Repository: facebook/react
Pull request: [compiler] Repros for missing memoization due to lack of phi type inference

Stack from [ghstack](https://github.com/ezyang/ghstack) (oldest at bottom):
* #30801
* #30797
* #30796
* #30795
* #30794
* __->__ #30793

This is a complex case: we not only need phi type inference but also need to be able infer the union of `MixedReadonly | Array`.

Changed files:
filename: compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error.todo-repro-missing-memoization-lack-of-phi-types.expect.md
patch: @@ -0,0 +1,42 @@
+
+## Input
+
+```javascript
+// @flow @validatePreserveExistingMemoizationGuarantees
+import {useMemo} from 'react';
+import {useFragment} from 'shared-runtime';
+
+function Component() {
+  const data = useFragment();
+  const nodes = data.nodes ?? [];
+  const flatMap = nodes.flatMap(node => node.items);
+  const filtered = flatMap.filter(item => item != null);
+  const map = useMemo(() => filtered.map(), [filtered]);
+  const index = filtered.findIndex(x => x === null);
+
+  return (
+    <div>
+      {map}
+      {index}
+    </div>
+  );
+}
+
+```
+
+
+## Error
+
+```
+   8 |   const flatMap = nodes.flatMap(node => node.items);
+   9 |   const filtered = flatMap.filter(item => item != null);
+> 10 |   const map = useMemo(() => filtered.map(), [filtered]);
+     |                                              ^^^^^^^^ CannotPreserveMemoization: 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 (10:10)
+
+CannotPreserveMemoization: React Compiler has skipped optimizing this component because the existing manual memoization could not be preserved. This value was memoized in source but not in compilation output. (10:10)
+  11 |   const index = filtered.findIndex(x => x === null);
+  12 |
+  13 |   return (
+```
+          
+      
\ No newline at end of file

filename: compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error.todo-repro-missing-memoization-lack-of-phi-types.js
patch: @@ -0,0 +1,19 @@
+// @flow @validatePreserveExistingMemoizationGuarantees
+import {useMemo} from 'react';
+import {useFragment} from 'shared-runtime';
+
+function Component() {
+  const data = useFragment();
+  const nodes = data.nodes ?? [];
+  const flatMap = nodes.flatMap(node => node.items);
+  const filtered = flatMap.filter(item => item != null);
+  const map = useMemo(() => filtered.map(), [filtered]);
+  const index = filtered.findIndex(x => x === null);
+
+  return (
+    <div>
+      {map}
+      {index}
+    </div>
+  );
+}

filename: compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/repro-missing-memoization-lack-of-phi-types-explicit-types.expect.md
patch: @@ -0,0 +1,108 @@
+
+## Input
+
+```javascript
+// @flow @validatePreserveExistingMemoizationGuarantees @enableUseTypeAnnotations
+import {useMemo} from 'react';
+import {useFragment} from 'shared-runtime';
+
+// This is a version of error.todo-repro-missing-memoization-lack-of-phi-types
+// with explicit type annotations and using enableUseTypeAnnotations to demonstrate
+// that type information is sufficient to preserve memoization in this example
+function Component() {
+  const data = useFragment();
+  const nodes: Array<any> = data.nodes ?? [];
+  const flatMap: Array<any> = nodes.flatMap(node => node.items);
+  const filtered: Array<any> = flatMap.filter(item => item != null);
+  const map: Array<any> = useMemo(() => filtered.map(), [filtered]);
+  const index: Array<any> = filtered.findIndex(x => x === null);
+
+  return (
+    <div>
+      {map}
+      {index}
+    </div>
+  );
+}
+
+```
+
+## Code
+
+```javascript
+import { c as _c } from "react/compiler-runtime";
+import { useMemo } from "react";
+import { useFragment } from "shared-runtime";
+
+function Component() {
+  const $ = _c(11);
+  const data = useFragment();
+  let t0;
+  if ($[0] !== data.nodes) {
+    t0 = data.nodes ?? [];
+    $[0] = data.nodes;
+    $[1] = t0;
+  } else {
+    t0 = $[1];
+  }
+  const nodes = t0;
+  let t1;
+  if ($[2] !== nodes) {
+    t1 = nodes.flatMap(_temp);
+    $[2] = nodes;
+    $[3] = t1;
+  } else {
+    t1 = $[3];
+  }
+  const flatMap = t1;
+  let t2;
+  if ($[4] !== flatMap) {
+    t2 = flatMap.filter(_temp2);
+    $[4] = flatMap;
+    $[5] = t2;
+  } else {
+    t2 = $[5];
+  }
+  const filtered = t2;
+  let t3;
+  let t4;
+  if ($[6] !== filtered) {
+    t4 = filtered.map();
+    $[6] = filtered;
+    $[7] = t4;
+  } else {
+    t4 = $[7];
+  }
+  t3 = t4;
+  const map = t3;
+  const index = filtered.findIndex(_temp3);
+  let t5;
+  if ($[8] !== map || $[9] !== index) {
+    t5 = (
+      <div>
+        {map}
+        {index}
+      </div>
+    );
+    $[8] = map;
+    $[9] = index;
+    $[10] = t5;
+  } else {
+    t5 = $[10];
+  }
+  return t5;
+}
+function _temp3(x) {
+  return x === null;
+}
+function _temp2(item) {
+  return item != null;
+}
+function _temp(node) {
+  return node.items;
+}
+
+```
+      
+### Eval output
+(kind: exception) Fixture not implemented
\ No newline at end of file

filename: compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/repro-missing-memoization-lack-of-phi-types-explicit-types.js
patch: @@ -0,0 +1,22 @@
+// @flow @validatePreserveExistingMemoizationGuarantees @enableUseTypeAnnotations
+import {useMemo} from 'react';
+import {useFragment} from 'shared-runtime';
+
+// This is a version of error.todo-repro-missing-memoization-lack-of-phi-types
+// with explicit type annotations and using enableUseTypeAnnotations to demonstrate
+// that type information is sufficient to preserve memoization in this example
+function Component() {
+  const data = useFragment();
+  const nodes: Array<any> = data.nodes ?? [];
+  const flatMap: Array<any> = nodes.flatMap(node => node.items);
+  const filtered: Array<any> = flatMap.filter(item => item != null);
+  const map: Array<any> = useMemo(() => filtered.map(), [filtered]);
+  const index: Array<any> = filtered.findIndex(x => x === null);
+
+  return (
+    <div>
+      {map}
+      {index}
+    </div>
+  );
+}