Cannot infer the return type of a generic function in a conditional type

[cshaa]

🔎 Search Terms

Conditional type inference, generic return type, type-level lambda, higher-kinded types, mapped types

🕗 Version & Regression Information

Present in TS 3.5.1 to 5.5.0-beta.

⚪️ Introduction

I need to write a piece of code that would transverse a deeply nested data structure and fetch required data. However, I would prefer to do it in a way that doesn't force me to couple it too strongly to the data structure's specific type. This would be easiest to achieve with a generic replacer. Trying to implement it, I came across a very unintuitive behavior: given a generic function and its type parameter, it is impossible to correctly infer its return type.

💻 Code

type Foo = (<T>(input: T) => T[]) extends ((a: string) => infer R) ? R : never;
// unknown[]

⏯ Playground Link

🙁 Actual behavior

Does not infer the type argument, replaces it with unknown.

🙂 Expected behavior

Should use the inferred type argument to correctly infer the return type: string[].

Additional information about the issue

It seems to me that adding inference here would be possible – after all if I actually call the function, the type argument does get infered, and so does the return type. The doors this would open are countless, as infering the return type of a generic function is essentially higher-kinded types in a trench coat.

Were this implemented, we could do things like these:

type GenericReturnType<F extends (arg: any) => any, P> = F extends (arg: P) => infer R ? R : never;

type Box<T> = { value: T };
type Mapped<T, F extends (arg: any) => any> = { [P in keyof T]: GenericReturnType<F, T[P]> };

type Boxified<T> = Mapped<T, <V>(v: V) => Box<V>>;

(Compare #27130)

Possibly Related Issues

#57351, #33594

[fatcerberus]

It seems to me that adding inference here would be possible – after all if I actually call the function, the type argument does get infered, and so does the return type.

That sentence makes this a duplicate of #40179.

higher-kinded types in a trench coat

This got a chuckle out of me. 😄

[typescript-bot]

This issue has been marked as "Duplicate" and has seen no recent activity. It has been automatically closed for house-keeping purposes.