vi.fn(mockImplementation) does not expose implementation

[Codex-]

Describe the bug

When using vi.fn() to create a mock:

const fnWithMock = vi.fn(() => 1)

The provided mock should be obtainable via getMockImplementation, but it is not:

> fnWithMock.getMockImplementation()
undefined

Upon some quick investigation this appears to be because of how the call to enhanceSpy from fn assumes that no implementation has been provided.

If you set an implementation after the initialisation, then you get the expected behaviour as the setter sets implementation.

We could do something like this:

export function fn<TArgs extends any[] = any[], R = any>(
  implementation?: (...args: TArgs) => R,
): Mock<TArgs, R> {
  const enhancedSpy = enhanceSpy(tinyspy.internalSpyOn({ fn: () => {} }, 'fn'))
  if (implementation)
    enhancedSpy.mockImplementation(implementation)

  return enhancedSpy as Mock
}

This would prevent breaking how spyOn works already, if you're happy with this, I can make a PR.

There are some existing tests in jest-mock.ts, however these assert expect(spy.getMockImplementation()).toBe(undefined) when it should actually be defined.

Reproduction

https://stackblitz.com/edit/vitest-dev-vitest-vuwwus?file=test/fn.test.ts

System Info

System:
    OS: macOS 13.3.1
    CPU: (8) arm64 Apple M1
    Memory: 43.36 MB / 16.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 16.19.0 - ~/.asdf/installs/nodejs/lts-gallium/bin/node
    Yarn: 1.22.19 - ~/.asdf/installs/nodejs/lts-gallium/.npm/bin/yarn
    npm: 8.19.3 - ~/.asdf/plugins/nodejs/shims/npm
  Browsers:
    Chrome: 112.0.5615.137
    Firefox Developer Edition: 112.0
    Safari: 16.4
  npmPackages:
    @vitest/browser: workspace:* => 0.30.1 
    @vitest/coverage-c8: workspace:* => 0.30.1 
    @vitest/coverage-istanbul: workspace:* => 0.30.1 
    @vitest/ui: workspace:* => 0.30.1 
    vite: ^4.2.1 => 4.2.1 
    vitest: workspace:* => 0.30.1

Used Package Manager

npm

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