diff --git a/docs/content/limitations/builtin-apis/assertions.mdx b/docs/content/limitations/builtin-apis/assertions.mdx
index 381fc3802..84c830e79 100644
--- a/docs/content/limitations/builtin-apis/assertions.mdx
+++ b/docs/content/limitations/builtin-apis/assertions.mdx
@@ -5,6 +5,8 @@
   `assert.AssertionError` used as a value outside `instanceof` throws `SC2020`.
   The comparison methods, `ok`, `fail` and `ifError` are function values that assert as the methods do; called through a stored value, `ok` without a message reports `value == true`, where Node quotes the call, and `fail` takes at most one argument.
   `throws`, `rejects`, `doesNotThrow`, `doesNotReject`, `match` and `doesNotMatch` as values throw `SC2020`.
+  `strictEqual` and `notStrictEqual` compare a number, string or boolean that can be `null` or `undefined`, such as `child.signalCode`, with any number, string, boolean, `null` or `undefined`.
+  `notStrictEqual` of another value that can be `null` or `undefined`, such as `child.stdin`, against `null` or `undefined` compiles; `strictEqual` of one throws `SC2020`.
   `assert.deepEqual` and `assert.notDeepEqual` follow Node's loose comparison, including Maps, Sets, Dates, regular expressions, typed arrays, and errors; operands whose type can be cyclic or holds functions inside a structure are refused.
   `assert.partialDeepStrictEqual` follows Node's partial comparison over the same values, with the same refusals.
   Checked-dynamic values compared deeply have no cycle detection.
diff --git a/docs/content/limitations/language-features/commonjs-require.mdx b/docs/content/limitations/language-features/commonjs-require.mdx
index f0bf67c4e..f13ca143f 100644
--- a/docs/content/limitations/language-features/commonjs-require.mdx
+++ b/docs/content/limitations/language-features/commonjs-require.mdx
@@ -7,3 +7,5 @@
   Defaults, rest elements, and nested patterns over other modules are unsupported.
   A require cycle compiles when each of its modules assigns all of its exports before its first `require` and uses the other modules of the cycle only inside functions that its top-level code can't call.
   Other require cycles are rejected with `SC1016`.
+  A top-level `return` ends the module's evaluation.
+  It is unsupported when the module assigns exports after it, or when a function that exists before it runs reads a binding declared after it, such as a callback created above the `return` or a function declaration.
diff --git a/packages/compiler/src/frontend/lowering/expressions/identifiers.ts b/packages/compiler/src/frontend/lowering/expressions/identifiers.ts
index 66cc8c1c7..7beff1c8f 100644
--- a/packages/compiler/src/frontend/lowering/expressions/identifiers.ts
+++ b/packages/compiler/src/frontend/lowering/expressions/identifiers.ts
@@ -24,6 +24,7 @@ import {
   funcOf,
   isUnitType,
   recordTextCodecClass,
+  typeEquals,
 } from "../../../ir/ir.js";
 import { lowerNodeModuleIdentifier } from "../lower-node-module.js";
 import { lowerCjsExportsObjectValue } from "../cjs-exports-object.js";
@@ -192,6 +193,11 @@ export function lowerIdentifierExpr(lowerer: Lowerer, expr: ts.Identifier, loc:
   // when the checker types this USE as a single arm, maybeNarrow
   // bridges the tagged representation with a unionNarrow.
   const local = lowerer.resolveLocal(expr);
+  {
+    const slot = local ?? lowerer.globalOf(expr);
+    const closureRead = slot ? closureDynRead(lowerer, expr, slot.id, slot.type, loc) : null;
+    if (closureRead) return closureRead;
+  }
   // A Date local holding its time value reads as that Date: only uses
   // that read the time value reach here (date-values.ts).
   if (local && lowerer.timeValueDateLocals.has(local)) {
@@ -1233,3 +1239,60 @@ export function uriFunctionValue(lowerer: Lowerer, which: string, loc: SrcLoc):
   }
   return { kind: "closure", fnName: name, captures: [], type, loc };
 }
+
+/** A read of a captured handle `let` (Lowerer.closureDynLets) inside a
+ * function nested in the binding's scope. The checker types it as the
+ * handle, but the closure can run before the first write. As a member
+ * receiver (`c.address()`, `child.stdin`) it reads the handle and throws
+ * Node's TypeError when the binding is still undefined; elsewhere it reads
+ * the checked-dynamic value of the handle or undefined. Null for other
+ * reads. */
+function closureDynRead(
+  lowerer: Lowerer,
+  expr: ts.Identifier,
+  id: string,
+  type: IrType,
+  loc: SrcLoc,
+): IrExpr | null {
+  if (type.kind !== "union" || lowerer.closureDynLets.size === 0) return null;
+  const symbol = lowerer.resolveValueSymbol(expr);
+  if (!symbol || !lowerer.closureDynLets.has(symbol)) return null;
+  const decl = symbol.valueDeclaration;
+  if (!decl) return null;
+  const scopeOf = (node: ts.Node): ts.Node | undefined => {
+    let at = node.parent;
+    while (at !== undefined && !ts.isFunctionLike(at) && !ts.isSourceFile(at)) at = at.parent;
+    return at;
+  };
+  if (scopeOf(expr) === scopeOf(decl)) return null;
+  const parent = expr.parent;
+  const handle = lowerer.unions.get(type.unionId)?.arms.find((arm) => !isUnitType(arm));
+  // Only a use the checker types as the handle narrows: inside a function
+  // declared before the first write the binding reads as `any`.
+  const typedAsHandle =
+    handle !== undefined && typeEquals(lowerer.mapTypeOf(lowerer.typeOf(expr)) ?? DYN, handle);
+  if (typedAsHandle && ts.isPropertyAccessExpression(parent) && parent.expression === expr) {
+    // An optional read (`c?.address`) tests the tagged value: undefined
+    // short-circuits and the handle arm narrows.
+    if (parent.questionDotToken) return { kind: "varRef", localId: id, type, loc };
+    const helper = lowerer.narrowedArmHelper(
+      type.unionId,
+      handle,
+      loc,
+      `Cannot read properties of undefined (reading '${parent.name.text}')`,
+    );
+    if (helper)
+      return lowerer.checkedNarrowCall(
+        helper,
+        { kind: "varRef", localId: id, type, loc },
+        handle,
+        loc,
+      );
+  }
+  return {
+    kind: "dynFrom",
+    value: { kind: "varRef", localId: id, type, loc },
+    type: DYN,
+    loc,
+  };
+}
diff --git a/packages/compiler/src/frontend/lowering/lower-assert.ts b/packages/compiler/src/frontend/lowering/lower-assert.ts
index 704314eaf..d167bb036 100644
--- a/packages/compiler/src/frontend/lowering/lower-assert.ts
+++ b/packages/compiler/src/frontend/lowering/lower-assert.ts
@@ -1290,12 +1290,28 @@ function lowerAssertEqual(
           ?.arms.every((arm) => instanceType(arm) || isUnitType(arm)) ??
           false))) &&
     lowerer.dynConvertible(e.type);
+  // A nullable number, string or boolean (`number | null`) against a
+  // scalar or null/undefined: under the strict pair each arm crosses by
+  // value, so the checked-dynamic SameValue compare answers, and renders
+  // its failure message, as Node does.
+  const strictArm = (t: IrType): boolean =>
+    t.kind === "f64" || t.kind === "string" || t.kind === "bool" || isUnitType(t);
+  const strictOperand = (e: IrExpr): boolean =>
+    !deep &&
+    (e.kind === "unitLit" ||
+      strictArm(e.type) ||
+      (e.type.kind === "union" &&
+        (lowerer.unions.get(e.type.unionId)?.arms.every(strictArm) ?? false))) &&
+    lowerer.dynConvertible(e.type);
+  const nullable = (e: IrExpr): boolean => e.type.kind === "union" || isUnitType(e.type);
+  const strictNullable = (nullable(a) || nullable(b)) && strictOperand(a) && strictOperand(b);
   if (
     a.type.kind === "dyn" ||
     b.type.kind === "dyn" ||
     a.type.kind === "moduleNs" ||
     b.type.kind === "moduleNs" ||
-    (instance(a) && instance(b))
+    (instance(a) && instance(b)) ||
+    strictNullable
   ) {
     const box = (e: IrExpr, node: ts.Expression): IrExpr => {
       if (e.type.kind === "dyn") return e;
@@ -1339,6 +1355,7 @@ function lowerAssertEqual(
         // re-encoding and losing identity (SEMANTICS.md).
         (k === "object" && e.type.kind === "object" && e.type.className === "%Error") ||
         instance(e) ||
+        strictOperand(e) ||
         // A symbol crosses as itself, so strict compares are identity.
         (!deep && k === "symbol") ||
         (deep && k !== "bytes" && k !== "object" && lowerer.dynConvertible(e.type)) ||
@@ -1472,6 +1489,66 @@ function lowerAssertEqual(
       loc,
     };
   }
+  // notStrictEqual of a nullable value whose other arms do not convert
+  // (`child.stdin`, a stream or null) against null or undefined fails only
+  // when the value is that unit, and the failure message renders only the
+  // unit. The value's tag test stands in for it: the unit when it matches,
+  // `false` otherwise, which the unit never equals. A union without that
+  // unit arm never equals it.
+  if (!deep && negated) {
+    const unitSide = isUnitType(a.type) ? a : isUnitType(b.type) ? b : null;
+    const other = unitSide === a ? b : a;
+    if (unitSide !== null && other.type.kind === "union") {
+      const tag = lowerer.armTag(other.type.unionId, unitSide.type);
+      const unit = unitSide.type.kind === "nullT" ? "null" : "undefined";
+      const unequal: IrExpr = { kind: "dynFrom", value: boolLit(false, loc), type: DYN, loc };
+      const tested: IrExpr =
+        tag < 0
+          ? {
+              kind: "seqExpr",
+              stmts: [{ kind: "exprStmt", expr: other, loc }],
+              result: unequal,
+              type: DYN,
+              loc,
+            }
+          : {
+              kind: "ternary",
+              cond: {
+                kind: "unionIsTag",
+                unionId: other.type.unionId,
+                tag,
+                negated: false,
+                value: other,
+                type: BOOL,
+                loc,
+              },
+              then: {
+                kind: "dynFrom",
+                value: { kind: "unitLit", unit, type: unitSide.type, loc },
+                type: DYN,
+                loc,
+              },
+              else_: unequal,
+              type: DYN,
+              loc,
+            };
+      const unitDyn: IrExpr = { kind: "dynFrom", value: unitSide, type: DYN, loc: unitSide.loc };
+      return {
+        kind: "libCall",
+        fn: "assert.eqDyn",
+        args: [
+          unitSide === a ? unitDyn : tested,
+          unitSide === a ? tested : unitDyn,
+          boolLit(true, loc),
+          boolLit(false, loc),
+          msg,
+          hasMsg,
+        ],
+        type: VOID,
+        loc,
+      };
+    }
+  }
   if (!deep) {
     const hint =
       a.type.kind === "union" || b.type.kind === "union"
diff --git a/packages/compiler/src/frontend/lowering/lower-modules.ts b/packages/compiler/src/frontend/lowering/lower-modules.ts
index a5fada25e..3afa145b3 100644
--- a/packages/compiler/src/frontend/lowering/lower-modules.ts
+++ b/packages/compiler/src/frontend/lowering/lower-modules.ts
@@ -137,6 +137,7 @@ import { funcTypeFromParamShapes, registerOverloadedCallableAlias } from "./call
 import {
   jsBindingHasOpenWrites,
   hasJsTypeAnnotation,
+  inferredEvolvingLetType,
   isVarDeclared,
   numericIteratorSourceOf,
   provenanceElidedConstDecl,
@@ -2465,10 +2466,16 @@ export function collectGlobals(
           ) {
             const symbol = lowerer.checker.getSymbolAtLocation(nameNode);
             if (symbol && !lowerer.globalsBySymbol.has(symbol)) {
+              // An initializer-less `let` whose writes all agree keeps
+              // their type, as a function's local does (lowerVarDecl).
+              const evolved =
+                isLet && !isVarDeclared(decl) && decl.initializer === undefined
+                  ? inferredEvolvingLetType(lowerer, decl)
+                  : null;
               const g: IrGlobal = {
                 id: `%g.${tag}${nsPrefix}${nameNode.text}`,
                 name: nameNode.text,
-                type: DYN,
+                type: evolved ?? DYN,
                 mutable: isLet,
                 source: bindingSource(nameNode),
               };
@@ -2477,7 +2484,7 @@ export function collectGlobals(
               // Mutable dyn globals hold the dyn undefined from module
               // entry (the main path's rule below): a closure called
               // above the declaration reads undefined instead of
-              // faulting on NULL.
+              // faulting on NULL. An undefined-armed union does the same.
               if (g.mutable) noteVarGlobalEntryInit(lowerer, sf, g);
             }
             continue;
diff --git a/packages/compiler/src/frontend/lowering/lower-stmts.ts b/packages/compiler/src/frontend/lowering/lower-stmts.ts
index 3aeda4db1..9a2e99e9c 100644
--- a/packages/compiler/src/frontend/lowering/lower-stmts.ts
+++ b/packages/compiler/src/frontend/lowering/lower-stmts.ts
@@ -100,6 +100,7 @@ import {
   BYTES_U8,
   CAUGHT,
   DATE_T,
+  DYN_HANDLE_KINDS,
   DYN,
   F64,
   type IrExpr,
@@ -1767,10 +1768,12 @@ const THROWN_AS_DYN: ReadonlySet<IrType["kind"]> = new Set([
 
 /** A `return` at the top level of a CommonJS module (preflight admits it
  * there only): it ends the module's evaluation, and its value is
- * evaluated and discarded, as Node's module wrapper does. Top-level
- * bindings and export assignments after it would be left uninitialized
- * where Node leaves them in their temporal dead zone or absent, so a
- * module with any keeps a refusal. Null for a return inside a function. */
+ * evaluated and discarded, as Node's module wrapper does. Bindings
+ * declared after it stay uninitialized when it is taken, which only code
+ * that exists by then can observe: a function created before the return,
+ * or a hoisted function declaration. A module whose such functions read a
+ * later binding, or that assigns exports after the return, keeps a
+ * refusal. Null for a return inside a function. */
 function lowerModuleLevelReturn(lowerer: Lowerer, stmt: ts.ReturnStatement): IrStmt | null {
   let top: ts.Node = stmt;
   while (top.parent !== undefined && !ts.isSourceFile(top.parent)) {
@@ -1780,19 +1783,34 @@ function lowerModuleLevelReturn(lowerer: Lowerer, stmt: ts.ReturnStatement): IrS
   const sf = top.parent;
   if (sf === undefined || !ts.isSourceFile(sf)) return null;
   const stmts = sf.statements.nodes;
-  const later = stmts.slice(stmts.indexOf(top as ts.Statement) + 1);
-  if (
-    later.some(
-      (s) =>
-        ts.isVariableStatement(s) || ts.isClassDeclaration(s) || cjsExportAssignmentOf(s) !== null,
-    )
-  ) {
+  const index = stmts.indexOf(top as ts.Statement);
+  const later = stmts.slice(index + 1);
+  if (later.some((s) => cjsExportAssignmentOf(s) !== null)) {
     lowerer.unsupported(
       "SC1090",
       stmt,
-      "a top-level return in a CommonJS module that declares variables or classes, or assigns exports, after it",
+      "a top-level return in a CommonJS module that assigns exports after it",
     );
   }
+  const laterBindings = laterModuleBindings(lowerer, later);
+  if (laterBindings.size > 0) {
+    // Functions that exist when the return is taken: every function in the
+    // statements up to the return, and the hoisted declarations after it.
+    const reachable = [
+      ...stmts.slice(0, index + 1),
+      ...later.filter((s) => ts.isFunctionDeclaration(s)),
+    ];
+    const read = reachable
+      .map((s) => functionReadOf(lowerer, s, laterBindings, ts.isFunctionDeclaration(s)))
+      .find((r) => r !== null);
+    if (read) {
+      lowerer.unsupported(
+        "SC1090",
+        read,
+        `reading '${read.text}', declared after a top-level CommonJS return, from a function that can run once the return is taken`,
+      );
+    }
+  }
   const loc = locOf(stmt);
   const ret: IrStmt = { kind: "return", value: null, loc };
   if (!stmt.expression) return ret;
@@ -1803,6 +1821,57 @@ function lowerModuleLevelReturn(lowerer: Lowerer, stmt: ts.ReturnStatement): IrS
   };
 }
 
+/** The module-scope bindings `stmts` declare: variables and classes, with
+ * `var` declarations in nested blocks. Bindings inside functions are not
+ * module-scope and are skipped. */
+function laterModuleBindings(lowerer: Lowerer, stmts: readonly ts.Statement[]): Set<ts.Symbol> {
+  const bindings = new Set<ts.Symbol>();
+  const addName = (name: ts.BindingName): void => {
+    if (ts.isIdentifier(name)) {
+      const symbol = lowerer.checker.getSymbolAtLocation(name);
+      if (symbol) bindings.add(symbol);
+      return;
+    }
+    for (const element of name.elements.nodes) {
+      if (ts.isBindingElement(element) && element.name) addName(element.name);
+    }
+  };
+  const visit = (node: ts.Node): void => {
+    if (ts.isFunctionLike(node) || ts.isClassStaticBlockDeclaration(node)) return;
+    if (ts.isVariableDeclaration(node)) addName(node.name);
+    if (ts.isClassDeclaration(node) && node.name) addName(node.name);
+    ts.forEachChild(node, visit);
+  };
+  for (const stmt of stmts) visit(stmt);
+  return bindings;
+}
+
+/** The first identifier inside a function or class in `node` (anywhere in
+ * `node` when `inFunction`) that reads one of `bindings`, or null. */
+function functionReadOf(
+  lowerer: Lowerer,
+  node: ts.Node,
+  bindings: ReadonlySet<ts.Symbol>,
+  inFunction: boolean,
+): ts.Identifier | null {
+  if (inFunction && ts.isIdentifier(node)) {
+    const symbol = lowerer.resolveValueSymbol(node);
+    return symbol && bindings.has(symbol) ? node : null;
+  }
+  // Class bodies count as functions: field initializers run per instance.
+  const nested =
+    inFunction ||
+    ts.isFunctionLike(node) ||
+    ts.isClassDeclaration(node) ||
+    ts.isClassExpression(node);
+  return (
+    ts.forEachChild(
+      node,
+      (child) => functionReadOf(lowerer, child, bindings, nested) ?? undefined,
+    ) ?? null
+  );
+}
+
 export function lowerStmt(lowerer: Lowerer, stmt: ts.Statement): IrStmt | IrStmt[] | null {
   if (ownPrototypeResetStatement(lowerer, stmt))
     return { kind: "block", body: [], loc: locOf(stmt) };
@@ -5924,15 +5993,24 @@ function evolvingArrayHolderType(lowerer: Lowerer, decl: ts.VariableDeclaration)
 /** A single static type for an initializer-less JavaScript `let` whose
  * later plain writes all agree. An evolving JS local reads as `undefined`
  * until control flow proves a write, so a scalar slot is sound only when
- * every ordinary use already has that concrete flow type. Any closure
- * capture stays checked-dynamic: it can observe the local before an outer
- * assignment, and the checker deliberately leaves captured evolving lets
- * broad. */
-function inferredEvolvingLetType(lowerer: Lowerer, decl: ts.VariableDeclaration): IrType | null {
+ * every ordinary use already has that concrete flow type. A write's call
+ * counts when its signature is declared in the standard library, whose
+ * return types are declared rather than inferred from JavaScript. A
+ * closure capture can observe the local before an outer assignment, and
+ * the checker leaves captured evolving lets broad: a captured runtime
+ * handle (`let child; if (...) child = spawn(...)`) takes the handle or
+ * `undefined`, which nested functions read as a checked-dynamic value
+ * (closureDynLets), and other captured bindings stay checked-dynamic. */
+export function inferredEvolvingLetType(
+  lowerer: Lowerer,
+  decl: ts.VariableDeclaration,
+): IrType | null {
   if (!ts.isIdentifier(decl.name)) return null;
   const symbol = lowerer.checker.getSymbolAtLocation(decl.name);
   const scope = decl.parent?.parent?.parent;
   if (symbol === undefined || (!ts.isBlock(scope) && !ts.isSourceFile(scope))) return null;
+  // Another module reads an exported binding, which this scan does not see.
+  if (ts.getCombinedModifierFlags(decl) & ts.ModifierFlags.Export) return null;
 
   // A shorthand property's name (`{ width }`) resolves to the property
   // symbol, but it reads the binding: the checker's shorthand-value symbol.
@@ -5967,9 +6045,19 @@ function inferredEvolvingLetType(lowerer: Lowerer, decl: ts.VariableDeclaration)
   const writes: IrType[] = [];
   const reads: ts.Identifier[] = [];
   let invalid = false;
-  ts.walkPreorder(scope, (node) => {
-    if (node !== scope && ts.isFunctionLike(node)) {
-      if (captures(node)) invalid = true;
+  let captured = false;
+  // The capturing closure being scanned: reads inside one carry the
+  // checker's broad type, so only its writes are collected.
+  let closure: ts.Node | null = null;
+  const visit = (node: ts.Node): void | "skip" | "stop" => {
+    if (node !== scope && node !== closure && ts.isFunctionLike(node)) {
+      if (captures(node)) {
+        captured = true;
+        const outer = closure;
+        closure = node;
+        ts.walkPreorder(node, visit);
+        closure = outer;
+      }
       return "skip";
     }
     if (ts.isBinaryExpression(node)) {
@@ -5980,11 +6068,7 @@ function inferredEvolvingLetType(lowerer: Lowerer, decl: ts.VariableDeclaration)
           invalid = true;
           return "stop";
         }
-        if (
-          ts.isCallExpression(node.right) &&
-          (implicitMethodCallInfersReturn(lowerer, node.right) ||
-            isJsSourceFile(ts.getSourceFileOfNode(node.right)))
-        ) {
+        if (ts.isCallExpression(node.right) && !stdlibDeclaredCall(lowerer, node.right)) {
           invalid = true;
           return "stop";
         }
@@ -6013,6 +6097,7 @@ function inferredEvolvingLetType(lowerer: Lowerer, decl: ts.VariableDeclaration)
       return "stop";
     }
     if (
+      closure === null &&
       ts.isIdentifier(node) &&
       node !== decl.name &&
       refersToBinding(node) &&
@@ -6021,16 +6106,32 @@ function inferredEvolvingLetType(lowerer: Lowerer, decl: ts.VariableDeclaration)
       reads.push(node);
     }
     return undefined;
-  });
+  };
+  ts.walkPreorder(scope, visit);
   const first = writes[0];
   if (invalid || first === undefined || !writes.every((write) => typeEquals(write, first)))
     return null;
-  return reads.every((read) => {
-    const mapped = lowerer.mapTypeOf(lowerer.typeOf(read));
-    return mapped !== null && typeEquals(mapped, first);
-  })
-    ? first
-    : null;
+  if (
+    !reads.every((read) => {
+      const mapped = lowerer.mapTypeOf(lowerer.typeOf(read));
+      return mapped !== null && typeEquals(mapped, first);
+    })
+  )
+    return null;
+  if (!captured) return first;
+  if (!DYN_HANDLE_KINDS.has(first.kind)) return null;
+  lowerer.closureDynLets.add(symbol);
+  return { kind: "union", unionId: lowerer.unions.intern([first, UNDEFINED_T]) };
+}
+
+/** Whether a call's resolved signature is declared in the standard library
+ * (`spawn(...)`), so its declared return type is the value's type. Calls of
+ * JavaScript functions, and implicit generic methods, infer theirs, and a
+ * `require(...)` result is a module rather than a value of its type. */
+function stdlibDeclaredCall(lowerer: Lowerer, call: ts.CallExpression): boolean {
+  if (implicitMethodCallInfersReturn(lowerer, call) || requireSpecOf(call) !== null) return false;
+  const declaration = lowerer.checker.getResolvedSignature(call)?.declaration;
+  return declaration !== undefined && lowerer.isStdlibFile(ts.getSourceFileOfNode(declaration));
 }
 
 export function lowerVarDecl(
diff --git a/packages/compiler/src/frontend/lowering/lowerer.ts b/packages/compiler/src/frontend/lowering/lowerer.ts
index 0fb5f9d8d..52d822fcc 100644
--- a/packages/compiler/src/frontend/lowering/lowerer.ts
+++ b/packages/compiler/src/frontend/lowering/lowerer.ts
@@ -4005,6 +4005,10 @@ export class Lowerer {
    * function called above the declaration statement reads that, never a
    * NULL slot. Filled by collectGlobals. */
   readonly varGlobalEntryInits = new Map<ts.SourceFile, IrGlobal[]>();
+  /** Initializer-less JavaScript lets holding a runtime handle or undefined
+   * that a closure captures (inferredEvolvingLetType). Reads inside a nested
+   * function lower as the checked-dynamic value. */
+  readonly closureDynLets = new Set<ts.Symbol>();
   /** Import-cycle bindings with an initialization flag, by global id, and
    * the top-level statements declaring them (cycle-initialization.ts). */
   readonly cycleInitFlags = new Map<string, CycleInitFlag>();
@@ -4422,6 +4426,7 @@ export class Lowerer {
       checkedTable("preparedClassFactories", () => this.preparedClassFactories.size),
       checkedTable("preparingClassFactories", () => this.preparingClassFactories.size),
       checkedTable("varGlobalEntryInits", () => this.varGlobalEntryInits.size),
+      checkedTable("closureDynLets", () => this.closureDynLets.size),
       checkedTable("cycleInitFlags", () => this.cycleInitFlags.size),
       checkedTable("classInitFlags", () => this.classInitFlags.size),
       checkedTable("cycleInitDeclarations", () => this.cycleInitDeclarations.size),
diff --git a/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/assert-strict-equal-nullable.ts.json b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/assert-strict-equal-nullable.ts.json
new file mode 100644
index 000000000..92d5e4440
--- /dev/null
+++ b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/assert-strict-equal-nullable.ts.json
@@ -0,0 +1,6 @@
+{
+ "order": [
+  "<repo>/tests/corpus/assert-strict-equal-nullable.ts"
+ ],
+ "diags": []
+}
diff --git a/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/cjs-top-level-return-later-bindings.cjs.json b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/cjs-top-level-return-later-bindings.cjs.json
new file mode 100644
index 000000000..938628e19
--- /dev/null
+++ b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/cjs-top-level-return-later-bindings.cjs.json
@@ -0,0 +1,6 @@
+{
+ "order": [
+  "<repo>/tests/corpus/cjs-top-level-return-later-bindings.cjs"
+ ],
+ "diags": []
+}
diff --git a/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/evolving-let-child-process.cjs.json b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/evolving-let-child-process.cjs.json
new file mode 100644
index 000000000..9ab5efe31
--- /dev/null
+++ b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/evolving-let-child-process.cjs.json
@@ -0,0 +1,6 @@
+{
+ "order": [
+  "<repo>/tests/corpus/evolving-let-child-process.cjs"
+ ],
+ "diags": []
+}
diff --git a/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/evolving-let-socket-optional-reads.cjs.json b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/evolving-let-socket-optional-reads.cjs.json
new file mode 100644
index 000000000..b5701c7db
--- /dev/null
+++ b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/evolving-let-socket-optional-reads.cjs.json
@@ -0,0 +1,6 @@
+{
+ "order": [
+  "<repo>/tests/corpus/evolving-let-socket-optional-reads.cjs"
+ ],
+ "diags": []
+}
diff --git a/tests/corpus/assert-strict-equal-nullable.ts b/tests/corpus/assert-strict-equal-nullable.ts
new file mode 100644
index 000000000..0a9051c56
--- /dev/null
+++ b/tests/corpus/assert-strict-equal-nullable.ts
@@ -0,0 +1,72 @@
+// assert.strictEqual and assert.notStrictEqual compare a nullable number,
+// string or boolean with SameValue against a value of another type or
+// null/undefined, and notStrictEqual compares a nullable stream against
+// null or undefined. Failures report Node's messages.
+import assert from "node:assert";
+import { spawn, spawnSync } from "node:child_process";
+
+function find(values: number[], value: number): number | null {
+  const index = values.indexOf(value);
+  return index < 0 ? null : index;
+}
+function label(n: number): string | null {
+  return n > 0 ? "positive" : null;
+}
+function ratio(n: number): number | undefined {
+  return n === 0 ? undefined : n / n;
+}
+function flag(n: number): boolean | null {
+  return n === 0 ? null : n > 0;
+}
+
+assert.strictEqual(find([1, 2, 3], 2), 1);
+assert.strictEqual(find([1, 2, 3], 9), null);
+assert.notStrictEqual(find([1, 2, 3], 3), null);
+assert.notStrictEqual(find([1, 2, 3], 3), 1);
+assert.strictEqual(label(1), "positive");
+assert.strictEqual(label(-1), null);
+assert.notStrictEqual(label(-1), undefined);
+assert.strictEqual(ratio(0), undefined);
+assert.strictEqual(ratio(-0), undefined);
+assert.strictEqual(flag(2), true);
+assert.strictEqual(flag(0), null);
+assert.strictEqual(null, find([], 1));
+console.log("passing comparisons ok");
+
+const nan: number | null = Number("x");
+assert.strictEqual(nan, NaN);
+
+function report(run: () => void): void {
+  try {
+    run();
+    console.log("no throw");
+  } catch (e) {
+    const error = e as assert.AssertionError;
+    console.log(error.name, error.code, error.operator, error.generatedMessage);
+    console.log(JSON.stringify(error.message));
+    console.log(String(error.actual), String(error.expected));
+  }
+}
+report(() => assert.strictEqual(find([1], 1), null));
+report(() => assert.strictEqual(find([1], 2), 0));
+report(() => assert.strictEqual(label(1), null));
+report(() => assert.strictEqual(label(-1), "positive", "label mismatch"));
+report(() => assert.notStrictEqual(find([1], 2), null));
+report(() => assert.notStrictEqual(label(3), "positive"));
+report(() => assert.strictEqual(flag(0), false));
+report(() => assert.strictEqual(find([0], 0), -0));
+
+const result = spawnSync("true");
+assert.strictEqual(result.status, 0);
+assert.strictEqual(result.signal, null);
+const child = spawn("true", [], { stdio: ["ignore", "pipe", "pipe"] });
+assert.notStrictEqual(child.stdout, null);
+assert.notStrictEqual(child.stdout, undefined);
+assert.notStrictEqual(child.stdin, undefined);
+report(() => assert.notStrictEqual(child.stdin, null));
+assert.strictEqual(child.signalCode, null);
+child.on("close", (code) => {
+  assert.strictEqual(child.exitCode, 0);
+  assert.strictEqual(child.signalCode, null);
+  console.log("child closed", code);
+});
diff --git a/tests/corpus/cjs-top-level-return-later-bindings.cjs b/tests/corpus/cjs-top-level-return-later-bindings.cjs
new file mode 100644
index 000000000..3cde1f32b
--- /dev/null
+++ b/tests/corpus/cjs-top-level-return-later-bindings.cjs
@@ -0,0 +1,28 @@
+'use strict';
+// A top-level return in a CommonJS module can precede declarations that
+// only the rest of the module reads. The timer and exit handler registered
+// before the return run, a callback's own binding shadows a later one of
+// the same name, and a function declared after the return is hoisted.
+const fs = require('fs');
+
+console.log('start', describe());
+setTimeout(() => {
+  const socket = 'inner socket';
+  console.log('timer', socket);
+}, 1);
+process.on('exit', (code) => console.log('exit', code));
+for (const name of ['a', 'b']) {
+  if (name === 'b' && !fs.existsSync('/scriptc-missing-path')) return;
+  console.log('visited', name);
+}
+
+const socket = 'outer socket';
+let count = 2;
+var total = count + 1;
+class Server {}
+const [first, second] = [count, total];
+for (var i = 0; i < 2; i++) console.log('not reached', i);
+console.log('not reached', socket, first, second, new Server());
+function describe() {
+  return 'hoisted';
+}
diff --git a/tests/corpus/evolving-let-child-process.cjs b/tests/corpus/evolving-let-child-process.cjs
new file mode 100644
index 000000000..2cdc34700
--- /dev/null
+++ b/tests/corpus/evolving-let-child-process.cjs
@@ -0,0 +1,46 @@
+// An initializer-less `let` assigned a child process on every branch keeps
+// the child's type, including when callbacks read it, as Node's
+// child_process tests do when they choose a command per platform.
+const { spawn } = require('child_process');
+
+let child;
+if (process.platform === 'win32') child = spawn('cmd.exe', ['/c', 'echo', 'hi']);
+else child = spawn('echo', ['hi']);
+let text = '';
+child.stdout.on('data', (chunk) => {
+  text += chunk;
+});
+
+let producer, consumer;
+if (process.platform === 'win32') {
+  producer = spawn('cmd.exe', ['/c', 'echo', 'hello&&', 'echo', 'node']);
+  consumer = spawn('findstr', ['o']);
+} else {
+  producer = spawn('printf', ['hello\nnode\nand\nworld\n']);
+  consumer = spawn('grep', ['o']);
+}
+producer.stdout.on('data', (data) => {
+  if (!consumer.stdin.write(data)) producer.stdout.pause();
+});
+consumer.stdin.on('drain', () => producer.stdout.resume());
+producer.stdout.on('end', () => consumer.stdin.end());
+let piped = '';
+consumer.stdout.on('data', (data) => {
+  piped += data.toString('utf8', 0, data.length);
+});
+
+let quiet;
+if (process.platform === 'win32') quiet = spawn('cmd.exe', ['/c', 'echo', 'quiet']);
+else quiet = spawn('echo', ['quiet']);
+quiet.stdout.setEncoding('utf8');
+quiet.stdout.resume();
+
+let count;
+if (process.argv.length > 10) count = Math.max(1, 2);
+else count = Math.min(3, 4);
+console.log(count.toFixed(1));
+
+process.on('exit', () => {
+  console.log(JSON.stringify(text), JSON.stringify(piped));
+  console.log(child.exitCode, producer.exitCode, consumer.exitCode, consumer.signalCode, quiet.exitCode);
+});
diff --git a/tests/corpus/evolving-let-socket-optional-reads.cjs b/tests/corpus/evolving-let-socket-optional-reads.cjs
new file mode 100644
index 000000000..959191850
--- /dev/null
+++ b/tests/corpus/evolving-let-socket-optional-reads.cjs
@@ -0,0 +1,20 @@
+'use strict';
+// Optional reads and calls on a captured `let` socket, before and after
+// its first write: undefined short-circuits, the socket answers.
+const net = require('net');
+let c;
+const server = net.createServer((s) => s.end());
+function report(label) {
+  console.log(label, c?.remoteAddress, c?.address()?.family, c?.remotePort > 0);
+}
+report('before');
+const early = () => console.log('arrow before', c?.remoteAddress ?? 'none');
+early();
+server.listen(0, '127.0.0.1', () => {
+  c = net.connect(server.address().port, '127.0.0.1', () => {
+    report('after');
+    console.log('inside', c?.remoteAddress, c?.address()?.family);
+    c.end();
+    server.close();
+  });
+});
diff --git a/tests/diagnostics/assert-fences.ts b/tests/diagnostics/assert-fences.ts
index 7cf96bc5b..0a8cd9c0e 100644
--- a/tests/diagnostics/assert-fences.ts
+++ b/tests/diagnostics/assert-fences.ts
@@ -1,7 +1,7 @@
 // The assert surface's honest fences: loose deep equality over recursive
 // types (equal/notEqual, deepEqual/notDeepEqual and assert/strict's aliases lower),
-// reference-equality strictEqual on objects,
-// union operands (narrow first), mismatched or uncomparable deep types
+// reference-equality strictEqual on objects, union operands with object
+// arms (narrow first), mismatched or uncomparable deep types
 // (typed arrays, class instances), namespace-object calls, unsupported
 // throws/rejects expectations (properties outside the static error
 // surface, empty shapes, non-literal objects, doesNotReject shapes —
@@ -19,9 +19,9 @@ assert.notDeepEqual([1], [2]);
 const box = { n: 1 };
 assert.strictEqual(box, box); // reference equality — deepStrictEqual hint
 
-let maybe: string | undefined = "x";
+let maybe: { n: number } | undefined = box;
 if ("x".length === 0) maybe = undefined;
-assert.strictEqual(maybe, "x"); // union operand — narrow first
+assert.strictEqual(maybe, box); // union operand with an object arm — narrow first
 
 const unk: unknown = JSON.parse("{}");
 assert.strictEqual(unk, { n: 1 }); // strict + composite static side: reference identity does not survive the unknown boundary
diff --git a/tests/harness/__snapshots__/assert-fences.ts.txt b/tests/harness/__snapshots__/assert-fences.ts.txt
index 041c20e9b..8ee90b796 100644
--- a/tests/harness/__snapshots__/assert-fences.ts.txt
+++ b/tests/harness/__snapshots__/assert-fences.ts.txt
@@ -7,10 +7,10 @@ assert-fences.ts:20:1 - error SC2020: 'assert.strictEqual of '{ n: number }' and
 
   hint: strictEqual on objects is reference equality — deepStrictEqual compares structure
 
-assert-fences.ts:24:1 - error SC2020: 'assert.strictEqual of 'string | undefined' and 'string' values' is part of the standard library types but has no scriptc lowering yet
+assert-fences.ts:24:1 - error SC2020: 'assert.strictEqual of '{ n: number } | undefined' and '{ n: number }' values' is part of the standard library types but has no scriptc lowering yet
 
   23 | if ("x".length === 0) maybe = undefined;
-  24 | assert.strictEqual(maybe, "x"); // union operand — narrow first
+  24 | assert.strictEqual(maybe, box); // union operand with an object arm — narrow first
      | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   25 | 
 
diff --git a/tests/harness/corpus-costs/assert-strict-equal-nullable.ts.json b/tests/harness/corpus-costs/assert-strict-equal-nullable.ts.json
new file mode 100644
index 000000000..f316dc50b
--- /dev/null
+++ b/tests/harness/corpus-costs/assert-strict-equal-nullable.ts.json
@@ -0,0 +1 @@
+{ "plain": 5208, "san": 7954 }
diff --git a/tests/harness/corpus-costs/cjs-top-level-return-later-bindings.cjs.json b/tests/harness/corpus-costs/cjs-top-level-return-later-bindings.cjs.json
new file mode 100644
index 000000000..5f011f8c6
--- /dev/null
+++ b/tests/harness/corpus-costs/cjs-top-level-return-later-bindings.cjs.json
@@ -0,0 +1 @@
+{ "plain": 3714, "san": 4365 }
diff --git a/tests/harness/corpus-costs/evolving-let-child-process.cjs.json b/tests/harness/corpus-costs/evolving-let-child-process.cjs.json
new file mode 100644
index 000000000..5196b4f3b
--- /dev/null
+++ b/tests/harness/corpus-costs/evolving-let-child-process.cjs.json
@@ -0,0 +1 @@
+{ "plain": 4340, "san": 4815 }
diff --git a/tests/harness/corpus-costs/evolving-let-socket-optional-reads.cjs.json b/tests/harness/corpus-costs/evolving-let-socket-optional-reads.cjs.json
new file mode 100644
index 000000000..1f406eb04
--- /dev/null
+++ b/tests/harness/corpus-costs/evolving-let-socket-optional-reads.cjs.json
@@ -0,0 +1 @@
+{ "plain": 5449, "san": 15501 }