fix(compiler-core): parenthesise v-for key in v-memo cache guard

[luc122c]

Closes #15723.

The v-memo + v-for cache guard inlined the :key expression without parentheses, so a ternary key parsed as cond ? a : b && _isMemoSame(...) and _isMemoSame(undefined, _memo) was called on the first render. Wraps the key expression in parentheses and covers it with a test.

Summary by CodeRabbit

• Bug Fixes
  • Fixed v-memo cache reuse for v-for items with conditional keys, ensuring the key comparison is evaluated correctly.

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Advanced

Run ID: b6eb1d06-1d94-468b-bdc6-c334e7bfd8ab

📥 Commits

Reviewing files that changed from the base of the PR and between 4ab865a and 2bbeecc.

⛔ Files ignored due to path filters (1)

• packages/compiler-core/__tests__/transforms/__snapshots__/vMemo.spec.ts.snap is excluded by !**/*.snap

📒 Files selected for processing (2)

• packages/compiler-core/__tests__/transforms/vMemo.spec.ts
• packages/compiler-core/src/transforms/vFor.ts

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📝 Walkthrough

Walkthrough

The compiler now wraps keyed v-for expressions in parentheses when it generates the v-memo cache guard. A compiler test checks the generated guard with a conditional key expression.

Changes

v-memo key guard

Layer / File(s)	Summary
Parenthesize the key in the memo guard packages/compiler-core/src/transforms/vFor.ts, packages/compiler-core/__tests__/transforms/vMemo.spec.ts	The generated cache guard now parenthesizes the key expression. A compiler test checks the generated comparison for a conditional key.

Priority: ➖ Normal

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Bug fix · Severity of issue fixed: Medium

Suggested reviewers: edison1105

Merge Risk: ⚪ Minimal · up to 2bbee

No actionable merge-blocking risk remains in the reviewed change.

Architecture Summary

Architecture risk: 🔵 Low · up to 2bbee

The change affects 1 system.

Changed systems: packages/compiler-core

Architecture concerns
No architecture-level concerns identified.

Review details

Systems and components

• observed — packages/compiler-core (library) was modified; 2 changed files map to changed impact.

Before / after behavior

• observed — Modified behavior in packages/compiler-core/tests/transforms/vMemo.spec.ts: Added a test compiling a v-for element with the conditional key x ? x : y and memo dependencies [x, y === z]; it asserts that generated code compares _cached.key with the conditional key expression.
• observed — Modified behavior in packages/compiler-core/src/transforms/vFor.ts: The memo cache condition now parenthesizes keyExp on the right side of the _cached.key equality check; the prior generated comparison emitted keyExp without parentheses.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Linked Issues check	⚠️ Warning	The compiler change in packages/compiler-core/src/transforms/vFor.ts parenthesizes the :key expression and addresses the precedence defect in issue #15723. The added compiler test checks generated…	Add a runtime regression test for v-memo with v-for and a ternary :key. Verify first-render mounting and SSR rendering without an exception.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: parenthesizing the v-for key in the v-memo cache guard.
Out of Scope Changes check	✅ Passed	The whole pull request changes only the v-memo and v-for compiler guard and adds a focused compiler test. Both changes directly support issue #15723. No unrelated change is shown.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 2…

Full details: Linked Issues check

Explanation

The compiler change in packages/compiler-core/src/transforms/vFor.ts parenthesizes the :key expression and addresses the precedence defect in issue #15723. The added compiler test checks generated code for a ternary key. The issue also requires the component to render without a first-render exception, including under SSR, and identifies a runtime regression test as coverage. The PR summary reports no runtime or SSR test, so that behavior is not verified by an automated test.

• Fix all pre-merge checks with AI

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create a new PR

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commit: 2bbeecc

[github-actions]

Size Report

Bundles

File	Size	Gzip	Brotli
runtime-dom.global.prod.js	109 kB	41.3 kB	37 kB
vue.global.prod.js	168 kB	61.5 kB	54.6 kB

Usages

Name	Size	Gzip	Brotli
createApp (CAPI only)	50.8 kB	19.6 kB	18 kB
createApp	58.9 kB	22.7 kB	20.8 kB
createSSRApp	63.7 kB	24.7 kB	22.5 kB
defineCustomElement	65.2 kB	24.6 kB	22.5 kB
overall	73.5 kB	28 kB	25.5 kB