fix(reactivity): allow async watchEffect callbacks

[Misrilal-Sah]

Issue link
[https://github.com//issues/14249

Description
This PR updates WatchEffect typing to allow async callbacks by accepting Promise in addition to void.

Why
watchEffect is documented and commonly used with async functions, but the previous type signature rejected this pattern and caused lint/type friction.

Changes
Updated WatchEffect type to support async callbacks.
Added dts regression coverage to ensure async watchEffect callbacks are accepted.

Summary by CodeRabbit

• New Features

  • watchEffect now supports async callbacks in addition to synchronous ones.

• Tests

  • Added type test coverage for async watchEffect callbacks with cleanup support.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: c34adcb8-c94b-4f0c-9a5c-5ecf48923be8

📥 Commits

Reviewing files that changed from the base of the PR and between fa23116 and af938c7.

📒 Files selected for processing (2)

• packages-private/dts-test/watch.test-d.ts
• packages/reactivity/src/watch.ts

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📝 Walkthrough

Walkthrough

This PR widens the WatchEffect type definition to permit async callbacks returning Promise<void> alongside synchronous callbacks, with an accompanying type test validating async watchEffect usage with cleanup functions.

Changes

Cohort / File(s)	Summary
Type Definition Widening packages/reactivity/src/watch.ts	Updated WatchEffect type to allow void | Promise<void> return type, enabling async callback support.
Type Test Addition packages-private/dts-test/watch.test-d.ts	Added test case for async watchEffect callback with onCleanup parameter invocation.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~3 minutes

Possibly related issues

• watchEffect typing does not support async callback #14249: Directly addresses the widening of WatchEffect type to support async callbacks with cleanup functions.

Suggested labels

ready to merge

Poem

🐰 Async effects now bloom so bright,
With promises held in watchful sight,
Cleanup functions dance in the async glow,
A type that flexes where effects flow! ✨

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title 'fix(reactivity): allow async watchEffect callbacks' clearly and concisely describes the main change: enabling async callback support in watchEffect by updating its type signature.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[Copilot]

Pull request overview

Updates Vue reactivity typings so watchEffect callbacks can be async (i.e., return a Promise<void>), matching documented/common usage and avoiding TS/ESLint “void-return expected” friction.

Changes:

• Broaden WatchEffect return type from void to void | Promise<void>.
• Add a DTS regression test ensuring async watchEffect callbacks type-check.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
packages/reactivity/src/watch.ts	Expands WatchEffect type to allow Promise<void> return.
packages-private/dts-test/watch.test-d.ts	Adds DTS coverage for async watchEffect usage (#14249).

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[edison1105]

duplicate of #14251