Mocking implementation with mockImplementation allows incorrect return types

[Namchee]

Describe the bug

Function mocking through .mockImplementation(Once) allows an incorrect Promise-wrapped type to be returned. For example, a function that returns number can be mocked to return Promise<number> as opposed to number. This is possible because .mockImplementation(Once) accepts an union-type of number | Promise<number> as the return type for the implementation.

Jest, a test runner with similar API doesn't allow Promise<number> to be returned by the mocked implementation and only allows number.

Reproduction

https://stackblitz.com/edit/vitest-dev-vitest-bayh5s?file=index.test.ts

System Info

System:
  OS: Linux 5.19 Ubuntu 22.04.2 LTS 22.04.2 LTS (Jammy Jellyfish)
  CPU: (12) x64 12th Gen Intel(R) Core(TM) i7-1255U
  Memory: 9.74 GB / 15.26 GB
  Container: Yes
  Shell: 5.8.1 - /usr/bin/zsh
Binaries:
  Node: 18.16.1
  npm: 9.5.1
  pnpm: 8.6.1
npmPackages:
  vite: ^4.3.9 => 4.3.9 
  vitest: ^0.32.4 => 0.32.4

Used Package Manager

pnpm

Validations

• Follow our Code of Conduct
• Read the Contributing Guidelines.
• Read the docs.
• Check that there isn't already an issue that reports the same bug to avoid creating a duplicate.
• Check that this is a concrete bug. For Q&A open a GitHub Discussion or join our Discord Chat Server.
• The provided reproduction is a minimal reproducible example of the bug.

[stackblitz]

Start a new pull request in StackBlitz Codeflow.

• vitest-dev-vitest-bayh5s (Open reproduction)