Generic type argument cannot be inferred by nested callback return type

[Igorbek]

TypeScript Version: 1.8.10

Code

declare function a<T>(f: (x: T) => ((y: T) => T)): T;

a<number>(x => y => 1); // works
a(x => y => 1);         // infers T as { }
a<string>(x => y => 1); // error, as expected

Expected behavior:
In a(x => y => 1), T is inferred as number based on nested callback return type.
As shows a<string>(x => y => 1) example, the just cannot be anything else.

Actual behavior:
In a(x => y => 1), T is inferred as { }.

[thorn0]

A similar (but not the same) situation: http://stackoverflow.com/questions/35613446/type-inference-for-functions-passed-to-generic-function-wrappers-like-debounce

[thorn0]

You can simplify your issue:

declare function a<T>(f: (x: T) => T): T;

a<number>(x => 1); // works
a(x => 1); // infers T as { }

[Igorbek]

Thanks, then it's dup of #9659. Closing this.