Overload with type unions

[domoritz]

TypeScript Version: 3.1.0-dev.201xxxxx

Search Terms:

overload, type union

Code

function f(a: number): number;
function f(a: string): string;
function f(a: string | number): string | number { 
  return a;
}

function foo(value: string | number) {
  f(value)
}

Expected behavior:

Should be valid.

Actual behavior:

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

Playground Link: https://www.typescriptlang.org/play/#src=function%20f(a%3A%20number)%3A%20number%3B%0D%0Afunction%20f(a%3A%20string)%3A%20string%3B%0D%0Afunction%20f(a%3A%20string%20%7C%20number)%3A%20string%20%7C%20number%20%7B%20%0D%0A%20%20return%20a%3B%0D%0A%7D%0D%0A%0D%0Afunction%20foo(value%3A%20string%20%7C%20number)%20%7B%0D%0A%20%20f(value)%0D%0A%7D%0D%0A

[domoritz]

Duplicate of #14107