[compiler] Repro for nested function local reassignment issue

Summary: Additional repro demonstrating a case that still exists after #30106

ghstack-source-id: 92b852068dd651cf52eb94b05a7b61e282ad25d5
Pull Request resolved: #30110

Author: mvitousek

Diff for: compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/todo.invalid-nested-function-reassign-local-variable-in-effect.expect.md

@@ -0,0 +1,101 @@
+
+## Input
+
+```javascript
+import { useEffect } from "react";
+function Component() {
+  let local;
+  const mk_reassignlocal = () => {
+    // Create the reassignment function inside another function, then return it
+    const reassignLocal = (newValue) => {
+      local = newValue;
+    };
+    return reassignLocal;
+  }
+  const reassignLocal = mk_reassignlocal();
+  const onMount = (newValue) => {
+    reassignLocal("hello");
+    if (local === newValue) {
+      // Without React Compiler, `reassignLocal` is freshly created
+      // on each render, capturing a binding to the latest `local`,
+      // such that invoking reassignLocal will reassign the same
+      // binding that we are observing in the if condition, and
+      // we reach this branch
+      console.log("`local` was updated!");
+    } else {
+      // With React Compiler enabled, `reassignLocal` is only created
+      // once, capturing a binding to `local` in that render pass.
+      // Therefore, calling `reassignLocal` will reassign the wrong
+      // version of `local`, and not update the binding we are checking
+      // in the if condition.
+      //
+      // To protect against this, we disallow reassigning locals from
+      // functions that escape
+      throw new Error("`local` not updated!");
+    }
+  };
+  useEffect(() => {
+    onMount();
+  }, [onMount]);
+  return "ok";
+}
+
+```
+
+## Code
+
+```javascript
+import { c as _c } from "react/compiler-runtime";
+import { useEffect } from "react";
+function Component() {
+  const $ = _c(4);
+  let local;
+  let t0;
+  if ($[0] === Symbol.for("react.memo_cache_sentinel")) {
+    const mk_reassignlocal = () => {
+      const reassignLocal = (newValue) => {
+        local = newValue;
+      };
+      return reassignLocal;
+    };
+
+    t0 = mk_reassignlocal();
+    $[0] = t0;
+  } else {
+    t0 = $[0];
+  }
+  const reassignLocal_0 = t0;
+  let t1;
+  if ($[1] === Symbol.for("react.memo_cache_sentinel")) {
+    t1 = (newValue_0) => {
+      reassignLocal_0("hello");
+      if (local === newValue_0) {
+        console.log("`local` was updated!");
+      } else {
+        throw new Error("`local` not updated!");
+      }
+    };
+    $[1] = t1;
+  } else {
+    t1 = $[1];
+  }
+  const onMount = t1;
+  let t2;
+  let t3;
+  if ($[2] === Symbol.for("react.memo_cache_sentinel")) {
+    t2 = () => {
+      onMount();
+    };
+    t3 = [onMount];
+    $[2] = t2;
+    $[3] = t3;
+  } else {
+    t2 = $[2];
+    t3 = $[3];
+  }
+  useEffect(t2, t3);
+  return "ok";
+}
+
+```
+      

Diff for: compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/todo.invalid-nested-function-reassign-local-variable-in-effect.js

@@ -0,0 +1,37 @@
+import { useEffect } from "react";
+function Component() {
+  let local;
+  const mk_reassignlocal = () => {
+    // Create the reassignment function inside another function, then return it
+    const reassignLocal = (newValue) => {
+      local = newValue;
+    };
+    return reassignLocal;
+  }
+  const reassignLocal = mk_reassignlocal();
+  const onMount = (newValue) => {
+    reassignLocal("hello");
+    if (local === newValue) {
+      // Without React Compiler, `reassignLocal` is freshly created
+      // on each render, capturing a binding to the latest `local`,
+      // such that invoking reassignLocal will reassign the same
+      // binding that we are observing in the if condition, and
+      // we reach this branch
+      console.log("`local` was updated!");
+    } else {
+      // With React Compiler enabled, `reassignLocal` is only created
+      // once, capturing a binding to `local` in that render pass.
+      // Therefore, calling `reassignLocal` will reassign the wrong
+      // version of `local`, and not update the binding we are checking
+      // in the if condition.
+      //
+      // To protect against this, we disallow reassigning locals from
+      // functions that escape
+      throw new Error("`local` not updated!");
+    }
+  };
+  useEffect(() => {
+    onMount();
+  }, [onMount]);
+  return "ok";
+}

Diff for: compiler/packages/snap/src/SproutTodoFilter.ts

@@ -506,6 +506,8 @@ const skipFilter = new Set([
 
   // needs to be executed as a module
   "meta-property",
+
+  "todo.invalid-nested-function-reassign-local-variable-in-effect",
 ]);
 
 export default skipFilter;