fix(javascript): preserve initializer execution and lexical ownership

src/domain/javascript/javascriptSemanticAnalysis.initialization_boundaries.test.ts

@@ -0,0 +1,213 @@
+import { expect, it } from "vitest";
+import { analyzeParsedJavaScriptSemantics } from "./javascriptSemanticAnalysis.js";
+import {
+  onlyCallable,
+  onlyBinding,
+} from "./javascriptSemanticAnalysis.fixture.js";
+import { parseJavaScriptSource } from "./javascriptSourceParser.js";
+
+const analyze = (source: string, path = "fixture.cjs") => {
+  const file = parseJavaScriptSource(source, path);
+  if (file === null) throw new Error("Expected parsed fixture");
+  return analyzeParsedJavaScriptSemantics(file);
+};
+const returned = (body: string) =>
+  onlyCallable(analyze(`exports.read = function read() { ${body} };`), "read")
+    .returnSites[0]?.value;
+
+it.each([
+  "return value; var value = 1;",
+  "var snapshot = value; var value = 1; return snapshot;",
+])("selects declaration-only bindings at their actual read: %s", (body) => {
+  expect(returned(body)?.status).toBe("unknown");
+});
+
+it.each([
+  "do { break; var value = 1; } while (false); return value;",
+  "do { if (flag) break; var value = 1; } while (false); return value;",
+  "do { continue; var value = 1; } while (false); return value;",
+  "label: { break label; var value = 1; } return value;",
+])(
+  "does not treat an initializer skipped by abrupt completion as definite: %s",
+  (body) => {
+    expect(returned(body)?.status).toBe("unknown");
+  },
+);
+
+it.each([
+  "var value = 1; return value;",
+  "do { var value = 1; break; } while (false); return value;",
+  "do { for (;;) { break; } var value = 1; } while (false); return value;",
+  "label: { break label; } var value = 1; return value;",
+  "function unused() { return; } var value = 1; return value;",
+])(
+  "preserves initializers that an earlier local exit does not skip: %s",
+  (body) => {
+    expect(returned(body)).toEqual({ status: "literal", value: 1 });
+  },
+);
+
+it.each([
+  "var value = 1; { function value() {} } exports.read = function read() { return value; };",
+  "exports.read = function read() { var value = 1; if (flag) { function value() {} } return value; };",
+])(
+  "indexes the legacy block-function write on its outer owner: %s",
+  (source) => {
+    expect(
+      onlyCallable(analyze(source), "read").returnSites[0]?.value.status,
+    ).not.toBe("literal");
+  },
+);
+
+it.each([
+  [
+    "fixture.cjs",
+    '"use strict"; var value = 1; { function value() {} } exports.read = function read() { return value; };',
+  ],
+  [
+    "fixture.cjs",
+    "let value = 1; { function value() {} } exports.read = function read() { return value; };",
+  ],
+  [
+    "fixture.mjs",
+    "var value = 1; { function value() {} } export function read() { return value; }",
+  ],
+])(
+  "does not apply legacy writes across a strict or lexical boundary: %s",
+  (path, source) => {
+    expect(
+      onlyCallable(analyze(source, path), "read").returnSites[0]?.value,
+    ).toEqual({ status: "literal", value: 1 });
+  },
+);
+
+it("assigns a catch-shadowed var initializer to the active lexical binding", () => {
+  const ir = analyze(`
+    function target() { return "target"; }
+    try { throw 0; } catch (handler) { var handler = target; handler(); }
+  `);
+  const target = onlyCallable(ir, "target");
+  expect(ir.callSites).toContainEqual(
+    expect.objectContaining({
+      resolution: "exact",
+      calleeCallableIds: [target.callableId],
+    }),
+  );
+  const hoisted = ir.bindings.find(
+    ({ name, kind }) => name === "handler" && kind === "variable",
+  );
+  expect(hoisted?.value.status).toBe("unknown");
+});
+
+it("keeps ordinary arguments local while arrows retain the outer binding", () => {
+  const ir = analyze(`
+    var arguments = { 0: "OUTER" };
+    function ordinary() { const selected = arguments[0]; return selected; }
+    const arrow = () => arguments[0];
+  `);
+  expect(onlyBinding(ir, "selected").value.status).toBe("unknown");
+  expect(onlyCallable(ir, "ordinary").returnSites[0]?.value.status).toBe(
+    "unknown",
+  );
+  expect(onlyCallable(ir, "arrow").returnSites[0]?.value).toEqual({
+    status: "literal",
+    value: "OUTER",
+  });
+  const outer = onlyBinding(ir, "arguments");
+  expect(
+    ir.closureCaptures.some(
+      ({ bindingId, callableId }) =>
+        bindingId === outer.bindingId &&
+        callableId === onlyCallable(ir, "ordinary").callableId,
+    ),
+  ).toBe(false);
+});
+
+it("does not resolve Promise sources through a future initializer", () => {
+  const early = analyze(
+    `
+    try { task.then(work); } catch (error) {}
+    const task = Promise.resolve(1);
+  `,
+    "fixture.mjs",
+  );
+  expect(
+    early.promiseOperations.find(({ method }) => method === "then"),
+  ).toMatchObject({ sourceResolution: "unresolved", sourcePromiseIds: [] });
+  const initialized = analyze(
+    "const task = Promise.resolve(1); task.then(work);",
+    "fixture.mjs",
+  );
+  expect(
+    initialized.promiseOperations.find(({ method }) => method === "then"),
+  ).toMatchObject({
+    sourceResolution: "complete",
+    sourcePromiseIds: [expect.any(String)],
+  });
+});
+
+it.each([
+  "globalThis.Promise = { resolve() { return 17; } };",
+  "Promise.resolve = () => 17;",
+])(
+  "requires the selected intrinsic Promise identity after ambient writes: %s",
+  (write) => {
+    const ir = analyze(
+      `${write} const task = Promise.resolve(1);`,
+      "fixture.mjs",
+    );
+    expect(
+      ir.promiseOperations.filter(({ method }) => method === "resolve"),
+    ).toEqual([]);
+    expect(
+      analyze("const task = Promise.resolve(1);", "fixture.mjs")
+        .promiseOperations,
+    ).toContainEqual(
+      expect.objectContaining({
+        method: "resolve",
+        sourceResolution: "complete",
+      }),
+    );
+  },
+);
+
+it("retains caught reference identity without tainting inert throws or later replacements", () => {
+  const cases = [
+    ["caught.value = 2;", "unknown"],
+    ["mutate(caught);", "unknown"],
+    ["", "literal"],
+    ["caught = {}; caught.value = 2;", "literal"],
+  ] as const;
+  for (const [body, status] of cases)
+    expect(
+      returned(
+        `const state = { value: 1 }; try { throw state; } catch (caught) { ${body} } return state.value;`,
+      )?.status,
+    ).toBe(status);
+});
+
+it("owns a class declaration's self reference separately from its outer variable", () => {
+  const ir = analyze(
+    `
+    class Service { static create() { return new Service(); } }
+    const Original = Service;
+    Service = function Replacement() {};
+  `,
+    "fixture.mjs",
+  );
+  const bindings = ir.bindings.filter(({ name }) => name === "Service");
+  expect(bindings).toHaveLength(2);
+  const create = onlyCallable(ir, "create");
+  expect(
+    ir.closureCaptures.some(
+      ({ callableId, bindingId }) =>
+        callableId === create.callableId &&
+        bindingId === bindings.find(({ mutable }) => mutable)?.bindingId,
+    ),
+  ).toBe(false);
+  expect(
+    ir.callSites.find(
+      ({ callerCallableId }) => callerCallableId === create.callableId,
+    )?.resolution,
+  ).toBe("exact");
+});

src/domain/javascript/javascriptSemanticAnalysis.ts

@@ -9,6 +9,7 @@ import * as t from "@babel/types";
 import {
   semanticComputedMethodKey,
   semanticBranchMayBeSkipped,
+  semanticPriorStatementMayExit,
   semanticIterationCardinality,
 } from "./javascriptSemanticEvaluationOrder.js";
 
@@ -116,6 +117,7 @@ import type { JavaScriptModuleResolutionMode } from "./javascriptModuleResolutio
 interface BindPatternInput {
   readonly iterationSources?: JavaScriptSemanticBindingState["initializers"][number]["iterationSources"];
   readonly pattern: t.Node;
+  readonly entryBody?: t.BlockStatement;
   readonly initializer: t.Node | null;
   readonly scope: JavaScriptSemanticScopeState;
   readonly state: JavaScriptSemanticAnalysisState;
@@ -1067,6 +1069,19 @@ function* collectDefinitionsSteps(
     node: t.CallExpression;
     scopeId: string;
   }[] = [];
+  const displacedVarInitializers: {
+    node: t.VariableDeclarator;
+    scope: JavaScriptSemanticScopeState;
+    owner: JavaScriptSemanticScopeState;
+  }[] = [];
+  const thrownValues: {
+    node: t.ThrowStatement;
+    ancestors: readonly t.Node[];
+  }[] = [];
+  const legacyBlockFunctions: {
+    node: t.FunctionDeclaration;
+    scope: JavaScriptSemanticScopeState;
+  }[] = [];
   let needsPrototypeFacts = false;
   const functionBodies: {
     readonly body: t.BlockStatement;
@@ -1117,6 +1132,21 @@ function* collectDefinitionsSteps(
         parentScope = dynamicScope;
       }
       bindOuterDeclaration(node, parentScope, state);
+      if (
+        t.isFunctionDeclaration(node) &&
+        node.id != null &&
+        parentScope.kind === "block"
+      )
+        legacyBlockFunctions.push({ node, scope: parentScope });
+      if (
+        t.isVariableDeclarator(node) &&
+        node.init != null &&
+        t.isIdentifier(node.id)
+      ) {
+        const owner = semanticVariableScope(parentScope, parent, state);
+        if (owner !== parentScope)
+          displacedVarInitializers.push({ node, scope: parentScope, owner });
+      }
       const nested = nestedScope(node, parent, parentScope, state);
       if (nested !== undefined) {
         stack.push(nested);
@@ -1141,6 +1171,8 @@ function* collectDefinitionsSteps(
         state,
       });
       bindInnerDeclaration(node, parent, scope, state);
+      if (t.isThrowStatement(node))
+        thrownValues.push({ node, ancestors: readAncestors() });
       // Every supported global eval origin starts with one of these decoded
       // identifiers, including computed properties and destructured aliases.
       if (t.isIdentifier(node) && globalEvalRoots.has(node.name))
@@ -1180,6 +1212,94 @@ function* collectDefinitionsSteps(
         stack.pop();
     },
   });
+  // A var declaration is instantiated in its variable environment, while its
+  // initializer assigns through the lexical environment at the source site.
+  for (const { node, scope, owner } of displacedVarInitializers) {
+    if (!t.isIdentifier(node.id) || node.init == null) continue;
+    const selected = resolveSemanticBindingFromScope(
+      scope,
+      node.id.name,
+      state,
+    );
+    const declared = owner.bindings.get(node.id.name);
+    if (
+      selected === undefined ||
+      selected === declared ||
+      declared === undefined
+    )
+      continue;
+    for (let index = declared.initializers.length - 1; index >= 0; index--)
+      if (declared.initializers[index]?.node === node.init)
+        declared.initializers.splice(index, 1);
+    addAssignment(
+      node.id,
+      node.init,
+      scope,
+      state,
+      state.conditionallySkippedExpressions.has(node.init),
+      node,
+    );
+  }
+  for (const { node, ancestors } of thrownValues) {
+    let child: t.Node = node;
+    for (let index = ancestors.length - 1; index >= 0; index--) {
+      const parent = ancestors[index]!;
+      if (t.isFunction(parent) || t.isClass(parent)) break;
+      if (
+        t.isTryStatement(parent) &&
+        parent.handler != null &&
+        child === parent.block
+      ) {
+        const scope = state.scopeByNode.get(parent.handler);
+        if (scope !== undefined && parent.handler.param != null)
+          bindPattern({
+            pattern: parent.handler.param,
+            initializer: node.argument,
+            scope,
+            state,
+            mutable: true,
+            referenceOnly: true,
+            entryBody: parent.handler.body,
+          });
+        break;
+      }
+      child = parent;
+    }
+  }
+  for (const { node, scope } of legacyBlockFunctions) {
+    if (
+      node.id == null ||
+      semanticStrictnessAt(state.parentsByNode.get(node) ?? node, state) ===
+        "strict"
+    )
+      continue;
+    let outer =
+      scope.parentScopeId === null
+        ? undefined
+        : state.scopesById.get(scope.parentScopeId);
+    while (outer !== undefined) {
+      const binding = outer.bindings.get(node.id.name);
+      if (
+        binding !== undefined &&
+        !binding.declarationKinds.has("var") &&
+        binding.kind !== "parameter"
+      )
+        break;
+      if (
+        outer.kind === "function" ||
+        outer.kind === "program" ||
+        outer.kind === "static-block"
+      ) {
+        if (binding !== undefined)
+          addAssignment(node.id, node, outer, state, false, node);
+        break;
+      }
+      outer =
+        outer.parentScopeId === null
+          ? undefined
+          : state.scopesById.get(outer.parentScopeId);
+    }
+  }
   if (needsPrototypeFacts)
     yield* indexPrototypeUncertaintySteps(program, state);
   // All declarations must exist before resolving writes, including hoisted
@@ -1365,7 +1485,11 @@ const writeMayBeSkipped = (
       t.isStaticBlock(parent)
     )
       return false;
-    if (semanticBranchMayBeSkipped(parent, child)) return true;
+    if (
+      semanticBranchMayBeSkipped(parent, child) ||
+      semanticPriorStatementMayExit(parent, child)
+    )
+      return true;
     child = parent;
   }
   return false;
@@ -1421,7 +1545,7 @@ const bindInnerDeclaration = (
         ? { declarationKind: parent.kind }
         : {}),
     });
-  else if (t.isClassExpression(node) && t.isIdentifier(node.id))
+  else if (t.isClass(node) && t.isIdentifier(node.id))
     addBinding({
       state,
       scope,
@@ -1588,7 +1712,8 @@ const mappedArgumentsOwner = (
   }
   if (!t.isIdentifier(value)) return undefined;
 
-  if (semanticBindingLookupAt(state, value, "arguments").kind !== "ambient")
+  const lookup = semanticBindingLookupAt(state, value, "arguments");
+  if (lookup.kind !== "implicit-arguments" && lookup.kind !== "ambient")
     return undefined;
   let owner: t.Function | undefined;
   for (
@@ -1634,12 +1759,12 @@ const mappedArgumentsOwner = (
   return {
     scope,
     parameters: mapped,
-    strictness: mappedArgumentsStrictness(owner, state),
+    strictness: semanticStrictnessAt(owner, state),
   };
 };
 
-const mappedArgumentsStrictness = (
-  owner: t.Function,
+const semanticStrictnessAt = (
+  owner: t.Node,
   state: JavaScriptSemanticAnalysisState,
 ): MappedArgumentsOwner["strictness"] => {
   for (
@@ -1737,6 +1862,9 @@ const nestedScope = (
     location: range(switchOwner ?? node),
     bindingsComplete: true,
     bindings: new Map(),
+    ...(t.isFunction(node) && !t.isArrowFunctionExpression(node)
+      ? { implicitArguments: true as const }
+      : {}),
   };
   state.scopes.push(scope);
   state.scopesById.set(scope.scopeId, scope);
@@ -1914,6 +2042,9 @@ const bindPattern = (input: BindPatternInput): void => {
         binding.referenceInitializers.push({
           node: initializer,
           projection,
+          ...(input.entryBody === undefined
+            ? {}
+            : { entryBody: input.entryBody }),
           ...(assignmentReference === undefined ? {} : { assignment: true }),
           ...(copyKind === undefined ? {} : { copyKind }),
           ...(copyProjectionOffset === undefined
@@ -2136,7 +2267,8 @@ const addAssignment = (
   const mutation = writeSite ?? initializer;
   if (mutation === null) return;
   const lookup = semanticBindingLookupFromScope(scope, identifier.name, state);
-  if (lookup.kind === "captured") return;
+  if (lookup.kind === "captured" || lookup.kind === "implicit-arguments")
+    return;
   const binding = "binding" in lookup ? lookup.binding : undefined;
   conditionallySkipped ||=
     lookup.kind !== "binding" && lookup.kind !== "ambient";

src/domain/javascript/javascriptSemanticEvaluationOrder.ts

@@ -6,6 +6,8 @@ import {
   unwrapJavaScriptExpression,
 } from "./javascriptAstValues.js";
 
+import { childNodes } from "./javascriptSemanticTraversal.js";
+
 const preserveGrouping = { stopAtParentheses: true } as const;
 
 export type JavaScriptSemanticIterationCardinality =
@@ -155,3 +157,94 @@ const optionalChainMayShortCircuit = (expression: t.Node): boolean => {
     else return false;
   }
 };
+
+interface AbruptCompletionSummary {
+  readonly breaks: ReadonlySet<string | null>;
+  readonly continues: ReadonlySet<string | null>;
+}
+const emptyAbruptLabels: ReadonlySet<string | null> = new Set();
+const emptyAbruptSummary: AbruptCompletionSummary = {
+  breaks: emptyAbruptLabels,
+  continues: emptyAbruptLabels,
+};
+const abruptSummaries = new WeakMap<t.Node, AbruptCompletionSummary>();
+const abruptPrefixes = new WeakMap<
+  t.BlockStatement | t.Program,
+  ReadonlySet<t.Node>
+>();
+
+/** Loop and label exits crossing a statement boundary, excluding nested callables. */
+const abruptSummary = (root: t.Node): AbruptCompletionSummary => {
+  const pending: { node: t.Node; visited: boolean }[] = [
+    { node: root, visited: false },
+  ];
+  while (pending.length > 0) {
+    const { node, visited } = pending.pop()!;
+    if (abruptSummaries.has(node)) continue;
+    const children =
+      t.isFunction(node) || t.isClass(node)
+        ? []
+        : childNodes(node).filter(
+            (child) =>
+              t.isStatement(child) ||
+              t.isSwitchCase(child) ||
+              t.isCatchClause(child),
+          );
+    if (!visited) {
+      pending.push({ node, visited: true });
+      for (const child of children)
+        if (!abruptSummaries.has(child))
+          pending.push({ node: child, visited: false });
+      continue;
+    }
+    let breaks: Set<string | null> | undefined,
+      continues: Set<string | null> | undefined;
+    if (t.isBreakStatement(node))
+      (breaks ??= new Set()).add(node.label?.name ?? null);
+    if (t.isContinueStatement(node))
+      (continues ??= new Set()).add(node.label?.name ?? null);
+    for (const child of children) {
+      const summary = abruptSummaries.get(child)!;
+      for (const label of summary.breaks) (breaks ??= new Set()).add(label);
+      for (const label of summary.continues)
+        (continues ??= new Set()).add(label);
+    }
+    if (t.isLoop(node) || t.isSwitchStatement(node)) breaks?.delete(null);
+    if (t.isLoop(node)) continues?.delete(null);
+    if (t.isLabeledStatement(node)) {
+      breaks?.delete(node.label.name);
+      continues?.delete(node.label.name);
+    }
+    abruptSummaries.set(
+      node,
+      (breaks?.size ?? 0) > 0 || (continues?.size ?? 0) > 0
+        ? {
+            breaks: breaks ?? emptyAbruptLabels,
+            continues: continues ?? emptyAbruptLabels,
+          }
+        : emptyAbruptSummary,
+    );
+  }
+  return abruptSummaries.get(root)!;
+};
+
+/** An earlier sibling may leave this body before reaching the selected statement. */
+export const semanticPriorStatementMayExit = (
+  parent: t.Node,
+  child: t.Node,
+): boolean => {
+  if (!t.isBlockStatement(parent) && !t.isProgram(parent)) return false;
+  let skipped = abruptPrefixes.get(parent);
+  if (skipped === undefined) {
+    const prefixes = new Set<t.Node>();
+    let exits = false;
+    for (const statement of parent.body) {
+      if (exits) prefixes.add(statement);
+      const summary = abruptSummary(statement);
+      exits ||= summary.breaks.size > 0 || summary.continues.size > 0;
+    }
+    skipped = prefixes;
+    abruptPrefixes.set(parent, skipped);
+  }
+  return skipped.has(child);
+};

src/domain/javascript/javascriptSemanticExpressionShapes.ts

@@ -53,7 +53,6 @@ export const createSemanticExpressionShapeResolver = (
   // Most modules have no named child arguments. Collect source-wide blockers
   // only when a query actually needs a named binding or ambient undefined.
   let blocked: boolean | undefined;
-  const blockFunctionNames = new Set<string>();
   const namedProofBlocked = (): boolean => {
     if (blocked !== undefined) return blocked;
     blocked = false;
@@ -62,13 +61,6 @@ export const createSemanticExpressionShapeResolver = (
         blocked ||=
           isErasedJavaScriptDeclaration(node) ||
           isUnmodeledJavaScriptRuntimeScope(node);
-        if (
-          javaScriptRuntimeContext(state.program) !== "module" &&
-          t.isFunctionDeclaration(node) &&
-          node.id != null &&
-          !hoistedFunction(node)
-        )
-          blockFunctionNames.add(node.id.name);
       },
     });
     return blocked;
@@ -105,13 +97,9 @@ export const createSemanticExpressionShapeResolver = (
         )
           return "unknown";
       }
-      // Annex B may copy a block function into an outer var without an indexed
-      // assignment. Keep same-named vars unknown rather than model that copy.
       if (
         namedProofBlocked() ||
-        semanticOpaqueBindingReadMayHaveChanged(binding, node, state) ||
-        (binding.declarationKinds.has("var") &&
-          blockFunctionNames.has(binding.name))
+        semanticOpaqueBindingReadMayHaveChanged(binding, node, state)
       )
         return "unknown";
       const initializer = semanticStableInitializer(binding, state);

src/domain/javascript/javascriptSemanticPromises.ts

@@ -13,11 +13,12 @@ import {
 } from "./javascriptAstValues.js";
 import {
   resolveSemanticBindingState,
-  semanticResolutionBlocked,
   type JavaScriptSemanticAnalysisState,
 } from "./javascriptSemanticState.js";
 import { compareUnicodeCodePoints } from "../unicodeCodePointOrder.js";
 import { traverseJavaScriptAst } from "./javascriptSemanticTraversal.js";
+import { semanticBindingAtRead } from "./javascriptSemanticMutationInitializers.js";
+import { isUnmodifiedGlobalReference } from "./javascriptSemanticAmbientWrites.js";
 import { range } from "./javascriptStaticAnalysisHelpers.js";
 import {
   assignedSemanticResultBindings,
@@ -140,7 +141,7 @@ const promiseCandidateDetails = (
   if (
     t.isNewExpression(node) &&
     t.isIdentifier(node.callee, { name: "Promise" }) &&
-    isUnshadowedGlobalPromise(node.callee, state)
+    isUnmodifiedGlobalReference(node.callee, state, "Promise")
   )
     return { kind: "constructor", method: "new" };
   if (t.isCallExpression(node) || t.isOptionalCallExpression(node)) {
@@ -149,7 +150,7 @@ const promiseCandidateDetails = (
     const method = semanticStaticPropertyName(member.property, member.computed);
     if (
       t.isIdentifier(member.object, { name: "Promise" }) &&
-      isUnshadowedGlobalPromise(member.object, state)
+      isUnmodifiedGlobalReference(node.callee, state, ["Promise", method ?? ""])
     ) {
       if (method === "all" || method === "allSettled")
         return { kind: "aggregate", method };
@@ -164,13 +165,6 @@ const promiseCandidateDetails = (
   return null;
 };
 
-const isUnshadowedGlobalPromise = (
-  node: t.Identifier,
-  state: JavaScriptSemanticAnalysisState,
-): boolean =>
-  resolveSemanticBindingState(state, node, "Promise") === undefined &&
-  !semanticResolutionBlocked(state, node, "Promise");
-
 const isPromiseProducer = (
   node: t.Node,
   state: JavaScriptSemanticAnalysisState,
@@ -447,14 +441,16 @@ const resolvePromiseExpression = (
     return { promiseIds: [direct.promiseId], status: "complete" };
   if (!t.isIdentifier(node)) return unresolvedPromise();
   const binding = resolveSemanticBindingState(state, node, node.name);
+  if (binding === undefined || seenBindings.has(binding.bindingId))
+    return unresolvedPromise();
+  const read = semanticBindingAtRead(binding, node, state.parentsByNode);
   if (
-    binding === undefined ||
-    binding.mutable ||
-    binding.initializers.length !== 1 ||
-    seenBindings.has(binding.bindingId)
+    read.status !== "ordered" ||
+    !read.hasDefiniteInitializer ||
+    read.binding.initializers.length !== 1
   )
     return unresolvedPromise();
-  const initializer = binding.initializers[0];
+  const initializer = read.binding.initializers[0];
   if (initializer === undefined || initializer.projection.length > 0)
     return unresolvedPromise();
   const nestedSeen = new Set(seenBindings);

src/domain/javascript/javascriptSemanticState.ts

@@ -117,6 +117,8 @@ export interface JavaScriptSemanticScopeState {
   readonly bindings: Map<string, JavaScriptSemanticBindingState>;
   /** Object environment introduced by `with`, active only for its body. */
   readonly dynamicWithStatement?: t.WithStatement;
+  /** Ordinary functions own `arguments`; arrow functions inherit it. */
+  readonly implicitArguments?: true;
 }
 
 /** Shared state for semantic collection and evaluation. */
@@ -301,7 +303,14 @@ type SemanticBindingLookup =
       readonly kind: "binding" | "uncertain-binding";
       readonly binding: JavaScriptSemanticBindingState;
     }
-  | { readonly kind: "ambient" | "uncertain-ambient" | "captured" | "unknown" };
+  | {
+      readonly kind:
+        | "ambient"
+        | "uncertain-ambient"
+        | "captured"
+        | "unknown"
+        | "implicit-arguments";
+    };
 
 /** Retain both a dynamic lookup's uncertainty and its possible outer write owner. */
 export const semanticBindingLookupFromScope = (
@@ -314,6 +323,8 @@ export const semanticBindingLookupFromScope = (
     const binding = scope.bindings.get(name);
     if (binding !== undefined)
       return { kind: uncertain ? "uncertain-binding" : "binding", binding };
+    if (name === "arguments" && scope.implicitArguments === true)
+      return { kind: "implicit-arguments" };
     if (!scope.bindingsComplete) {
       if (scope.dynamicWithStatement === undefined) return { kind: "unknown" };
       const lookup = semanticWithRequireLookup(

src/domain/javascript/javascriptSemanticValues.ts

@@ -658,12 +658,16 @@ const evaluateExpressionValue = (
     const binding = resolveSemanticBindingState(context.state, node, node.name);
     if (binding === undefined)
       return { status: "unknown", reason: `Unbound identifier ${node.name}.` };
-    const read =
+    const read = semanticBindingAtRead(
+      binding,
+      node,
+      context.state.parentsByNode,
+    );
+    if (
+      read.status === "unresolved" &&
       binding.mutable &&
       binding.definitions.some(({ kind }) => kind === "assignment")
-        ? semanticBindingAtRead(binding, node, context.state.parentsByNode)
-        : undefined;
-    if (read?.status === "unresolved") {
+    ) {
       const value = evaluateBinding(binding, nestedContext(context));
       if (
         value.status === "unknown" ||
@@ -676,7 +680,7 @@ const evaluateExpressionValue = (
         reason: `Binding ${binding.name} has unresolved ordering at this read.`,
       };
     }
-    const atRead = read?.binding ?? binding;
+    const atRead = read.binding;
     if (
       semanticOpaqueBindingReadMayHaveChanged(
         binding,