fix: native compiler segfault when closure mutation error is inside a function

src/semantic/closure-mutation-checker.ts

@@ -97,7 +97,11 @@ class ClosureMutationChecker {
       // Simple-name reassignment after capture is the error we're looking for.
       // Member-access assignments (obj.x = y) don't reassign the binding itself.
       if (capturedNames.indexOf(assign.name) !== -1) {
-        this.reportError(assign.name, assign.loc);
+        // NOTE: `assign.loc` is intentionally not passed here. No AssignmentStatement
+        // creation site in parser-ts/parser-native sets `loc`, so reading `assign.loc`
+        // in the native self-hosted compiler GEPs past the struct and segfaults.
+        // See the PR that introduced this comment for the full root cause.
+        this.reportError(assign.name, undefined);
       }
       this.scanExprForCaptures(assign.value, scopeVarNames, capturedNames);
     } else if (stype === "if") {

tests/fixtures/closures/closure-mutation-inside-arrow-body.ts

@@ -0,0 +1,10 @@
+// @test-compile-error: variable 'x' is captured by a closure but reassigned after capture
+// @test-description: closure mutation detected when assignment is inside the arrow body itself
+function outer() {
+  let x = 0;
+  const g = () => {
+    x = 1;
+  };
+  g();
+}
+outer();

tests/fixtures/closures/closure-mutation-inside-function.ts

@@ -0,0 +1,9 @@
+// @test-compile-error: variable 'x' is captured by a closure but reassigned after capture
+// @test-description: closure mutation detected inside a function body (not just top-level)
+function outer() {
+  let x = 0;
+  const g = () => x;
+  x = 1;
+  g();
+}
+outer();