Infer works differently for interface and type alias

[UnVuTWFu]

🔎 Search Terms

infer, index signature

🕗 Version & Regression Information

• This won't compile.
• This is the behavior in every version I tried,.

⏯ Playground Link

Playground link with relevant code

💻 Code

interface IntA {
    id: string;
}

type TypeA = {
    id: string
}

type Get<T> = T extends { [index: string]: infer A } ? true : false;
type Res1 = Get<IntA>
// Res1 is false
type Res2 = Get<TypeA>
// Res2 is true

🙁 Actual behavior

When using interface,I cannot get type of A(as example above).But if instead of interface,the infer works.

🙂 Expected behavior

I'd expect the same behaviour in both cases,or I can control pass type or interface.

Additional information about the issue

maybe related #15300

[mkantor]

maybe related #15300

This is a direct consequence of #15300. You're trying to infer out of an index signature, but interfaces don't have implicit index signatures as described in #15300.

If you add an explicit index signature to IntA then your code works.

[UnVuTWFu]

Thanks so rapid response.For my actual use,I want pass a generic type and extract specific types,just like this

type Generic = { [name: string]: any }
type ExtractARule<R extends Generic> = R extends { [name: string]: infer VALUE } ? VALUE extends string | number ? R : never : never
type ExtractBRule<R extends Generic> = R extends { [name: string]: infer VALUE } ? VALUE extends boolean ? R : never : never

As far as I konw,if generic type = {[index:string]:any},interface and type will have same behaviour shown in playground

For my use,type works well,but interface only return never without any error or warning.Maybe there should have a way to declare one type only need type or interface if program rely on this behaviour.

[mkantor]

Are you looking for something like this?

type ExtractARule<R> = R[keyof R] extends string | number ? R : never

[MartinJohns]

🕗 Version & Regression Information

• This won't compile.

Compiles just fine.