mockRestore does not restore the original implementation

[andrew-w-ross]

Describe the bug

According to the docs mockRestore should restore the original implementation.

I would expect the following test to pass:

import { expect, test, vi } from 'vitest';

const testFn = vi.fn(() => true);

test('.mockRestore() should restore initial implementation', () => {
  expect(testFn()).toBe(true);

  testFn.mockReturnValue(false);
  expect(testFn()).toBe(false);

  testFn.mockRestore();
  expect(testFn()).toBe(true);
});

I get the following error on stack blitz:

AssertionError: expected undefined to be true // Object.is equality

It seems that instead of restoring the implementation it's wiped completely.
I'd expect that from mockReset not mockRestore.

Reproduction

StackBlitz Reproduction

System Info

System:
    OS: macOS 13.3.1
    CPU: (8) x64 Apple M1
    Memory: 25.78 MB / 16.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 18.12.1 - ~/.nvm/versions/node/v18.12.1/bin/node
    Yarn: 3.3.0 - ~/.nvm/versions/node/v18.12.1/bin/yarn
    npm: 8.19.2 - ~/.nvm/versions/node/v18.12.1/bin/npm
  Browsers:
    Chrome: 113.0.5672.92
    Safari: 16.4
  npmPackages:
    vitest: ^0.31.0 => 0.31.0

Used Package Manager

yarn

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• vitest-dev-vitest-kgywe2 (Open reproduction)