(call(probe));
+console.log(probe({ v: 5, w: "e" }));
diff --git a/tests/corpus/8230-readline-interface-constructor.cjs b/tests/corpus/8230-readline-interface-constructor.cjs
new file mode 100644
index 000000000..6d7eea0e4
--- /dev/null
+++ b/tests/corpus/8230-readline-interface-constructor.cjs
@@ -0,0 +1,51 @@
+// `new readline.Interface(...)`: Node's createInterface calls this
+// constructor with the same arguments, and a JavaScript file may call it
+// directly. Its declaration is protected, which JavaScript does not
+// enforce.
+"use strict";
+const readline = require("readline");
+const readlinePromises = require("readline/promises");
+const { Interface } = require("readline");
+const { PassThrough } = require("stream");
+
+// The options form.
+const a = new PassThrough();
+const rlA = new readline.Interface({ input: a, terminal: false });
+console.log("instanceof", rlA instanceof readline.Interface);
+const linesA = [];
+rlA.on("line", (line) => {
+  linesA.push(line);
+});
+rlA.on("close", () => {
+  console.log("options", JSON.stringify(linesA));
+});
+a.write("one\ntwo\n");
+a.end("three");
+
+// The positional form, through a destructured binding.
+const b = new PassThrough();
+const outB = new PassThrough();
+const rlB = new Interface(b, outB);
+const linesB = [];
+rlB.on("line", (line) => {
+  linesB.push(line);
+});
+rlB.on("close", () => {
+  console.log("positional", JSON.stringify(linesB));
+});
+b.write("four\r\nfive\n");
+b.end();
+
+// readline/promises' Interface.
+const c = new PassThrough();
+const rlC = new readlinePromises.Interface({ input: c, crlfDelay: Infinity });
+console.log("promises instanceof", rlC instanceof readlinePromises.Interface);
+const linesC = [];
+rlC.on("line", (line) => {
+  linesC.push(line);
+});
+rlC.on("close", () => {
+  console.log("promises", JSON.stringify(linesC));
+});
+c.end("six\nseven");
+
diff --git a/tests/corpus/8231-builtin-member-binding-new.cjs b/tests/corpus/8231-builtin-member-binding-new.cjs
new file mode 100644
index 000000000..df1b2ebb3
--- /dev/null
+++ b/tests/corpus/8231-builtin-member-binding-new.cjs
@@ -0,0 +1,60 @@
+// `new` of a builtin class bound through a member of a require() result
+// (`const SD = require("string_decoder").StringDecoder`, the spelling of
+// Node's own tests): the binding resolves to the class, as the
+// destructured `const { StringDecoder } = require(...)` does, also for the
+// X509Certificate record.
+"use strict";
+const SD = require("string_decoder").StringDecoder;
+const Readable = require("stream").Readable;
+const PassThrough = require("stream").PassThrough;
+const Transform = require("stream").Transform;
+const X509 = require("crypto").X509Certificate;
+
+const decoder = new SD("utf8");
+console.log(decoder.encoding);
+console.log(JSON.stringify(decoder.write(Buffer.from([0xe2, 0x82]))));
+console.log(decoder.end(Buffer.from([0xac])));
+const hex = new SD("hex");
+console.log(hex.encoding, hex.write(Buffer.from("hi")));
+console.log(new SD().encoding);
+
+const r = new Readable({ read() {} });
+r.on("data", (chunk) => console.log("readable", String(chunk)));
+r.on("end", () => console.log("readable end"));
+r.push("abc");
+r.push(null);
+
+const p = new PassThrough();
+p.on("data", (chunk) => console.log("passthrough", String(chunk)));
+p.end("xyz");
+
+const upper = new Transform({
+  transform(chunk, encoding, callback) {
+    callback(null, String(chunk).toUpperCase());
+  },
+});
+upper.on("data", (chunk) => console.log("transform", String(chunk)));
+upper.end("shout");
+
+// The X509Certificate record constructs through the same binding.
+const PEM = `-----BEGIN CERTIFICATE-----
+MIIDEzCCAfugAwIBAgIUMsBCFtPck8TATMJzfikgW963q3AwDQYJKoZIhvcNAQEL
+BQAwGDEWMBQGA1UEAwwNdHNuYXRpdmUtdGVzdDAgFw0yNjA3MTcxNzUyMTFaGA8y
+MTI2MDYyMzE3NTIxMVowGDEWMBQGA1UEAwwNdHNuYXRpdmUtdGVzdDCCASIwDQYJ
+KoZIhvcNAQEBBQADggEPADCCAQoCggEBALRspe2XjtKvNU7Ws5JfqzmTCPmWtfw0
+1Uha271hNp/30VstOxWLbpqPrXDvIwJMN4o8CRp92Y1hcmS36m9QS5qfXJFw+n+S
+SWGpFdMNvp6hro0OoNBA+anuKWoTsMyHSwe8ND0fyobQbgAXJ0+snmuK0E2ii+CM
+IoUqYbK57nHeyHfISLupgrRoNGXjDHv4v+RSr3MJDag6BLA+/yVjkBOBOT6kaE4K
+oyNXYYsfFxGnB/4pDSanV/UsfJ4wHXkVn18ndZHIzTsZMUkV4BecF8UaMrP+u8NI
+c5wce0uWLFFEneK4q15iVvk9TaY0g1Uj69ryRT3L58W7RYYuHu7cVB0CAwEAAaNT
+MFEwHQYDVR0OBBYEFLT2GqaTmxDhntPxMWaUZvA3s9V5MB8GA1UdIwQYMBaAFLT2
+GqaTmxDhntPxMWaUZvA3s9V5MA8GA1UdEwEB/wQFMAMBAf8wDQYJKoZIhvcNAQEL
+BQADggEBAFcTLVJIyhbIKHfOnGM5T+Kzu5IoDlqzr+Rdx/0IKFLVDqEA9ihEme07
+K/d246pOW5LBceVDY2eMikDpIOjYPEoe9C+Ftv5zrCaDw1wtXrq8a7/eS5B+3TzL
+k8c1SVVDPIbmViAZNRmnkwCPBsIF9xIfMFhxz4BTJrm/Lt2Hv1GcRBQx/yRK89sj
+JiGnsTna//4mPGRGYw+dRCq4tbyr8HbBKdJmbzOCb4CPKfwAMKUpd4e0Ao7jh/BU
+EML7B3BEzv5Jw/BT7B39wAfDyhq32PnRnZefF+7pAmpkklHbckBtZtWn02zwQazv
+5z62tszZee2i4BouJiyFnb30F6Pebs4=
+-----END CERTIFICATE-----`;
+const cert = new X509(PEM);
+console.log(cert.fingerprint, cert.validFrom, cert.validTo);
diff --git a/tests/corpus/8241-literal-result-slices.ts b/tests/corpus/8241-literal-result-slices.ts
index 11de82819..05deb40a5 100644
--- a/tests/corpus/8241-literal-result-slices.ts
+++ b/tests/corpus/8241-literal-result-slices.ts
@@ -79,3 +79,12 @@ for (const item of list) {
 // The producer keeps working for every other caller.
 const whole = split("kept", 2);
 console.log("kept", whole[0], whole[1], whole.length);
+
+// Fields that write a parameter: each inlined call writes its own copy.
+function bump(a: number, s: string): [number, number, string] {
+  return [a++, (a = a * 2), s.slice(a - 3)];
+}
+for (const start of [2, 3]) {
+  const [before, doubled, tailOf] = bump(start, "abcdefgh");
+  console.log("writes", start, before, doubled, tailOf);
+}
diff --git a/tests/diagnostics/function-value-arguments-length.ts b/tests/diagnostics/function-value-arguments-length.ts
new file mode 100644
index 000000000..ab0b53f82
--- /dev/null
+++ b/tests/diagnostics/function-value-arguments-length.ts
@@ -0,0 +1,36 @@
+// `arguments.length` has no single value when the calls pass varying
+// counts: a function expression or declaration passed where the expected
+// function type has optional or rest parameters. Methods and nested
+// functions used as values, and arrows inside a function used as a value,
+// keep the fence.
+const replaced = "a-b".replace(/-/, function (match) {
+  return String(arguments.length) + match;
+});
+function call(f: (a: number, b?: number) => number): number {
+  return f(1, 2);
+}
+const counted = call(function (a) {
+  return arguments.length + a;
+});
+function offset(value: number): number {
+  return value + arguments.length;
+}
+const offsets = call(offset);
+class Scale {
+  static by(value: number): number {
+    return value * arguments.length;
+  }
+}
+const scaled = [2].map(Scale.by);
+function outer(): number[] {
+  function inner(value: number): number {
+    return value + arguments.length;
+  }
+  return [1].map(inner);
+}
+function deferred(value: number): number {
+  const read = (): number => arguments.length;
+  return value + read();
+}
+const late = [1].map(deferred);
+export { replaced, counted, offsets, scaled, outer, late };
diff --git a/tests/harness/__snapshots__/function-value-arguments-length.ts.txt b/tests/harness/__snapshots__/function-value-arguments-length.ts.txt
new file mode 100644
index 000000000..adcb48d2a
--- /dev/null
+++ b/tests/harness/__snapshots__/function-value-arguments-length.ts.txt
@@ -0,0 +1,43 @@
+function-value-arguments-length.ts:7:17 - error SC1090: 'arguments.length' in function expressions passed where the expected function type has optional or rest parameters (the count varies per call) are not supported yet
+
+  6 | const replaced = "a-b".replace(/-/, function (match) {
+  7 |   return String(arguments.length) + match;
+    |                 ^~~~~~~~~~~~~~~~
+  8 | });
+
+function-value-arguments-length.ts:13:10 - error SC1090: 'arguments.length' in function expressions passed where the expected function type has optional or rest parameters (the count varies per call) are not supported yet
+
+  12 | const counted = call(function (a) {
+  13 |   return arguments.length + a;
+     |          ^~~~~~~~~~~~~~~~
+  14 | });
+
+function-value-arguments-length.ts:18:22 - error SC1090: functions that read 'arguments.length' used as values where the expected function type has optional or rest parameters (the count varies per call) are not supported yet
+
+  17 | }
+  18 | const offsets = call(offset);
+     |                      ^~~~~~
+  19 | class Scale {
+
+function-value-arguments-length.ts:21:20 - error SC1090: 'arguments.length' in methods, nested, async and generator functions used as values (a caller of the value can pass more arguments than the function declares) are not supported yet
+
+  20 |   static by(value: number): number {
+  21 |     return value * arguments.length;
+     |                    ^~~~~~~~~~~~~~~~
+  22 |   }
+
+function-value-arguments-length.ts:32:30 - error SC1090: 'arguments.length' in an arrow function inside a function that is also used as a value is not supported yet
+
+  31 | function deferred(value: number): number {
+  32 |   const read = (): number => arguments.length;
+     |                              ^~~~~~~~~~~~~~~~
+  33 |   return value + read();
+
+function-value-arguments-length.ts:33:18 - error SC2004: uses of 'read' inherit the blocker on its declaration
+
+  32 |   const read = (): number => arguments.length;
+  33 |   return value + read();
+     |                  ^~~~
+  34 | }
+
+  hint: the declaration of 'read' did not compile — fix the diagnostic reported there and these sites clear with it
\ No newline at end of file
diff --git a/tests/harness/corpus-costs/7381-function-value-arguments-length.ts.json b/tests/harness/corpus-costs/7381-function-value-arguments-length.ts.json
new file mode 100644
index 000000000..b4659b9a0
--- /dev/null
+++ b/tests/harness/corpus-costs/7381-function-value-arguments-length.ts.json
@@ -0,0 +1 @@
+{ "plain": 3400, "san": 3300 }
diff --git a/tests/harness/corpus-costs/7503-pipeline-untyped-sources.mjs.json b/tests/harness/corpus-costs/7503-pipeline-untyped-sources.mjs.json
new file mode 100644
index 000000000..d2d431b0f
--- /dev/null
+++ b/tests/harness/corpus-costs/7503-pipeline-untyped-sources.mjs.json
@@ -0,0 +1 @@
+{ "plain": 5499, "san": 7259 }
diff --git a/tests/harness/corpus-costs/7504-pipeline-untyped-source-errors.mjs.json b/tests/harness/corpus-costs/7504-pipeline-untyped-source-errors.mjs.json
new file mode 100644
index 000000000..4fd20681f
--- /dev/null
+++ b/tests/harness/corpus-costs/7504-pipeline-untyped-source-errors.mjs.json
@@ -0,0 +1 @@
+{ "plain": 3431, "san": 4529 }
diff --git a/tests/harness/corpus-costs/7505-pipeline-untyped-sources.cjs.json b/tests/harness/corpus-costs/7505-pipeline-untyped-sources.cjs.json
new file mode 100644
index 000000000..3bb5df076
--- /dev/null
+++ b/tests/harness/corpus-costs/7505-pipeline-untyped-sources.cjs.json
@@ -0,0 +1 @@
+{ "plain": 3197, "san": 4220 }
diff --git a/tests/harness/corpus-costs/7506-pipeline-any-source.ts.json b/tests/harness/corpus-costs/7506-pipeline-any-source.ts.json
new file mode 100644
index 000000000..454dcb2c0
--- /dev/null
+++ b/tests/harness/corpus-costs/7506-pipeline-any-source.ts.json
@@ -0,0 +1 @@
+{ "plain": 3250, "san": 4741 }
diff --git a/tests/harness/corpus-costs/7507-stream-write-error-drain.mjs.json b/tests/harness/corpus-costs/7507-stream-write-error-drain.mjs.json
new file mode 100644
index 000000000..35f298bef
--- /dev/null
+++ b/tests/harness/corpus-costs/7507-stream-write-error-drain.mjs.json
@@ -0,0 +1 @@
+{ "plain": 2735, "san": 4076 }
diff --git a/tests/harness/corpus-costs/7985-untyped-collection-types.mjs.json b/tests/harness/corpus-costs/7985-untyped-collection-types.mjs.json
new file mode 100644
index 000000000..6915a85fe
--- /dev/null
+++ b/tests/harness/corpus-costs/7985-untyped-collection-types.mjs.json
@@ -0,0 +1 @@
+{ "plain": 2400, "san": 2600 }
diff --git a/tests/harness/corpus-costs/8223-argument-count-destructured-parameter.ts.json b/tests/harness/corpus-costs/8223-argument-count-destructured-parameter.ts.json
new file mode 100644
index 000000000..664b49416
--- /dev/null
+++ b/tests/harness/corpus-costs/8223-argument-count-destructured-parameter.ts.json
@@ -0,0 +1 @@
+{ "plain": 4700, "san": 4200 }
diff --git a/tests/harness/corpus-costs/8230-readline-interface-constructor.cjs.json b/tests/harness/corpus-costs/8230-readline-interface-constructor.cjs.json
new file mode 100644
index 000000000..9b459042f
--- /dev/null
+++ b/tests/harness/corpus-costs/8230-readline-interface-constructor.cjs.json
@@ -0,0 +1 @@
+{ "plain": 3500, "san": 3000 }
diff --git a/tests/harness/corpus-costs/8231-builtin-member-binding-new.cjs.json b/tests/harness/corpus-costs/8231-builtin-member-binding-new.cjs.json
new file mode 100644
index 000000000..54e11997a
--- /dev/null
+++ b/tests/harness/corpus-costs/8231-builtin-member-binding-new.cjs.json
@@ -0,0 +1 @@
+{ "plain": 2500, "san": 2500 }
diff --git a/tests/harness/self-hosting-native-toolchain.test.ts b/tests/harness/self-hosting-native-toolchain.test.ts
index c5476328e..f4121bf88 100644
--- a/tests/harness/self-hosting-native-toolchain.test.ts
+++ b/tests/harness/self-hosting-native-toolchain.test.ts
@@ -109,7 +109,7 @@ for (const backend of ["llvm"] as const) {
           object_format: target.objectFormat,
           minimum_os: target.minimumOs,
         },
-        runtime_abi: { version: 37, marker: "scr_runtime_abi_v37" },
+        runtime_abi: { version: 42, marker: "scr_runtime_abi_v42" },
         compiler: { command: "fixture", identity: "fixture", target: target.llvmTriple },
         macros: { executable: [], excluded: [], sanitizer: "external-toolchain-required" },
         flavors: {