From 8deecf508559f064f9c1d1bf2b82ec0d90c9b4d0 Mon Sep 17 00:00:00 2001
From: Joe Savona <joesavona@fb.com>
Date: Fri, 14 Nov 2025 11:23:42 -0800
Subject: [PATCH] [compiler] Repro for false positive mutation of a value
 derived from props

Repro from the compiler WG (Thanks Cody!) of a case where the compiler incorrectly thinks a value is mutable.
---
 ...ure-from-prop-with-default-value.expect.md | 43 +++++++++++++++++++
 ...estructure-from-prop-with-default-value.js | 15 +++++++
 2 files changed, 58 insertions(+)
 create mode 100644 compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/new-mutability/error.todo-repro-destructure-from-prop-with-default-value.expect.md
 create mode 100644 compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/new-mutability/error.todo-repro-destructure-from-prop-with-default-value.js

diff --git a/compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/new-mutability/error.todo-repro-destructure-from-prop-with-default-value.expect.md b/compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/new-mutability/error.todo-repro-destructure-from-prop-with-default-value.expect.md
new file mode 100644
index 0000000000000..9bd31999f6216
--- /dev/null
+++ b/compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/new-mutability/error.todo-repro-destructure-from-prop-with-default-value.expect.md
@@ -0,0 +1,43 @@
+
+## Input
+
+```javascript
+export function useFormatRelativeTime(opts = {}) {
+  const {timeZone, minimal} = opts;
+  const format = useCallback(function formatWithUnit() {}, [minimal]);
+  // We record `{timeZone}` as capturing timeZone into the object,
+  // then assume that dateTimeFormat() mutates that object,
+  // which in turn can mutate timeZone and the object it came from,
+  // which means that the value `minimal` is derived from can change.
+  //
+  // The bug is that we shouldn't be recording a Capture effect given
+  // that `opts` is known maybe-frozen. If we correctly recorded
+  // an ImmutableCapture, this wouldn't be mistaken as mutating
+  // opts/minimal
+  dateTimeFormat({timeZone});
+  return format;
+}
+
+```
+
+
+## Error
+
+```
+Found 1 error:
+
+Compilation Skipped: Existing memoization could not be preserved
+
+React Compiler has skipped optimizing this component because the existing manual memoization could not be preserved. This dependency may be mutated later, which could cause the value to change unexpectedly.
+
+error.todo-repro-destructure-from-prop-with-default-value.ts:3:60
+  1 | export function useFormatRelativeTime(opts = {}) {
+  2 |   const {timeZone, minimal} = opts;
+> 3 |   const format = useCallback(function formatWithUnit() {}, [minimal]);
+    |                                                             ^^^^^^^ This dependency may be modified later
+  4 |   // We record `{timeZone}` as capturing timeZone into the object,
+  5 |   // then assume that dateTimeFormat() mutates that object,
+  6 |   // which in turn can mutate timeZone and the object it came from,
+```
+          
+      
\ No newline at end of file
diff --git a/compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/new-mutability/error.todo-repro-destructure-from-prop-with-default-value.js b/compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/new-mutability/error.todo-repro-destructure-from-prop-with-default-value.js
new file mode 100644
index 0000000000000..bcbd85231f599
--- /dev/null
+++ b/compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/new-mutability/error.todo-repro-destructure-from-prop-with-default-value.js
@@ -0,0 +1,15 @@
+export function useFormatRelativeTime(opts = {}) {
+  const {timeZone, minimal} = opts;
+  const format = useCallback(function formatWithUnit() {}, [minimal]);
+  // We record `{timeZone}` as capturing timeZone into the object,
+  // then assume that dateTimeFormat() mutates that object,
+  // which in turn can mutate timeZone and the object it came from,
+  // which means that the value `minimal` is derived from can change.
+  //
+  // The bug is that we shouldn't be recording a Capture effect given
+  // that `opts` is known maybe-frozen. If we correctly recorded
+  // an ImmutableCapture, this wouldn't be mistaken as mutating
+  // opts/minimal
+  dateTimeFormat({timeZone});
+  return format;
+}