Generic return type of generic function gets not resolved correctly with *typeof*

[lochbrunner]

TypeScript Version: 2.4.2

Code

function foo<T>(input: T): T {
    if (typeof input === 'number') {
        return 10;
    }
    if (typeof input === 'string') {
        return 'Hallo';
    }
    return input;
}

Expected behavior:

No compiler error.

Actual behavior:

Type '10' is not assignable to type 'T'.
Type '"Hallo"' is not assignable to type 'T'.

[aluanhaddad]

10 is assignable to type number.
When the first if condition is met, we know that, whatever T is, it is also assignable to type number.
This does not imply that 10 is assignable to T.

[mhegazy]

Narrowing currently works on a single variable, i.e. type of input inside these blocks would be number & T or string & T, but it does not narrow related types, i.e. T itself to a number or string. doing such non-local narrowing is fairly expensive.

[mhegazy]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.