Discriminated Unions Incompatible with Intersection Types

[MikeyBurkman]

TypeScript Version: 2.0.3 AND nightly (2.1.0-dev.20161102)

Code

interface Foo {
 kind: 'foo';
 a: number;
}

interface Bar {
 kind: 'bar';
 b: string;
}

function test(x: Bar|Foo) {
 if (x.kind === 'foo') {
 console.log(x.a); // Valid, as expected
 }
}

interface Qux {
 c: boolean;
}

function test2(x: (Bar|Foo)&Qux) {
 if (x.kind === 'foo') {
 // Type of x should be Foo&Qux now, but is still (Bar|Foo)&Qux
 console.log(x.a); // Error
 }
}

Expected behavior:
In test2(), the discriminated union check should narrow the type down to (Foo)&Qux, letting you access property a, since a is guaranteed to be there if x.type === 'foo'.

Actual behavior:
No type narrowing is done whatsoever. The type of x remains (Foo|Bar)&Qux.

[ahejlsberg]

This looks like a duplicate of #9919, which was fixed in #11717. I can compile your example with no error in the nightly build and x is narrowed to Foo & Qux as expected. Can you reconfirm that there's an issue in the nightly?

[MikeyBurkman]

Anders Hejlsberg (@ahejlsberg) Ah I stand corrected. tsc on the command line does indeed work, it's just that it's still be reported as an error in VS Code. Not sure whether to report that as a different issue or not.

Also I do have another, different but related bug that is reproduceable on the command line on the nightly build :)