TypeScript errors when mocking overloaded functions

[Tobbe]

Describe the bug

I found #6085 and have a very similar issue.
So I thought it would have been solved by #6086. But unfortunately I'm still running into issues.

This actually looks to be a regression introduced with 2.0.3
Here it is without any TS errors with 2.0.2 https://stackblitz.com/edit/vitest-dev-vitest-vmu2hy?file=test%2Fbasic.test.ts
And here it is with 2.0.3: https://stackblitz.com/edit/vitest-dev-vitest-huxpgg?file=test%2Fbasic.test.ts

This is my code (also available at the above StackBlitz links)
(With inlined error messages)

And in text form, for easier Google indexing:

With string

vi.spyOn(fs, 'readFileSync').mockImplementation(() => {
  return 'string'
})

Error:

Argument of type '() => string' is not assignable to parameter of type '{ (path: PathOrFileDescriptor, options?: { encoding?: null | undefined; flag?: string | undefined; } | null | undefined): Buffer; (path: PathOrFileDescriptor, options: BufferEncoding | { ...; }): string; (path: PathOrFileDescriptor, options?: (ObjectEncodingOptions & { ...; }) | ... 2 more ... | undefined): string |...'.
  Type 'string' is not assignable to type 'Buffer'.

Trying Buffer instead, as that's what suggested by the error message

vi.spyOn(fs, 'readFileSync').mockImplementation(() => {
  return Buffer.from('string')
})

Error:

Argument of type '() => Buffer' is not assignable to parameter of type '{ (path: PathOrFileDescriptor, options?: { encoding?: null | undefined; flag?: string | undefined; } | null | undefined): Buffer; (path: PathOrFileDescriptor, options: BufferEncoding | { ...; }): string; (path: PathOrFileDescriptor, options?: (ObjectEncodingOptions & { ...; }) | ... 2 more ... | undefined): string |...'.
  Type 'Buffer' is not assignable to type 'string'.

Trying the combination of Buffer | string as that's what the actual implementation has

vi.spyOn(fs, 'readFileSync').mockImplementation(() => {
  return 'string' as Buffer | string
})

Error:

Argument of type '() => Buffer | string' is not assignable to parameter of type '{ (path: PathOrFileDescriptor, options?: { encoding?: null | undefined; flag?: string | undefined; } | null | undefined): Buffer; (path: PathOrFileDescriptor, options: BufferEncoding | { ...; }): string; (path: PathOrFileDescriptor, options?: (ObjectEncodingOptions & { ...; }) | ... 2 more ... | undefined): string |...'.
  Type 'string | Buffer' is not assignable to type 'Buffer'.
    Type 'string' is not assignable to type 'Buffer'.

Reproduction

https://stackblitz.com/edit/vitest-dev-vitest-huxpgg?file=test%2Fbasic.test.ts

System Info

System:
    OS: macOS 14.1.1
    CPU: (12) arm64 Apple M3 Pro
    Memory: 53.80 MB / 36.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 20.15.1 - /usr/local/bin/node
    Yarn: 4.1.1 - /usr/local/bin/yarn
    npm: 10.8.2 - /usr/local/bin/npm
    bun: 1.1.6 - /opt/homebrew/bin/bun
  Browsers:
    Safari: 17.1

Used Package Manager

yarn

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