How to infer return type of function with provided arguments?

Question

Related to https://github.com/Microsoft/TypeScript/issues/26043

const f = <T extends any>(a: T) => a;

// How to implement ReturnTypeWithArguments ???

type r = ReturnTypeWithArguments<typeof f, number> // number

Answer 1

There is no syntax for applying type parameters to a generic function in type annotation. We can declare a function in which we invoke the generic function (which we won't use) and get the return value of that:

const f = <T extends any>(a: T) => a;

const fForNumber = () => f(0)
type r = ReturnType<typeof fForNumber> // number