The simplified `MockInstance<T>` type does not work with function overloads

[xsjcTony]

Describe the bug

It won't work with function overloads, refer to the reproduction.

Reproduction

type error in v2
https://stackblitz.com/edit/vitest-dev-vitest-hlq3zj?file=test%2Fbasic.test.ts

No type error in v1, but seems the other overload signature is being ignored
https://stackblitz.com/edit/vitest-dev-vitest-cayu3q?file=test%2Fbasic.test.ts

System Info

System:
    OS: Windows 11 10.0.22631
    CPU: (8) x64 11th Gen Intel(R) Core(TM) i5-1135G7 @ 2.40GHz
    Memory: 7.79 GB / 31.69 GB
  Binaries:
    Node: 20.11.0 - C:\Program Files\nodejs\node.EXE
    npm: 10.8.1 - C:\Program Files\nodejs\npm.CMD
    pnpm: 9.5.0 - ~\AppData\Local\pnpm\pnpm.EXE
  Browsers:
    Edge: Chromium (126.0.2592.87)
    Internet Explorer: 11.0.22621.3527

Used Package Manager

npm

Validations

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• The provided reproduction is a minimal reproducible example of the bug.

[hi-ogawa]

Thanks for the report. I haven't thought about this case.

Similar code on Jest has no type errors, but it looks like it still has some limitations with function overloads.

test("overload", () => {
  // they have dedicated `Spied` type to align `spyOn` return type.
  // so assignment works fine
  let spy: jest.Spied<Element["scrollTo"]>
  spy = jest.spyOn(Element.prototype, "scrollTo");

  // but only 2nd overload argument types
  spy.mock.calls satisfies [x: number, y: number][];

 
  // for `Mock`, probably Vitest and Jest work same.
  let mockFn: jest.Mock<Element["scrollTo"]>;
  mockFn = jest.fn(Element.prototype.scrollTo);
  
  mockFn(0, 1); // 2nd overload ok
  
  // @ts-expect-error
  mockFn({ top: 0 }); // 1st overload error
})

Probably this comes down to Parameters picking up only one of the overload

type OverloadFn = {
  (x: number, y: number): void;
  (someOptions?: { top?: number }): void;
}

// this becomes `[x: number, y: number]`
type OverloadArgs = Parameters<Element["scrollTo"]>

[xsjcTony]

That's true. Like I provided above, even in v1, because that I'm using Parameters<T> manually, hence there's no type error because only 1 overload is picked up, but truly it is the limitation, since .mockImplementation() won't support another overload in that case.

[xsjcTony]

Seems like a hard limitation there, emmm,..... 🤦‍♀️

See this also: microsoft/TypeScript#32164 (comment)

[xsjcTony]

But I think at least we should try to find a way to avoid the type error there, and maybe document the TS limitation on function overloads

[hi-ogawa]

This type mismatch is probably because we split types here MockInstance<(this: R, ...args: A) => R>. If we can somehow preserve it as MockInstance<T[M]>, then typescript won't probably complain.

vitest/packages/spy/src/index.ts

Lines 401 to 408 in 99a12ae

	export function spyOn<T, M extends Classes<Required<T>> | Methods<Required<T>>>(
	obj: T,
	methodName: M
	): Required<T>[M] extends
	| { new (...args: infer A): infer R }
	| ((...args: infer A) => infer R)
	? MockInstance<(this: R, ...args: A) => R>
	: never