Type inference for `Function.call` return type on generic functions

🔎 Search Terms

type infer function call generic

⏯ Playground Link

Playground link with relevant code

💻 Code

function numberEcho(x: number): number {
    return x;
}

function genericEcho<T>(x: T): T {
    return x;
}

// This works
const resultNumber: number = numberEcho.call(this, 1);

// Type Error: Type 'unknown' is not assignable to type 'string'.(2322)
const resultString: string = genericEcho.call(this, 'hello');

🙁 Actual behavior

Type inference of genericEcho.call return type failed even when all the argument types are known at call site.

Of course explicitly hinting the return type would work but it's not good:

const resultString: string = genericEcho.call<typeof this, [string], string>(this, 'hello');

🙂 Expected behavior

I would expect type inference of generic function return type to work if all arguments types are known.

I think #40179 is related but the example I provided here is much more common since it exists in standard library.

Start with the linked TypeScript Playground and compare the inferred return types for the direct and generic Function.call examples. Done means the generic call infers the argument type and accepts the result as a string, with the existing behavior still covered by an appropriate regression check.