expectTypeOf allows an incorrect any value instead of the specified type

[kwangure]

When I use expectTypeOf, I expect it to fail when I pass any as input. For example, toBeString() should strictly only allow strings and not any other type...including any.

expectTypeOf(1).toBeString(); //  Fails 👍
expectTypeOf(1 as any).toBeString(); // Passes 👎

This is particular useful in cases when your types reason about any, I need to strictly know that a type will fail if its any or not. For example:

type IsAny<T> = 0 extends 1 & T ? true : false;

export type CustomType<T> = IsAny<T> extends true ? number: any;

If I use CustomType somewhere, I'd like to test expectTypeOf(value).toBeNumber(); and have it fail if it's any when I don't expect it to be.

This issue was originally opened at vitest-dev/vitest#4205.

[mmkal]

This will be fixed by #16. And looks to be a duplicate of #32. If you're using expect-type directly, you can try the next dist tag now. The version in vitest will be bumped by vitest-dev/vitest#4206. Simplest thing to do will just be to wait for that.

[mmkal]

Copy-pasted from docs added in #16, this will be the new behaviour:

.toBe... methods allow for types which extend the expected type:

expectTypeOf<number>().toBeNumber()
expectTypeOf<1>().toBeNumber()

expectTypeOf<any[]>().toBeArray()
expectTypeOf<number[]>().toBeArray()

expectTypeOf<string>().toBeString()
expectTypeOf<'foo'>().toBeString()

expectTypeOf<boolean>().toBeBoolean()
expectTypeOf<true>().toBeBoolean()

.toBe... methods protect against any:

const goodIntParser = (s: string) => Number.parseInt(s, 10)
const badIntParser = (s: string) => JSON.parse(s) // uh-oh - works at runtime if the input is a number, but return 'any'

expectTypeOf(goodIntParser).returns.toBeNumber()
// @ts-expect-error - if you write a test like this, `.toBeNumber()` will let you know your implementation returns `any`.
expectTypeOf(badIntParser).returns.toBeNumber()