Vitest 4 mock types don't allow class implementation

[notaphplover]

Describe the bug

Docs suggests classes as contructor mock implementations:

const cart = {
  Apples: class Apples {
    getApples() {
      return 42
    }
  }
}

const Spy = vi.spyOn(cart, 'Apples')
  // with a function keyword
  .mockImplementation(function () {
    this.getApples = () => 0
  })
  // with a custom class
  .mockImplementation(class MockApples {
    getApples() {
      return 0
    }
  })

const mock = new Spy()

However, it seems mock types do not allow providing such implementations

Reproduction

Consider this sample code:

import { vitest, type Mocked, type Mock } from 'vitest';

class Foo {
  constructor(public readonly bar: string) {}

  public getBar(): string {
    return this.bar;
  }
}

class FooMock implements Mocked<Foo> {
  readonly barMock: Mock<() => string> = vitest.fn();

  public get bar(): string {
    return this.barMock();
  }

  public getBar: Mock<() => string> = vitest
    .fn()
    .mockImplementation(() => this.barMock());
}

vitest.mocked(Foo).mockImplementation(FooMock);
vitest.mocked(Foo).mockImplementation(Foo);

Typescript complains with the following errors:

Argument of type 'typeof FooMock' is not assignable to parameter of type '(bar: string) => Foo'.
  Type 'typeof FooMock' provides no match for the signature '(bar: string): Foo'.

Argument of type 'typeof Foo' is not assignable to parameter of type '(bar: string) => Foo'.
  Type 'typeof Foo' provides no match for the signature '(bar: string): Foo'.

Repro available here

System Info

System:
    OS: Linux 6.14 Ubuntu 24.04.3 LTS 24.04.3 LTS (Noble Numbat)
    CPU: (16) x64 AMD Ryzen 9 4900HS with Radeon Graphics
    Memory: 31.22 GB / 38.60 GB
    Container: Yes
    Shell: 5.2.21 - /bin/bash
  Binaries:
    Node: 24.10.0 - /home/bob/.nvm/versions/node/v24.10.0/bin/node
    npm: 11.6.1 - /home/bob/.nvm/versions/node/v24.10.0/bin/npm
    pnpm: 10.19.0 - /home/bob/.nvm/versions/node/v24.10.0/bin/pnpm
  Browsers:
    Chrome: 141.0.7390.122
    Firefox: 144.0
    Firefox Developer Edition: 144.0

Used Package Manager

npm

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.

[notaphplover]

After some trial and error, it seems vitest.mocked does returns some unexpected types:

vitest.mocked(Foo) returns MockInstance<(bar: string) => Foo> instead of MockInstance<typeof Foo>. Keeping this in mind:

(vitest.mocked(Foo) as unknown as MockInstance<typeof Foo>).mockImplementation(
  FooMock
);

Is a workaround to deal with this issue until it's solved.