From dc25c988283a222b8c7cb7c4af1b4d974622f498 Mon Sep 17 00:00:00 2001
From: Raashish Aggarwal <94279692+raashish1601@users.noreply.github.com>
Date: Sat, 2 May 2026 10:44:07 +0530
Subject: [PATCH] Infer filter as a mixed readonly array method

---
 .../src/TypeInference/InferTypes.ts           |  9 ++
 ...local-mutation-in-filter-usememo.expect.md | 99 +++++++++++++++++++
 .../local-mutation-in-filter-usememo.js       | 23 +++++
 3 files changed, 131 insertions(+)
 create mode 100644 compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/local-mutation-in-filter-usememo.expect.md
 create mode 100644 compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/local-mutation-in-filter-usememo.js

diff --git a/compiler/packages/babel-plugin-react-compiler/src/TypeInference/InferTypes.ts b/compiler/packages/babel-plugin-react-compiler/src/TypeInference/InferTypes.ts
index 190e3d3a7a3e..b244fd987e5b 100644
--- a/compiler/packages/babel-plugin-react-compiler/src/TypeInference/InferTypes.ts
+++ b/compiler/packages/babel-plugin-react-compiler/src/TypeInference/InferTypes.ts
@@ -108,6 +108,9 @@ function equation(left: Type, right: Type): TypeEquation {
   };
 }
 
+// These method names only exist on arrays in the MixedReadonly model.
+const ARRAY_METHODS_ON_MIXED_READONLY = new Set(['filter']);
+
 function* generate(
   func: HIRFunction,
 ): Generator<TypeEquation, void, undefined> {
@@ -321,6 +324,12 @@ function* generateInstructionTypes(
     }
 
     case 'PropertyLoad': {
+      if (ARRAY_METHODS_ON_MIXED_READONLY.has(String(value.property))) {
+        yield equation(value.object.identifier.type, {
+          kind: 'Object',
+          shapeId: BuiltInMixedReadonlyId,
+        });
+      }
       yield equation(left, {
         kind: 'Property',
         objectType: value.object.identifier.type,
diff --git a/compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/local-mutation-in-filter-usememo.expect.md b/compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/local-mutation-in-filter-usememo.expect.md
new file mode 100644
index 000000000000..dcc5c6319ed7
--- /dev/null
+++ b/compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/local-mutation-in-filter-usememo.expect.md
@@ -0,0 +1,99 @@
+
+## Input
+
+```javascript
+import React from 'react';
+
+export function Component({items}) {
+  const stuff = React.useMemo(() => {
+    let isCool = false;
+    const someItems = items.filter(cause => {
+      if (cause.foo) {
+        isCool = true;
+      }
+      return true;
+    });
+
+    if (someItems.length > 0) {
+      return {someItems, isCool};
+    }
+  }, [items]);
+  return <div>{stuff?.someItems.length}</div>;
+}
+
+export const FIXTURE_ENTRYPOINT = {
+  fn: Component,
+  params: [{items: [{foo: true}, {foo: false}]}],
+};
+
+```
+
+## Code
+
+```javascript
+import { c as _c } from "react/compiler-runtime";
+import React from "react";
+
+export function Component(t0) {
+  const $ = _c(8);
+  const { items } = t0;
+  let t1;
+  bb0: {
+    let isCool;
+    let t2;
+    if ($[0] !== items) {
+      isCool = false;
+      t2 = items.filter((cause) => {
+        if (cause.foo) {
+          isCool = true;
+        }
+
+        return true;
+      });
+      $[0] = items;
+      $[1] = isCool;
+      $[2] = t2;
+    } else {
+      isCool = $[1];
+      t2 = $[2];
+    }
+    const someItems = t2;
+
+    if (someItems.length > 0) {
+      let t3;
+      if ($[3] !== isCool || $[4] !== someItems) {
+        t3 = { someItems, isCool };
+        $[3] = isCool;
+        $[4] = someItems;
+        $[5] = t3;
+      } else {
+        t3 = $[5];
+      }
+      t1 = t3;
+      break bb0;
+    }
+    t1 = undefined;
+  }
+  const stuff = t1;
+
+  const t2 = stuff?.someItems.length;
+  let t3;
+  if ($[6] !== t2) {
+    t3 = <div>{t2}</div>;
+    $[6] = t2;
+    $[7] = t3;
+  } else {
+    t3 = $[7];
+  }
+  return t3;
+}
+
+export const FIXTURE_ENTRYPOINT = {
+  fn: Component,
+  params: [{ items: [{ foo: true }, { foo: false }] }],
+};
+
+```
+      
+### Eval output
+(kind: ok) <div>2</div>
\ No newline at end of file
diff --git a/compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/local-mutation-in-filter-usememo.js b/compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/local-mutation-in-filter-usememo.js
new file mode 100644
index 000000000000..d2277fb58ae8
--- /dev/null
+++ b/compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/local-mutation-in-filter-usememo.js
@@ -0,0 +1,23 @@
+import React from 'react';
+
+export function Component({items}) {
+  const stuff = React.useMemo(() => {
+    let isCool = false;
+    const someItems = items.filter(cause => {
+      if (cause.foo) {
+        isCool = true;
+      }
+      return true;
+    });
+
+    if (someItems.length > 0) {
+      return {someItems, isCool};
+    }
+  }, [items]);
+  return <div>{stuff?.someItems.length}</div>;
+}
+
+export const FIXTURE_ENTRYPOINT = {
+  fn: Component,
+  params: [{items: [{foo: true}, {foo: false}]}],
+};