Repository navigation
Discriminated Unions Incompatible with Intersection Types #11989
Description
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.
Activity
ahejlsberg commentedon Nov 2, 2016
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
xis narrowed toFoo & Quxas expected. Can you reconfirm that there's an issue in the nightly?MikeyBurkman commentedon Nov 2, 2016
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 :)