Conditional Types and Narrowing Type Fail to Return

[MrDesjardins]

TypeScript Version: 2.8-rc

Search Terms:
typescript Conditional Types narrow type primitive union
Code

type SwapType<T extends number | string> = T extends number ? string : number;

function swapper<T extends number | string>(id: T): SwapType<T>{
 switch (typeof id) {
		case "number": 
			return id.toString();
		case "string": 
			return Number(id);
		default:
			throw new TypeError("Not supported");
	}
}

const idNumber = swapper("123");
const idString = swapper(123);
console.log(`Number ${typeof idNumber} and string ${typeof idString}`);

Expected behavior:
To be able to return the narrowed type from the union and have the compiler not mentioning that the returned type is not assignable the type that has conditional type.
Actual behavior:
The compiler produce this error:

src/conditionalType.ts(6,4): error TS2322: Type 'string' is not assignable to type 'SwapType<T>'.
src/conditionalType.ts(8,4): error TS2322: Type 'number' is not assignable to type 'SwapType<T>'.

The generated JavaScript is good.

Playground Link:
Link

[jack-williams]

Related comment: here

[RyanCavanaugh]

There's no possible way we could soundly reason about this in anything except the most trivial special cases.