diff --git a/packages/compiler/src/backend/llvm/emitter.ts b/packages/compiler/src/backend/llvm/emitter.ts
index 8ce49a03d..a15782b31 100644
--- a/packages/compiler/src/backend/llvm/emitter.ts
+++ b/packages/compiler/src/backend/llvm/emitter.ts
@@ -811,7 +811,14 @@ export class LlEmitter {
     this.ffiExtendNarrowIntegers =
       options.wasi === true || ffiExtendsNarrowIntegers(options.targetTriple);
     this.wasi = options.wasi === true;
-    this.inlineTrunc = /^(x86_64|amd64)\b/i.test(options.targetTriple ?? "");
+    // An empty triple targets the host, as for executableTls below: the
+    // Node-hosted CLI passes none, the native CLI passes its toolchain's, and
+    // both must emit the same module for the same target.
+    this.inlineTrunc =
+      !this.wasi &&
+      (options.targetTriple
+        ? /^(x86_64|amd64)\b/i.test(options.targetTriple)
+        : process.arch === "x64");
     this.executableTls =
       mod.workers === true &&
       mod.lib === undefined &&
diff --git a/packages/compiler/src/frontend/lowering/builtins/strings.ts b/packages/compiler/src/frontend/lowering/builtins/strings.ts
index 57e650feb..b117c24ff 100644
--- a/packages/compiler/src/frontend/lowering/builtins/strings.ts
+++ b/packages/compiler/src/frontend/lowering/builtins/strings.ts
@@ -3,6 +3,7 @@ import * as ts from "../../ts7/adapter.js";
 import { type Lowerer } from "../lowerer.js";
 import { locOf } from "../../program.js";
 import { lowerOptionalArgument, lowerStringSearchArgument } from "../optional-arguments.js";
+import { lowerToNumberArgument } from "../lower-exprs.js";
 import { F64, type IrExpr, STRING, arrayOf, isUnitType, typeEquals } from "../../../ir/ir.js";
 
 /** `String.fromCharCode/fromCodePoint(...codes)` on THE String global: every argument
@@ -97,7 +98,9 @@ export function lowerStringStaticCall(
     const packed = lowerer.lowerExprExpecting(spread.expression, arrayOf(F64));
     return { kind: "libCall", fn: `string.${member}`, args: [packed], type: STRING, loc };
   }
-  const elems = call.arguments.map((a) => lowerer.lowerExprExpecting(a, F64));
+  // Each code converts with ToNumber: a missing value is NaN (code unit 0
+  // for fromCharCode, a RangeError for fromCodePoint), exactly Node.
+  const elems = call.arguments.map((a) => lowerToNumberArgument(lowerer, a));
   const packed: IrExpr = { kind: "arrayLit", elems, type: arrayOf(F64), loc };
   return { kind: "libCall", fn: `string.${member}`, args: [packed], type: STRING, loc };
 }
diff --git a/packages/compiler/src/frontend/lowering/containers/bytes.ts b/packages/compiler/src/frontend/lowering/containers/bytes.ts
index 5b8285f12..255165530 100644
--- a/packages/compiler/src/frontend/lowering/containers/bytes.ts
+++ b/packages/compiler/src/frontend/lowering/containers/bytes.ts
@@ -32,6 +32,7 @@ import {
   lowerStaticallyUndefinedArgument,
 } from "../optional-arguments.js";
 import { lowerCheckedArrayFrom } from "./array-construction.js";
+import { lowerToNumberArgument } from "../lower-exprs.js";
 
 /** Uint8Array.prototype.toSorted. The receiver/comparator expressions are
  * evaluated before entering the helper; the helper snapshots with
@@ -486,7 +487,16 @@ export function lowerBytesMethodCall(
       lowerer.noLowering(`.${name} with ${nArgs} arguments on typed arrays`, call);
     }
     const receiver = lowerer.lowerExprExpecting(access.expression, receiverIr);
-    const args = call.arguments.map((a) => lowerer.lowerExprExpecting(a, F64));
+    // A missing start is 0 and a missing end is the length (+Infinity
+    // clamps to it), as for omitted arguments.
+    const args = call.arguments.map((a, i) =>
+      lowerOptionalArgument(lowerer, a, F64, {
+        kind: "numLit",
+        value: i === 0 ? 0 : Infinity,
+        type: F64,
+        loc,
+      }),
+    );
     // subarray is a VIEW (TypedArray.prototype.subarray aliases), and
     // Buffer's slice() is subarray's deprecated Node alias — resolved by
     // where the member is declared, the toString discipline below. Only
@@ -796,7 +806,8 @@ export function lowerBytesMethodCall(
       lowerer.noLowering(`.${name} with ${nArgs} arguments`, call);
     }
     const receiver = lowerer.lowerExprExpecting(access.expression, receiverIr);
-    const args = [lowerer.lowerExprExpecting(call.arguments[0]!, F64)];
+    // ToIndex(undefined) is ToIndex(NaN): a missing offset reads offset 0.
+    const args = [lowerToNumberArgument(lowerer, call.arguments[0]!)];
     if (nArgs === 2) args.push(lowerer.lowerExprExpecting(call.arguments[1]!, BOOL));
     return { kind: "bytesIntrinsic", method: dvGetter.method, receiver, args, type: F64, loc };
   }
@@ -829,9 +840,11 @@ export function lowerBytesMethodCall(
       lowerer.noLowering(`.${name} with ${nArgs} arguments`, call);
     }
     const receiver = lowerer.lowerExprExpecting(access.expression, receiverIr);
+    // The offset (ToIndex) and the value (ToNumber) treat a missing value
+    // as NaN.
     const args = [
-      lowerer.lowerExprExpecting(call.arguments[0]!, F64),
-      lowerer.lowerExprExpecting(call.arguments[1]!, F64),
+      lowerToNumberArgument(lowerer, call.arguments[0]!),
+      lowerToNumberArgument(lowerer, call.arguments[1]!),
     ];
     if (nArgs === 3) args.push(lowerer.lowerExprExpecting(call.arguments[2]!, BOOL));
     return { kind: "bytesIntrinsic", method: dvSetter.method, receiver, args, type: VOID, loc };
diff --git a/packages/compiler/src/frontend/lowering/lower-calls.ts b/packages/compiler/src/frontend/lowering/lower-calls.ts
index 6335a317e..7db085f22 100644
--- a/packages/compiler/src/frontend/lowering/lower-calls.ts
+++ b/packages/compiler/src/frontend/lowering/lower-calls.ts
@@ -107,8 +107,10 @@ import { lowerProcStreamMethodCall } from "./builtins/process.js";
 import { lowerReflectCall } from "./builtins/reflection.js";
 import {
   lowerAbsenceProbe,
+  lowerOptionalNumber,
   lowerPromiseAllTupleCall,
   lowerPromiseRejectCall,
+  lowerToNumberArgument,
   stringWrapperToString,
   symbolFieldInfo,
   templateRawTextOf,
@@ -9520,7 +9522,8 @@ export function lowerStaticNumberGlobal(
       );
     }
     if (name === "isNaN") {
-      const x = lowerer.lowerExprExpecting(args[0]!, F64);
+      // The global's ToNumber turns a missing value into NaN.
+      const x = lowerToNumberArgument(lowerer, args[0]!);
       return { kind: "libCall", fn: "num.isNaN", args: [x], type: BOOL, loc };
     }
     const radix: IrExpr = args[1]
@@ -9650,6 +9653,17 @@ export function lowerStaticNumberGlobal(
     if (name === "isFinite" && probed?.type.kind === "f64") {
       return { kind: "libCall", fn: "number.isFinite", args: [probed], type: BOOL, loc };
     }
+    // A number the checker trusts but storage may hold missing (an
+    // out-of-range read): the global's ToNumber makes undefined NaN.
+    if (
+      name === "isFinite" &&
+      probed?.type.kind === "union" &&
+      lowerer.armTag(probed.type.unionId, UNDEFINED_T) >= 0 &&
+      lowerer.stripUndefinedArm(probed.type).kind === "f64"
+    ) {
+      const x = lowerOptionalNumber(lowerer, probed, loc, args[0]);
+      return { kind: "libCall", fn: "number.isFinite", args: [x], type: BOOL, loc };
+    }
   }
   return null;
 }
diff --git a/packages/compiler/src/frontend/lowering/lower-exprs.ts b/packages/compiler/src/frontend/lowering/lower-exprs.ts
index 677b29e67..2bae9c82e 100644
--- a/packages/compiler/src/frontend/lowering/lower-exprs.ts
+++ b/packages/compiler/src/frontend/lowering/lower-exprs.ts
@@ -2595,6 +2595,10 @@ function lowerExprInner(lowerer: Lowerer, expr: ts.Expression): IrExpr {
         lowerer.isRuntimeOptionalField(handled.shapeId, handled.field)
       )
         return handled;
+      // A class field assigned a missing read: the checker narrows this
+      // read to the assigned value's bare type, but the stored undefined
+      // arm is real (`obj.n = bytes[i]; obj.n` is undefined in Node).
+      if (runtimeOptionalClassField(lowerer, handled, expr)) return handled;
       return lowerer.maybeNarrow(handled, expr);
     }
     // `globalThis.<name>` that no lowering above claimed
@@ -6487,6 +6491,33 @@ export function lowerOptionalNumber(
     : { kind: "seqExpr", stmts: prefix, result: value, type: F64, loc };
 }
 
+/** A number argument of a builtin that converts it with ToNumber (Math
+ * functions, String.fromCharCode, the global isNaN/isFinite, DataView
+ * offsets and values). ToNumber(undefined) is NaN, so a missing value — an
+ * out-of-range read, a local or field holding one, or a hole — converts to
+ * NaN instead of trusting the checker's number type. */
+export function lowerToNumberArgument(lowerer: Lowerer, node: ts.Expression): IrExpr {
+  let inner = node;
+  while (ts.isParenthesizedExpression(inner)) inner = inner.expression;
+  const loc = locOf(inner);
+  const isOptionalNumber = (value: IrExpr): boolean =>
+    value.type.kind === "union" &&
+    lowerer.armTag(value.type.unionId, UNDEFINED_T) >= 0 &&
+    lowerer.stripUndefinedArm(value.type).kind === "f64";
+  const optional = ts.isIdentifier(inner)
+    ? runtimeOptionalStorageOperand(lowerer, inner)
+    : ts.isElementAccessExpression(inner)
+      ? lowerAbsenceProbe(lowerer, inner)
+      : null;
+  if (optional && isOptionalNumber(optional))
+    return lowerOptionalNumber(lowerer, optional, loc, inner);
+  // A conditional builds each branch for the destination.
+  if (ts.isConditionalExpression(inner)) return lowerer.lowerExprExpecting(node, F64);
+  const value = lowerer.lowerExpr(node);
+  if (isOptionalNumber(value)) return lowerOptionalNumber(lowerer, value, loc, inner);
+  return lowerer.coerceInto(node, value, F64);
+}
+
 /** `a[i]` reads. Only f64 indices into array receivers are modeled; JS
  * string-key element access (`a["length"]`) and string indexing (`s[0]`
  * typechecks against the lib's index signature; use .charAt) stay out. */
@@ -7595,6 +7626,24 @@ export function runtimeOptionalStorageOperand(
   return optional.value;
 }
 
+/** A `T | undefined` class field read that the checker narrowed to `T`
+ * where the field was assigned a value that may be missing (an unchecked
+ * indexed read). TypeScript's assignment narrowing is not a runtime fact
+ * there, so the read keeps the stored union instead of extracting `T`. */
+function runtimeOptionalClassField(
+  lowerer: Lowerer,
+  value: IrExpr,
+  node: ts.PropertyAccessExpression,
+): boolean {
+  if (value.kind !== "fieldGet" || value.type.kind !== "union") return false;
+  if (lowerer.armTag(value.type.unionId, UNDEFINED_T) < 0) return false;
+  if ((lowerer.unions.get(value.type.unionId)?.arms.length ?? 0) !== 2) return false;
+  const symbol = lowerer.checker.getSymbolAtLocation(node.name);
+  if (!symbol || !lowerer.runtimeOptionalAssignedSymbols.has(symbol)) return false;
+  const narrowed = lowerer.mapTypeOf(lowerer.typeOf(node));
+  return narrowed !== null && typeEquals(narrowed, lowerer.stripUndefinedArm(value.type));
+}
+
 /** A presence-test operand: an absence-aware element or field read, an
  * identifier's runtime-optional storage, or the ordinary lowering. */
 export function lowerPresenceOperand(lowerer: Lowerer, node: ts.Expression): IrExpr {
@@ -14380,6 +14429,15 @@ function classFieldTarget(
   const lowerObjectReceiver = (): IrExpr => {
     let obj = represented;
     if (obj.type.kind === "union") {
+      // A receiver that may hold a missing value (a field assigned an
+      // unchecked read): Node's member-read TypeError for the unit arm.
+      const present = lowerer.runtimeOptionalPropertyReceiver(
+        receiverNode,
+        obj,
+        receiverType,
+        fieldName,
+      );
+      if (present) return present;
       const helper = lowerer.narrowedArmHelper(obj.type.unionId, receiverType, locOf(receiverNode));
       obj = helper
         ? lowerer.checkedNarrowCall(helper, obj, receiverType, locOf(receiverNode))
diff --git a/packages/compiler/src/frontend/lowering/lower-island.ts b/packages/compiler/src/frontend/lowering/lower-island.ts
index 5f9301c76..88c2d34df 100644
--- a/packages/compiler/src/frontend/lowering/lower-island.ts
+++ b/packages/compiler/src/frontend/lowering/lower-island.ts
@@ -5,11 +5,7 @@ import { InternalCompilerError } from "../../errors.js";
  * surface (Math and number/string methods under --dynamic), and the npm
  * package boundary fences for node_modules-declared symbols. */
 import * as ts from "../ts7/adapter.js";
-import {
-  lowerAbsenceProbe,
-  lowerOptionalNumber,
-  runtimeOptionalStorageOperand,
-} from "./lower-exprs.js";
+import { lowerToNumberArgument } from "./lower-exprs.js";
 import type { Lowerer } from "./lowerer.js";
 import {
   arrayOf,
@@ -3401,23 +3397,10 @@ export function lowerIslandMethodCall(
   // Static numeric Math calls precede the island path. The scalar methods
   // use their declared arity; min/max and hypot accept their variadic forms.
   const staticMath = isMath ? own(STATIC_MATH_FNS, name) : undefined;
-  // A missing element (a hole bound by a loop, a local holding a missing
-  // read, or the read itself) converts to NaN, ToNumber's answer for
-  // undefined, instead of trusting the checker's number type.
-  const mathArg = (a: ts.Expression): IrExpr => {
-    let inner = a;
-    while (ts.isParenthesizedExpression(inner)) inner = inner.expression;
-    const optional = ts.isIdentifier(inner)
-      ? runtimeOptionalStorageOperand(lowerer, inner)
-      : ts.isElementAccessExpression(inner)
-        ? lowerAbsenceProbe(lowerer, inner)
-        : null;
-    return optional &&
-      optional.type.kind === "union" &&
-      lowerer.stripUndefinedArm(optional.type).kind === "f64"
-      ? lowerOptionalNumber(lowerer, optional, loc, inner)
-      : lowerer.lowerExprExpecting(a, F64);
-  };
+  // A missing element (a hole bound by a loop, a local or field holding a
+  // missing read, or the read itself) converts to NaN, ToNumber's answer
+  // for undefined, instead of trusting the checker's number type.
+  const mathArg = (a: ts.Expression): IrExpr => lowerToNumberArgument(lowerer, a);
   if (staticMath && name === "hypot") {
     const elems = call.arguments.map((a) =>
       ts.isSpreadElement(a) ? lowerer.lowerExprExpecting(a.expression, arrayOf(F64)) : mathArg(a),
diff --git a/packages/compiler/src/frontend/lowering/lower-stmts.ts b/packages/compiler/src/frontend/lowering/lower-stmts.ts
index ab34a1eb5..266a79d81 100644
--- a/packages/compiler/src/frontend/lowering/lower-stmts.ts
+++ b/packages/compiler/src/frontend/lowering/lower-stmts.ts
@@ -4901,6 +4901,18 @@ export function lowerVarDecl(
     init.type.kind === "func"
   )
     type = init.type;
+  // A TypeScript lambda whose closure returns the runtime result (a missing
+  // read, a `??` default of another type) instead of the checker's bare
+  // return type: the binding keeps the checker's parameters and takes the
+  // closure's result.
+  if (
+    !decl.type &&
+    type?.kind === "func" &&
+    init.type.kind === "func" &&
+    (ts.isArrowFunction(decl.initializer) || ts.isFunctionExpression(decl.initializer)) &&
+    lowerer.runtimeOptionalFunctionReturns.has(decl.initializer)
+  )
+    type = { ...type, ret: init.type.ret };
   if (
     lowerer.implicitParamTypes !== null &&
     init.type.kind === "dyn" &&
diff --git a/packages/compiler/src/frontend/lowering/lowerer.ts b/packages/compiler/src/frontend/lowering/lowerer.ts
index 9cea42ed9..184ffc8a4 100644
--- a/packages/compiler/src/frontend/lowering/lowerer.ts
+++ b/packages/compiler/src/frontend/lowering/lowerer.ts
@@ -4223,6 +4223,23 @@ export class Lowerer {
       if (!symbol) return false;
       return noteFieldSymbol(symbol, name);
     };
+    /** An instance field of a TypeScript class whose declared storage is
+     * exactly `T | undefined`: an assignment from a missing read stores the
+     * undefined arm while the checker narrows later reads to `T`. */
+    const optionalInstanceField = (symbol: ts.Symbol): boolean => {
+      const decl = this.checker.valueDeclarationOf(symbol);
+      if (!decl || !ts.isPropertyDeclaration(decl) || !ts.isIdentifier(decl.name)) return false;
+      if (ts.getCombinedModifierFlags(decl) & ts.ModifierFlags.Static) return false;
+      const sf = decl.getSourceFile();
+      if (isJsSourceFile(sf) || sf.isDeclarationFile) return false;
+      const declared = typeOrNull(decl.name);
+      const mapped = declared ? this.mapTypeOf(declared) : null;
+      if (mapped?.kind !== "union" || this.armTag(mapped.unionId, UNDEFINED_T) < 0) return false;
+      const arms = this.unions.get(mapped.unionId)?.arms ?? [];
+      return (
+        arms.length === 2 && arms.every((arm) => arm.kind === "undefinedT" || !isUnitType(arm))
+      );
+    };
     const noteFieldSymbol = (symbol: ts.Symbol, name: string): boolean => {
       const set = optionalFields.get(symbol) ?? new Set<string>();
       const before = set.size;
@@ -4353,12 +4370,29 @@ export class Lowerer {
       return found;
     };
     const jsLambdas: ts.FunctionLikeDeclaration[] = [];
+    // TypeScript lambdas keep the checker's inferred return type, which
+    // spells a missing read as its bare element type and `read ?? "text"`
+    // as a number. Their closures return the runtime result instead.
+    const tsLambdas: ts.FunctionLikeDeclaration[] = [];
     for (const sf of sourceFiles) {
-      if (!isJsSourceFile(sf)) continue;
+      const js = isJsSourceFile(sf);
+      if (!js && sf.isDeclarationFile) continue;
       ts.walkPreorder(sf, (node) => {
-        if (ts.isArrowFunction(node) || ts.isFunctionExpression(node)) jsLambdas.push(node);
+        if (ts.isArrowFunction(node) || ts.isFunctionExpression(node))
+          (js ? jsLambdas : tsLambdas).push(node);
       });
     }
+    /** The primitive result union of a lambda's returns (see
+     * optionalPrimitiveResultType), as declared functions record it. */
+    const lambdaArithmeticReturn = (fn: ts.FunctionLikeDeclaration): IrType | null => {
+      if (fn.body && !ts.isBlock(fn.body))
+        return optionalPrimitiveResultType(fn.body as ts.Expression);
+      for (const expression of returnsOf(fn)) {
+        const result = optionalPrimitiveResultType(expression);
+        if (result) return result;
+      }
+      return null;
+    };
     for (const [symbol, signature] of signatureBySymbol) {
       const declaration = functionDeclBySymbol.get(symbol);
       if (!declaration || !isJsSourceFile(declaration.getSourceFile())) continue;
@@ -5049,6 +5083,13 @@ export class Lowerer {
               optionalSymbols.add(field);
               provenance?.note("static-field", node.left, node.right);
               changed = true;
+            } else if (field && !optionalSymbols.has(field) && optionalInstanceField(field)) {
+              // The checker narrows the field to the assigned read's bare
+              // type, but the stored value can be undefined: reads keep the
+              // stored union (runtimeOptionalClassField).
+              optionalSymbols.add(field);
+              provenance?.note("assigned-field", node.left, node.right);
+              changed = true;
             } else if (ts.isIdentifier(node.left.expression)) {
               const symbol = symbolOf(node.left.expression);
               if (symbol && noteFieldSymbol(symbol, node.left.name.text)) {
@@ -5383,6 +5424,54 @@ export class Lowerer {
       for (const sf of sourceFiles) if (scanFile(sf)) changed = true;
       for (const [symbol, decl] of functionDeclBySymbol)
         if (scanReturns(symbol, decl)) changed = true;
+      for (const fn of tsLambdas) {
+        // An annotated return is the author's contract, as for parameters.
+        if (fn.type) continue;
+        const mapped = this.mapTypeOf(this.typeOf(fn));
+        if (mapped?.kind !== "func") continue;
+        const previous = this.runtimeOptionalFunctionReturnType(fn, mapped.ret);
+        // Async, generator and void bodies keep their own result shapes.
+        if (
+          previous.kind === "promise" ||
+          previous.kind === "generator" ||
+          previous.kind === "void"
+        )
+          continue;
+        const arithmetic = lambdaArithmeticReturn(fn);
+        const next =
+          arithmetic &&
+          (previous.kind === "f64" || previous.kind === "string" || previous.kind === "bool")
+            ? arithmetic
+            : bodyReturnsOptional(fn)
+              ? this.runtimeOptionalType(previous)
+              : previous;
+        if (!typeEquals(previous, next)) {
+          this.runtimeOptionalFunctionReturns.set(fn, next);
+          provenance?.note("lambda-return", fn, fn.body);
+          changed = true;
+        }
+        // Calls through the lambda's own binding (`const get = () => xs[i];
+        // get()`) produce the same result as calls of a declared function.
+        const binding = fn.parent;
+        if (
+          next !== previous &&
+          ts.isVariableDeclaration(binding) &&
+          binding.initializer === fn &&
+          !binding.type &&
+          ts.isIdentifier(binding.name)
+        ) {
+          const symbol = symbolOf(binding.name);
+          if (symbol && next.kind === "union" && this.armTag(next.unionId, UNDEFINED_T) >= 0) {
+            if (!optionalReturns.has(symbol)) {
+              optionalReturns.add(symbol);
+              changed = true;
+            }
+          } else if (symbol && !arithmeticReturns.has(symbol)) {
+            arithmeticReturns.set(symbol, next);
+            changed = true;
+          }
+        }
+      }
       for (const fn of jsLambdas) {
         if (!bodyReturnsOptional(fn)) continue;
         const mapped = this.mapTypeOf(this.typeOf(fn));
@@ -5493,11 +5582,11 @@ export class Lowerer {
         const symbol = symbolOf(node.name);
         if (!symbol) return;
         if (
-          isJsSourceFile(sf) &&
           !node.type &&
           !hasJsTypeAnnotation(node) &&
           node.initializer &&
-          (ts.isArrowFunction(node.initializer) || ts.isFunctionExpression(node.initializer))
+          (ts.isArrowFunction(node.initializer) || ts.isFunctionExpression(node.initializer)) &&
+          (isJsSourceFile(sf) || this.runtimeOptionalFunctionReturns.has(node.initializer))
         ) {
           const global = this.globalsBySymbol.get(symbol);
           if (global?.type.kind === "func")
@@ -9591,14 +9680,32 @@ export class Lowerer {
   /* ── scoping and captures ─────────────────────────────────────────── */
 
   declareLocal(nameNode: ts.Node, name: string, type: IrType, mutable: boolean): IrLocal {
-    return declareContextLocal(
+    const symbol = this.checker.getSymbolAtLocation(nameNode);
+    const local = declareContextLocal(
       this.ctx,
       name,
       type,
       mutable,
-      this.checker.getSymbolAtLocation(nameNode),
+      symbol,
       bindingSource(nameNode),
     );
+    // A binding declared `T | undefined` that receives a missing read: the
+    // checker narrows it to `T` after the assignment, but the stored value
+    // can be undefined. Track it like widened storage so bare reads keep
+    // the union until a guard proves presence.
+    if (
+      symbol &&
+      this.runtimeOptionalAssignedSymbols.has(symbol) &&
+      type.kind === "union" &&
+      this.armTag(type.unionId, UNDEFINED_T) >= 0 &&
+      this.unions.get(type.unionId)?.arms.length === 2 &&
+      !isUnitType(this.stripUndefinedArm(type))
+    ) {
+      const root = this.runtimeOptionalRootOf(local);
+      this.runtimeOptionalStorageLocals.add(root);
+      this.runtimeOptionalLocals.add(root);
+    }
+    return local;
   }
 
   /** A never-returning call in a value position of type `expected`: the
diff --git a/packages/compiler/test/byte-number-emission.test.ts b/packages/compiler/test/byte-number-emission.test.ts
index 18c41814e..fc36fd23d 100644
--- a/packages/compiler/test/byte-number-emission.test.ts
+++ b/packages/compiler/test/byte-number-emission.test.ts
@@ -45,7 +45,8 @@ test("numeric byte pipelines retain checked fallbacks and use field widths on bo
       expect(wide).toContain("phi i64");
       expect(wide).toContain("@llvm.bswap.i64");
       const offsets = body(ll, "offsets");
-      expect(offsets).toContain("@llvm.trunc.f64");
+      // Truncation is the intrinsic, or inline on an x86-64 host (trunc.ts).
+      expect(offsets).toMatch(/@llvm\.trunc\.f64|@llvm\.copysign\.f64/);
       expect(offsets).toContain("@scr_dataview_set");
       expect(offsets).toContain("@scr_dataview_get");
       expect(offsets).toMatch(/icmp ule i(?:32|64)/);
diff --git a/packages/compiler/test/ts7/baselines/order-parity.json b/packages/compiler/test/ts7/baselines/order-parity.json
index b99f04249..7fdd6d5c5 100644
--- a/packages/compiler/test/ts7/baselines/order-parity.json
+++ b/packages/compiler/test/ts7/baselines/order-parity.json
@@ -10493,6 +10493,24 @@
    ],
    "diags": []
   },
+  "<repo>/tests/corpus/missing-read-lambda-results.ts": {
+   "order": [
+    "<repo>/tests/corpus/missing-read-lambda-results.ts"
+   ],
+   "diags": []
+  },
+  "<repo>/tests/corpus/missing-read-narrowed-storage.ts": {
+   "order": [
+    "<repo>/tests/corpus/missing-read-narrowed-storage.ts"
+   ],
+   "diags": []
+  },
+  "<repo>/tests/corpus/missing-read-number-conversions.ts": {
+   "order": [
+    "<repo>/tests/corpus/missing-read-number-conversions.ts"
+   ],
+   "diags": []
+  },
   "<repo>/tests/corpus/module-fallback-functions/main.js": {
    "order": [
     "<repo>/tests/corpus/module-fallback-functions/platform.js",
diff --git a/tests/corpus/missing-read-lambda-results.ts b/tests/corpus/missing-read-lambda-results.ts
new file mode 100644
index 000000000..0d9f047d2
--- /dev/null
+++ b/tests/corpus/missing-read-lambda-results.ts
@@ -0,0 +1,74 @@
+// A missing read (an out-of-range typed-array or array read the checker
+// types as `number`) defaulted with `??` or `||` to a string, or returned
+// as is, gives a lambda a result its inferred return type does not spell:
+// TypeScript types `read ?? "text"` and `() => xs[i]` as number. The
+// closure returns the runtime value, through bindings, callbacks taking
+// `unknown`, calls of the binding, and the enclosing function's return.
+const base: number = process.argv.length;
+const chars = new Uint16Array(4);
+chars[0] = 108;
+const nums: number[] = [7];
+const far = base + 100;
+const a = chars[far];
+const c0 = chars[0];
+
+// The scanbox probe table: every arrow is typed `() => number` by
+// inference and stored in a `() => unknown` slot.
+const probes: [string, () => unknown][] = [
+  ["plus", () => a + 1],
+  ["or", () => a | 0],
+  ["max", () => Math.max(a, 1)],
+  ["fcc", () => JSON.stringify(String.fromCharCode(c0, a))],
+  ["and", () => c0 && 5],
+  ["or2", () => chars[far] || 7],
+  ["nullish", () => a ?? "dflt"],
+  ["nullish present", () => c0 ?? "dflt"],
+  ["tern", () => (a ? "yes" : "no")],
+  ["bang", () => !a],
+  ["concat", () => a + "s"],
+  ["missing", () => nums[far]],
+];
+for (const [name, f] of probes) {
+  try {
+    console.log(name, f());
+  } catch (e) {
+    console.log(name, "threw", (e as Error).message);
+  }
+}
+
+function take(f: () => unknown): unknown {
+  return f();
+}
+console.log("take nullish", take(() => a ?? "d1"));
+console.log("take missing", take(() => nums[far]));
+
+const bound = () => a ?? "d2";
+console.log("bound", bound());
+const block = () => {
+  return a ?? "d3";
+};
+console.log("block", block());
+const expression = function () {
+  return a || "d4";
+};
+console.log("expression", expression());
+const read = () => nums[far];
+console.log("read", read(), read() === undefined);
+
+function viaBinding(i: number): number {
+  const get = () => nums[i];
+  return get();
+}
+console.log("via binding", viaBinding(far), viaBinding(0));
+
+function declared() {
+  return a ?? "d5";
+}
+console.log("declared", declared());
+
+class Lexer {
+  peek(i: number) {
+    return chars[i] ?? "eof";
+  }
+}
+console.log("method", new Lexer().peek(far), new Lexer().peek(0));
diff --git a/tests/corpus/missing-read-narrowed-storage.ts b/tests/corpus/missing-read-narrowed-storage.ts
new file mode 100644
index 000000000..97be7214a
--- /dev/null
+++ b/tests/corpus/missing-read-narrowed-storage.ts
@@ -0,0 +1,88 @@
+// A `T | undefined` slot assigned a missing read (an out-of-range
+// typed-array or array read the checker types as `T`) holds undefined,
+// even though TypeScript narrows later reads to `T`. Reads keep the stored
+// value: printing shows undefined, arithmetic gives NaN, and a member read
+// throws Node's TypeError. Class fields (through `obj.f` and `this.f`) and
+// locals both follow it; a presence guard still narrows.
+const base: number = process.argv.length;
+const chars = new Uint16Array([5, 6]);
+const nums: number[] = [7];
+const far = base + 100;
+
+function twice(n: number): number {
+  return n * 2;
+}
+
+class Cursor {
+  token: number | undefined = 0;
+  peek(i: number): number {
+    return chars[i];
+  }
+  advance(i: number): string {
+    this.token = chars[i];
+    return `${this.token} ${this.token === undefined} ${this.token + 1}`;
+  }
+}
+
+function fields(i: number, label: string): void {
+  const cursor = new Cursor();
+  cursor.token = chars[i];
+  console.log(label, "direct", cursor.token, cursor.token === undefined);
+  console.log(label, "ops", cursor.token + 1, -cursor.token, cursor.token < 3, `${cursor.token}`);
+  console.log(label, "defaults", cursor.token ?? "none", cursor.token || "falsy");
+  console.log(label, "call", twice(cursor.token), [cursor.token]);
+  cursor.token = cursor.peek(i);
+  console.log(label, "method", cursor.token, typeof cursor.token);
+  cursor.token = nums[i];
+  console.log(label, "array", cursor.token, String(cursor.token));
+  console.log(label, "this", cursor.advance(i));
+  if (cursor.token !== undefined) console.log(label, "guarded", cursor.token * 2);
+  else console.log(label, "guarded absent");
+}
+fields(far, "missing");
+fields(0, "present");
+
+function locals(i: number, label: string): void {
+  let code: number | undefined = 0;
+  code = chars[i];
+  console.log(label, "local", code, code + 1, `${code}`, [code], twice(code));
+  let late: number | undefined;
+  late = nums[i];
+  console.log(label, "late", late, late === undefined, late ?? "none");
+  let branch: number | undefined = 1;
+  if (base > 0) branch = chars[i];
+  console.log(label, "branch", branch, typeof branch);
+  if (code !== undefined) console.log(label, "local guarded", code * 3);
+  const viaExpr = (code = chars[i]);
+  console.log(label, "assign expr", viaExpr, code);
+}
+locals(far, "missing");
+locals(1, "present");
+
+class Node2 {
+  v: number;
+  constructor(v: number) {
+    this.v = v;
+  }
+}
+const pool: Node2[] = [new Node2(4)];
+
+class Holder {
+  node: Node2 | undefined = undefined;
+}
+
+function objects(i: number, label: string): void {
+  const holder = new Holder();
+  holder.node = pool[i];
+  console.log(label, "object field", holder.node === undefined);
+  try {
+    console.log(label, "object member", holder.node.v);
+  } catch (e) {
+    console.log(label, "object member threw", (e as Error).name, (e as Error).message);
+  }
+  let current: Node2 | undefined = new Node2(1);
+  current = pool[i];
+  console.log(label, "object local", current === undefined);
+}
+objects(far, "missing");
+objects(0, "present");
diff --git a/tests/corpus/missing-read-number-conversions.ts b/tests/corpus/missing-read-number-conversions.ts
new file mode 100644
index 000000000..abbe50879
--- /dev/null
+++ b/tests/corpus/missing-read-number-conversions.ts
@@ -0,0 +1,95 @@
+// Builtins that convert a number argument with ToNumber see a missing
+// read (an out-of-range typed-array or array read the checker types as
+// `number`) as NaN, ToNumber's answer for undefined: String.fromCharCode
+// yields code unit 0, fromCodePoint throws a RangeError, Math answers NaN,
+// isNaN is true. The missing value reaches them from a module binding, a
+// local, a class field and a record field.
+const base: number = process.argv.length;
+const chars = new Uint16Array(4);
+chars[0] = 108;
+chars[1] = 66;
+const nums: number[] = [7];
+const far = base + 100;
+
+const missing = chars[far];
+const present = chars[1];
+const missingNum = nums[far];
+
+function show(label: string, value: unknown): void {
+  console.log(label, typeof value === "string" ? JSON.stringify(value) : value);
+}
+
+function attempt(label: string, run: () => unknown): void {
+  try {
+    show(label, run());
+  } catch (e) {
+    console.log(label, "threw", (e as Error).name, (e as Error).message);
+  }
+}
+
+// String.fromCharCode / fromCodePoint.
+show("fcc module", String.fromCharCode(chars[0], missing));
+show("fcc single", String.fromCharCode(missing));
+show("fcc present", String.fromCharCode(chars[0], present));
+show("fcc array", String.fromCharCode(missingNum, 65));
+attempt("fcp module", () => String.fromCodePoint(missing));
+show("fcp present", String.fromCodePoint(present));
+
+// Math functions.
+show("abs", Math.abs(missing));
+show("floor", Math.floor(missingNum));
+show("max", Math.max(missing, 1));
+show("min3", Math.min(1, 2, missing));
+show("imul", Math.imul(missing, 3));
+show("pow", Math.pow(present, 2));
+show("hypot", Math.hypot(missing, 3));
+
+// The global number predicates convert; the Number statics do not.
+show("isNaN", isNaN(missing));
+show("isNaN present", isNaN(present));
+show("isFinite", isFinite(missing));
+show("isFinite present", isFinite(present));
+show("Number.isNaN", Number.isNaN(missing));
+show("Number.isFinite", Number.isFinite(missing));
+
+// DataView offsets (ToIndex) and values (ToNumber).
+const view = new DataView(new ArrayBuffer(4));
+view.setUint8(1, 9);
+show("dv get", view.getUint8(missing));
+view.setUint8(missing, 5);
+show("dv set offset", view.getUint8(0));
+view.setUint8(2, missing);
+show("dv set value", view.getUint8(2));
+
+// Typed-array ranges: a missing start is 0, a missing end is the length.
+show("slice start", chars.slice(missing).length);
+show("slice end", chars.slice(1, missing).length);
+show("subarray start", chars.subarray(missing).length);
+show("subarray end", chars.subarray(2, missing).length);
+
+// The same reads held in a local, a class field and a record field.
+function local(i: number, label: string): void {
+  let code: number | undefined = 0;
+  code = chars[i];
+  show(`local fcc ${label}`, String.fromCharCode(97, code));
+  show(`local max ${label}`, Math.max(code, 1));
+  show(`local isNaN ${label}`, isNaN(code));
+}
+local(far, "missing");
+local(1, "present");
+
+class Cursor {
+  code: number | undefined = 0;
+}
+const cursor = new Cursor();
+cursor.code = chars[far];
+show("field fcc", String.fromCharCode(97, cursor.code));
+show("field abs", Math.abs(cursor.code));
+cursor.code = chars[1];
+show("field fcc present", String.fromCharCode(97, cursor.code));
+
+const record: { code: number | undefined } = { code: 0 };
+record.code = chars[far];
+show("record fcc", String.fromCharCode(97, record.code));
+show("record max", Math.max(record.code, 1));
+show("record isNaN", isNaN(record.code));