type guard does not narrow intersection over union

[siegebell]

TypeScript Version: 2.0.8
Code

interface A {tag: "a", a};
interface B {tag: "b", b};
interface C {c};
type ABC1 = (A & C) | (B & C);
let y : ABC1 = {tag:"a", a: "foo", c: "bar"}
if(y.tag === "a")
 y.a // this is accepted
type ABC2 = (A | B) & C;
let x : ABC2 = {tag:"a", a: "foo", c: "bar"}
if(x.tag === "a")
 x.a // but this is currently an error :(

Expected behavior:
After checking x.tag==="a", the type of x should be narrowed to A & C
Actual behavior:
There is no narrowing, so x.a results in a type error.

A workaround is given by ABC1, but in practice, this results in very long and redundant type definitions.

[siegebell]

Hmm. It seems that ABC1 and ABC2 are not even considered equivalent types:

x = y; // okay: y:ABC1 is a subtype of x:ABC2
y = x; // error

Is this intended behavior? I.e. is there a case where x and y might not be compatible?

[RyanCavanaugh]

This has been fixed by #11717

[siegebell]

Ryan Cavanaugh (@RyanCavanaugh) phew -- Thanks!