How to infer return type of a generic function with default types

Issue

Given the following code:

declare function test<A = number, B = string>(a: A, b: B): { a: A, b: B }

type T1 = ReturnType<typeof test>         // { a: unknown; b: unknown }
type T2 = ReturnType<typeof test<number>> // { a: number; b: string }

Why can't TS infer T1 correctly? Is it a bug? Are there any workarounds besides T2?

Solution

Typescript infers the type unknown correctly, since the types of A and B are not defined as long as the function is not called with arguments. You must define the types with extends like this:

declare function test<A extends number = number, B extends string = string>(a: A, b: B): { a: A, b: B }

Answered By - joas