TypeScript 2.0.3 narrows enum type and causes compile error

[dsehnal]

TypeScript Version: 2.0.3

Code

enum Enum { A, B }
function modify(x: { a: Enum }) { x.a = Enum.B; }
function doStuff() {
    let x = { a: Enum.A };
    if (x.a === Enum.A) {
        modify(x);
        if (x.a === Enum.B) {
            // ...
        }
    }
}

Expected behavior:

Expected to compile, code was working in version 1.8.4.

Actual behavior:

Error at line if (x.a === Enum.B): Operator '===' cannot be applied to types 'Enum.A' and 'Enum.B'.

[mhegazy]

looks like a duplicate of #10665. please see #10665 (comment) for more details.