Inconsistent Treatment of Shorthand Functions

[ChrisGrohHS]

TypeScript Version: 2.0.3

Code

// A *self-contained* demonstration of the problem follows...
function testFunction(x: string | Function): any {
    if (typeof x === 'string') {
        return {
            key: function (a: any, b: any) {
                // Fine, x is string
                return a[x];
            }
        };
        return {
            key(a: any, b: any) {
                // Fails, x is string | Function here
                return a[x];
            }
        };
    }
}

Expected behavior:
Compiles fine

Actual behavior:
Error: An index expression argument must be of type 'string', 'number', 'symbol', or 'any'.

[mhegazy]

duplicate of #10501