fix: correctly compile post-increment assignment

[everettbu]

Mirror of facebook/react#35304
Original author: tennisleng

---

Fixes #35205. Explicitly captures the previous value of the member expression in a temporary before the update occurs, ensuring that post-increment operations return the original value.

[greptile-apps]

Greptile Overview

Greptile Summary

Fixes post-increment operator compilation by explicitly capturing the pre-increment value in a separate temporary before the store operation occurs. This prevents aliasing issues where obj.property++ would incorrectly return the incremented value instead of the original value.

• Fixed UpdateExpression handling in BuildHIR.ts by adding capturedPreviousValue temporary that loads from previousValuePlace before mutation
• Added test case reproducing the bug where agg.count++ should return [0,1,2] but was returning [1,2,3]
• Test added to skip list as expected output file shows incorrect behavior and may need regeneration

Confidence Score: 3/5

• This PR is mostly safe but has one concerning issue with the test expectation file
• The BuildHIR.ts fix correctly addresses the aliasing issue by capturing the pre-increment value. However, the .expect.md file shows incorrect compiled output (returns incremented values instead of pre-increment values), suggesting the test expectations need to be regenerated or the fix may need additional work
• Pay close attention to bug-post-increment-assignment.expect.md - the expected output still demonstrates the bug rather than showing the correct behavior

Important Files Changed

File Analysis

Filename	Score	Overview
compiler/packages/babel-plugin-react-compiler/src/HIR/BuildHIR.ts	4/5	Fixed post-increment to capture pre-increment value in separate temporary, preventing aliasing issues
compiler/packages/babel-plugin-react-compiler/src/tests/fixtures/compiler/bug-post-increment-assignment.expect.md	2/5	Expected output shows incorrect behavior - needs regeneration after BuildHIR fix is complete

[greptile-apps]

_{4 files reviewed, 1 comment}

_{Edit Code Review Agent Settings | Greptile}

[greptile-apps]

logic: Expected output still shows incorrect behavior: const current = agg.count assigns the incremented value (1, 2, 3) instead of pre-increment values (0, 1, 2). Expectation file may need regeneration after fix is verified.

Suggested change

	agg.count = agg.count + 1;
	const current = agg.count;
	const current = agg.count - 1;
	agg.count = agg.count + 1;

Prompt To Fix With AI

This is a comment left during a code review.
Path: compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/bug-post-increment-assignment.expect.md
Line: 67:68

Comment:
**logic:** Expected output still shows incorrect behavior: `const current = agg.count` assigns the incremented value (1, 2, 3) instead of pre-increment values (0, 1, 2). Expectation file may need regeneration after fix is verified.

```suggestion
  const current = agg.count - 1;
  agg.count = agg.count + 1;
```

How can I resolve this? If you propose a fix, please make it concise.