From 2bcbf254f168ddec567156f802d19315e64e4aa8 Mon Sep 17 00:00:00 2001
From: Joseph Savona <6425824+josephsavona@users.noreply.github.com>
Date: Mon, 20 Oct 2025 08:42:04 -0700
Subject: [PATCH] [compiler] Fix false positive for useMemo reassigning context
 vars (#34904)

Within a function expression local variables may use StoreContext for
local context variables, so the reassignment check here was firing too
often. We should only report an error for variables that are declared
outside the function, ie part of its `context`.

---
[//]: # (BEGIN SAPLING FOOTER)
Stack created with [Sapling](https://sapling-scm.com). Best reviewed
with [ReviewStack](https://reviewstack.dev/facebook/react/pull/34904).
* #34903
* __->__ #34904
---
 .../src/Validation/ValidateUseMemo.ts         | 31 +++++++------
 .../reassign-variable-in-usememo.expect.md    | 45 +++++++++++++++++++
 .../compiler/reassign-variable-in-usememo.js  | 12 +++++
 3 files changed, 74 insertions(+), 14 deletions(-)
 create mode 100644 compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/reassign-variable-in-usememo.expect.md
 create mode 100644 compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/reassign-variable-in-usememo.js

diff --git a/compiler/packages/babel-plugin-react-compiler/src/Validation/ValidateUseMemo.ts b/compiler/packages/babel-plugin-react-compiler/src/Validation/ValidateUseMemo.ts
index 05a4b4b91f..2bccda3a2e 100644
--- a/compiler/packages/babel-plugin-react-compiler/src/Validation/ValidateUseMemo.ts
+++ b/compiler/packages/babel-plugin-react-compiler/src/Validation/ValidateUseMemo.ts
@@ -184,25 +184,28 @@ function validateNoContextVariableAssignment(
   fn: HIRFunction,
   errors: CompilerError,
 ): void {
+  const context = new Set(fn.context.map(place => place.identifier.id));
   for (const block of fn.body.blocks.values()) {
     for (const instr of block.instructions) {
       const value = instr.value;
       switch (value.kind) {
         case 'StoreContext': {
-          errors.pushDiagnostic(
-            CompilerDiagnostic.create({
-              category: ErrorCategory.UseMemo,
-              reason:
-                'useMemo() callbacks may not reassign variables declared outside of the callback',
-              description:
-                'useMemo() callbacks must be pure functions and cannot reassign variables defined outside of the callback function',
-              suggestions: null,
-            }).withDetails({
-              kind: 'error',
-              loc: value.lvalue.place.loc,
-              message: 'Cannot reassign variable',
-            }),
-          );
+          if (context.has(value.lvalue.place.identifier.id)) {
+            errors.pushDiagnostic(
+              CompilerDiagnostic.create({
+                category: ErrorCategory.UseMemo,
+                reason:
+                  'useMemo() callbacks may not reassign variables declared outside of the callback',
+                description:
+                  'useMemo() callbacks must be pure functions and cannot reassign variables defined outside of the callback function',
+                suggestions: null,
+              }).withDetails({
+                kind: 'error',
+                loc: value.lvalue.place.loc,
+                message: 'Cannot reassign variable',
+              }),
+            );
+          }
           break;
         }
       }
diff --git a/compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/reassign-variable-in-usememo.expect.md b/compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/reassign-variable-in-usememo.expect.md
new file mode 100644
index 0000000000..29dc3afcd6
--- /dev/null
+++ b/compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/reassign-variable-in-usememo.expect.md
@@ -0,0 +1,45 @@
+
+## Input
+
+```javascript
+// @flow
+export hook useItemLanguage(items) {
+  return useMemo(() => {
+    let language: ?string = null;
+    items.forEach(item => {
+      if (item.language != null) {
+        language = item.language;
+      }
+    });
+    return language;
+  }, [items]);
+}
+
+```
+
+## Code
+
+```javascript
+import { c as _c } from "react/compiler-runtime";
+export function useItemLanguage(items) {
+  const $ = _c(2);
+  let language;
+  if ($[0] !== items) {
+    language = null;
+    items.forEach((item) => {
+      if (item.language != null) {
+        language = item.language;
+      }
+    });
+    $[0] = items;
+    $[1] = language;
+  } else {
+    language = $[1];
+  }
+  return language;
+}
+
+```
+      
+### Eval output
+(kind: exception) Fixture not implemented
\ No newline at end of file
diff --git a/compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/reassign-variable-in-usememo.js b/compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/reassign-variable-in-usememo.js
new file mode 100644
index 0000000000..4ed89bfc64
--- /dev/null
+++ b/compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/reassign-variable-in-usememo.js
@@ -0,0 +1,12 @@
+// @flow
+export hook useItemLanguage(items) {
+  return useMemo(() => {
+    let language: ?string = null;
+    items.forEach(item => {
+      if (item.language != null) {
+        language = item.language;
+      }
+    });
+    return language;
+  }, [items]);
+}