expectType generic causes incorrect inference

[mmkal]

expectType doesn't work properly with generic functions. Here's a repro:

import {expectType} from 'tsd'

declare const inferrable: <T = 'SomeDefaultValue'>() => T

expectType<number>(inferrable()) // passes, should fail

I'm not sure if it's possible to fix given the API design, but here's the same test using expect-type which fails as it should.

import {expectTypeOf} from 'expect-type'

declare const inferrable: <T = 'SomeDefaultValue'>() => T

expectTypeOf(inferrable()).toEqualTypeOf<number>() // fails as expected, because `inferrable()` returns type `'SomeDefaultValue'`

[ilchenkoArtem]

I have this issue too

[tommy-mitchell]

Added a test. @sindresorhus do you have any idea how to go about fixing this? From the conversation in dot-prop it seems no, but it could be worth further looking into.

[sindresorhus]

do you have any idea how to go about fixing this?

No

[sindresorhus]

Added a test

Can you submit it it (commented out)?

[tommy-mitchell]

By submit it do you mean as a PR?

[sindresorhus]

Yes

[tommy-mitchell]

See #196 for discussion on this.

[SamVerschueren]

Given how TypeScript and generics work, I think currently the only way of solving this is by storing the result of the inferrable() call in a variable first

declare const inferrable: <T = 'SomeDefaultValue'>() => T

const val = inferrable();

expectType<number>(val);

TypeScript forces the generic to fit in the argument. And there doesn't seem to be a way to prevent this. It would be fixed by something like expectNumber() or expectString(). But if you have much more complex types, tsd can't offer an expectXXX for everything 🤷 . I'll think about a possible solution 🤔 .