Indexed type using in

[ersimont]

Hello, and thank you for the expect-type library!

I ran into what might be an issue. Or maybe I misunderstand something. Should these pass:

    const normal: { [x: string]: Date } = {};
    const usingIn: { [x in string]: Date } = {};
    expectTypeOf(normal).toEqualTypeOf(usingIn);
    expectTypeOf(usingIn).toEqualTypeOf(normal);

They both show this error:

TS2345: Argument of type '{ [x: string]: Date; }' is not assignable to parameter of type 'never'.

[mmkal]

Hi @ersimont - thanks for the report. I hadn't considered this case, but my first instinct is to say that .toEqualTypeOf should be as strict as possible in any doubt. So I am ok with the fact these showing an error for now.

A couple other things worth noting:

• .toMatchTypeOf passes in both cases
• the typescript-eslint rule consistent-indexed-object-style autofixes normal in your example with Record<string, Date> which makes the tests as you have them show no errors, so arguably if following "best practices" this is a non-issue.

You have highlighted a UX problem though - the error message could be more helpful. Unfortunately TypeScript doesn't currently allow for dynamic type errors (see mmkal/ts#152) so in general there is some debugging required when expect-type finds any errors. But I'll keep this open and see if I can find a way to make the error message give a better hint of where to start looking/what it's actually highlighting.