fix: correctly compile post-increment assignment

compiler/packages/babel-plugin-react-compiler/src/HIR/BuildHIR.ts

@@ -2596,11 +2596,17 @@ function lowerExpression(
 
         // Store the previous value to a temporary
         const previousValuePlace = lowerValueToTemporary(builder, value);
+        const capturedPreviousValue = lowerValueToTemporary(builder, {
+          kind: 'LoadLocal',
+          place: {...previousValuePlace},
+          loc: exprLoc,
+        });
+
         // Store the new value to a temporary
         const updatedValue = lowerValueToTemporary(builder, {
           kind: 'BinaryExpression',
           operator: binaryOperator,
-          left: {...previousValuePlace},
+          left: {...capturedPreviousValue},
           right: lowerValueToTemporary(builder, {
             kind: 'Primitive',
             value: 1,
@@ -2633,7 +2639,7 @@ function lowerExpression(
           kind: 'LoadLocal',
           place: expr.node.prefix
             ? {...newValuePlace}
-            : {...previousValuePlace},
+            : {...capturedPreviousValue},
           loc: exprLoc,
         };
       }

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/bug-post-increment-assignment.expect.md

@@ -0,0 +1,79 @@
+
+## Input
+
+```javascript
+// @enableNewMutationAliasingModel:false
+/**
+ * Bug repro:
+ * Found differences in evaluator results
+ *   Non-forget (expected):
+ *   (kind: ok) {"count":3,"res":[0,1,2]}
+ *   Forget:
+ *   (kind: ok) {"count":3,"res":[1,2,3]}
+ *
+ * The post-increment operator `agg.count++` should return the value 
+ * BEFORE incrementing, but the compiler's optimization incorrectly
+ * causes the incremented value to be used.
+ */
+function Component(props) {
+  const items = [0, 1, 2];
+  return items.reduce((agg, item) => {
+    const current = agg.count++;
+    agg.res.push(current);
+    return agg;
+  }, {count: 0, res: []});
+}
+
+export const FIXTURE_ENTRYPOINT = {
+  fn: Component,
+  params: [{}],
+};
+
+```
+
+## Code
+
+```javascript
+import { c as _c } from "react/compiler-runtime"; // @enableNewMutationAliasingModel:false
+/**
+ * Bug repro:
+ * Found differences in evaluator results
+ *   Non-forget (expected):
+ *   (kind: ok) {"count":3,"res":[0,1,2]}
+ *   Forget:
+ *   (kind: ok) {"count":3,"res":[1,2,3]}
+ *
+ * The post-increment operator `agg.count++` should return the value
+ * BEFORE incrementing, but the compiler's optimization incorrectly
+ * causes the incremented value to be used.
+ */
+function Component(props) {
+  const $ = _c(1);
+  let t0;
+  if ($[0] === Symbol.for("react.memo_cache_sentinel")) {
+    const items = [0, 1, 2];
+    t0 = items.reduce(
+      _temp,
+
+      { count: 0, res: [] },
+    );
+    $[0] = t0;
+  } else {
+    t0 = $[0];
+  }
+  return t0;
+}
+function _temp(agg, item) {
+  agg.count = agg.count + 1;
+  const current = agg.count;
+  agg.res.push(current);
+  return agg;
+}
+
+export const FIXTURE_ENTRYPOINT = {
+  fn: Component,
+  params: [{}],
+};
+
+```
+

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/bug-post-increment-assignment.js

@@ -0,0 +1,26 @@
+// @enableNewMutationAliasingModel:false
+/**
+ * Bug repro:
+ * Found differences in evaluator results
+ *   Non-forget (expected):
+ *   (kind: ok) {"count":3,"res":[0,1,2]}
+ *   Forget:
+ *   (kind: ok) {"count":3,"res":[1,2,3]}
+ *
+ * The post-increment operator `agg.count++` should return the value 
+ * BEFORE incrementing, but the compiler's optimization incorrectly
+ * causes the incremented value to be used.
+ */
+function Component(props) {
+  const items = [0, 1, 2];
+  return items.reduce((agg, item) => {
+    const current = agg.count++;
+    agg.res.push(current);
+    return agg;
+  }, {count: 0, res: []});
+}
+
+export const FIXTURE_ENTRYPOINT = {
+  fn: Component,
+  params: [{}],
+};

compiler/packages/snap/src/SproutTodoFilter.ts

@@ -460,6 +460,7 @@ const skipFilter = new Set([
   'fbt/bug-fbt-plural-multiple-function-calls',
   'fbt/bug-fbt-plural-multiple-mixed-call-tag',
   'bug-invalid-phi-as-dependency',
+  'bug-post-increment-assignment',
   'bug-ref-prefix-postfix-operator',
 
   // 'react-compiler-runtime' not yet supported