Union type as argument to overloaded function

[mchenzero]

TypeScript Version: 2.0.3

Code

function fn(str: string): void;
function fn(num: number): void;
function fn(arg) {
  // do something with arg
}

function test(arg: string | number) {
  fn(arg); // from whatever source I get a (string | number)
}

Expected behavior: This code looks logically valid.

Actual behavior: It results in a compile error: error TS2345: Argument of type 'string | number' is not assignable to parameter of type 'number'. Type 'string' is not assignable to type 'number'.

[RyanCavanaugh]

Duplicate #1805