[compiler] Special-case phi inference for mixed readonly type

This allows us to handle common operations such as `useFragment(...).edges.nodes ?? []` where we have a `Phi(MixedReadonly, Array)`. The underlying pattern remains general-purpose and not Relay-specific, and any API that returns transitively "mixed" data (primitives, arrays, plain objects) can benefit from the same type refinement.

ghstack-source-id: 5128310
Pull Request resolved: #30797

Author: josephsavona

compiler/packages/babel-plugin-react-compiler/src/TypeInference/InferTypes.ts

@@ -25,6 +25,7 @@ import {
   BuiltInArrayId,
   BuiltInFunctionId,
   BuiltInJsxId,
+  BuiltInMixedReadonlyId,
   BuiltInObjectId,
   BuiltInPropsId,
   BuiltInRefValueId,
@@ -496,8 +497,13 @@ class Unifier {
         if (candidateType === null) {
           candidateType = resolved;
         } else if (!typeEquals(resolved, candidateType)) {
-          candidateType = null;
-          break;
+          const unionType = tryUnionTypes(resolved, candidateType);
+          if (unionType === null) {
+            candidateType = null;
+            break;
+          } else {
+            candidateType = unionType;
+          }
         } // else same type, continue
       }
 
@@ -650,3 +656,39 @@ const RefLikeNameRE = /^(?:[a-zA-Z$_][a-zA-Z$_0-9]*)Ref$|^ref$/;
 function isRefLikeName(t: PropType): boolean {
   return RefLikeNameRE.test(t.objectName) && t.propertyName === 'current';
 }
+
+function tryUnionTypes(ty1: Type, ty2: Type): Type | null {
+  let readonlyType: Type;
+  let otherType: Type;
+  if (ty1.kind === 'Object' && ty1.shapeId === BuiltInMixedReadonlyId) {
+    readonlyType = ty1;
+    otherType = ty2;
+  } else if (ty2.kind === 'Object' && ty2.shapeId === BuiltInMixedReadonlyId) {
+    readonlyType = ty2;
+    otherType = ty1;
+  } else {
+    return null;
+  }
+  if (otherType.kind === 'Primitive') {
+    /**
+     * Union(Primitive | MixedReadonly) = MixedReadonly
+     *
+     * For example, `data ?? null` could return `data`, the fact that RHS
+     * is a primitive doesn't guarantee the result is a primitive.
+     */
+    return readonlyType;
+  } else if (
+    otherType.kind === 'Object' &&
+    otherType.shapeId === BuiltInArrayId
+  ) {
+    /**
+     * Union(Array | MixedReadonly) = Array
+     *
+     * In practice this pattern means the result is always an array. Given
+     * that this behavior requires opting-in to the mixedreadonly type
+     * (via moduleTypeProvider) this seems like a reasonable heuristic.
+     */
+    return otherType;
+  }
+  return null;
+}

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/error.todo-repro-missing-memoization-lack-of-phi-types.expect.md

@@ -0,0 +1,105 @@
+
+## 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>
+  );
+}
+
+```
+
+## 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