Selecting a case from a discriminated union using intersection does not work

[rklaehn]

TypeScript Version: 2.7.0-dev.201xxxxx

Search Terms:
discriminated union, narrowing, intersection

Code

type DU = { t: '1', a: 2 } | { t: '2', c: 3 }
type C1 = DU & { t: '1' }
declare const foo: C1
foo.a // <= does not compile

Expected behavior:
C1 should be inferred to { t: '1', a: 2 }, so it should be clear that a can be accessed

Actual behavior:
C1 is inferred to ({ t: "1"; a: 2; } & { t: "1"; }) | ({ t: "2"; c: 3; } & { t: "1"; })

Note the impossible type ({ t: "2"; c: 3; } & { t: "1"; }) is not removed!

Playground Link:
link

Related Issues:

[DanielRosenwasser]

I believe we discussed this at #18339. I believe the issue was two-fold:

1. Some of these intersection types are vacuous, but it is computationally intensive to try to discover which types are uninhabitable.
2. It becomes difficult to find out just how you got to a never type when you have an uninhabitable intersection.

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.