Describe the bug

Here's how Vitest docs recommend declaring types for custom matchers.

interface CustomMatchers<R = unknown> {
  toBeFoo: () => R
}

While this works with symmetric matchers (expect(a).toBeFoo()), when you apply this to an asymmetric one, the unknown return type of the matcher will result in a type error.

interface O {
  s: string
}

expect({ s: 'foo' }).toEqual<O>({ s: expect.toBeFoo() })

Type 'unknown' is not assignable to type '{ s string; }'.ts(2322)

Suggested solutions

1. Consider defaulting to any instead of unknown. The two are not the same and are not interchangeable^[1]. Here, it looks like any may be a better fit.

In fact, I think it should've been any from the start. The matcher itself cannot infer or guarantee a type of the value (that'd be out of its scope). The best it can do is be anything.

Reproduction

https://stackblitz.com/edit/vitest-dev-vitest-2narajwa?file=vite.config.ts,test%2Fbasic.test.ts&initialPath=__vitest__/

Go to test/basic.test.ts and see the type error when asserting on the s property. It must not be there.

System Info

Irrelevant.

Used Package Manager

npm

Validations

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