Type guards don't work with interface members

[notenoughneon]

TypeScript Version:

1.8.9

Code

interface I {
    m: string | {a: string};
}
var x: I;
var y: string;
if (typeof x.m === 'string') {
    y = x.m;
} else {
    y = x.m.a;
}

Expected behavior:
no compile errors

Actual behavior:
test.ts(7,5): error TS2322: Type 'string | { a: string; }' is not assignable to type 'string'.
Type '{ a: string; }' is not assignable to type 'string'.
test.ts(9,13): error TS2339: Property 'a' does not exist on type 'string | { a: string; }'.

Pulling x.m out into a separate variable causes the type guard to work as expected.

[yortus]

Duplicate of #3812?

[notenoughneon]

Yes, I think it may be a duplicate. Sorry!

[RyanCavanaugh]

Yep, this should work once #3812 is fixed