TypeScript Version: 2.4.1

Code

Case 1

// overloaded functions
declare function f(a: number): number;
declare function f(a: string): string;

function g<T extends number | string>(a: T) {
    return f(a); // either of arguments is acceptable
//           ~
// Argument of type 'T' is not assignable to parameter of type 'string'.
//  Type 'string | number' is not assignable to type 'string'.
//    Type 'number' is not assignable to type 'string'.
}

Case 2

declare function f2<T>(a: Promise<T>): number;
declare function f2<T>(a: T): string;

function g2<T>(a: T) {
    const result: string = f2(a); // first overload is ignored, but the argument can be a Promise
    return result;
}

Expected behavior:

Case 1: compiles with no errors
Case 2: error that type string | number is not assignable to type string

Actual behavior:

Case 1: error

Argument of type 'T' is not assignable to parameter of type 'string'.
  Type 'string | number' is not assignable to type 'string'.
    Type 'number' is not assignable to type 'string'

Case 2: compiles without errors

Note

I remember here were some discussions about that, so that verifying all possible paths may result in N*M complexity (N overloads, M constituent types in unions). I could not find it.

The second case seems unsafe at all, because skips a possibly valid overload which may effect on return type. I expect that f2(a) would be of type number | string because either of these two overloads can play. It actually has the same result with a: any.