Generic property type widened by conditional

[alexfoxgill]

TypeScript Version: 3.2.1

Search Terms: generic, conditional type, widened

Code

type A = { foo: "a" }
type B = { foo: "b" }
type C = A | B

type Resolve<T extends "a" | "b"> = "a" | "b" extends T ? never : T

type GenericResolve<T extends C> = Resolve<T["foo"]>

type X = Resolve<"a"> // "a"
type Y = Resolve<A["foo"]> // "a"
type Z = GenericResolve<A> // never

type ResolveWithoutCondition<T extends "a" | "b"> = T
type GenericResolveWithoutCondition<T extends C> = ResolveWithoutCondition<T["foo"]>

type ZZ = GenericResolveWithoutCondition<A> // "a"

Expected behavior:
Z should be "a"

Actual behavior:
Z is never

Playground Link: http://www.typescriptlang.org/play/#src=type%20A%20%3D%20%7B%20foo%3A%20%22a%22%20%7D%0D%0Atype%20B%20%3D%20%7B%20foo%3A%20%22b%22%20%7D%0D%0Atype%20C%20%3D%20A%20%7C%20B%0D%0A%0D%0Atype%20Resolve%3CT%20extends%20%22a%22%20%7C%20%22b%22%3E%20%3D%20%22a%22%20%7C%20%22b%22%20extends%20T%20%3F%20never%20%3A%20T%0D%0A%0D%0Atype%20GenericResolve%3CT%20extends%20C%3E%20%3D%20Resolve%3CT%5B%22foo%22%5D%3E%0D%0A%0D%0Atype%20X%20%3D%20Resolve%3C%22a%22%3E%20%2F%2F%20%22a%22%0D%0Atype%20Y%20%3D%20Resolve%3CA%5B%22foo%22%5D%3E%20%2F%2F%20%22a%22%0D%0Atype%20Z%20%3D%20GenericResolve%3CA%3E%20%2F%2F%20never%0D%0A%0D%0Atype%20ResolveWithoutCondition%3CT%20extends%20%22a%22%20%7C%20%22b%22%3E%20%3D%20T%0D%0Atype%20GenericResolveWithoutCondition%3CT%20extends%20C%3E%20%3D%20ResolveWithoutCondition%3CT%5B%22foo%22%5D%3E%0D%0A%0D%0Atype%20ZZ%20%3D%20GenericResolveWithoutCondition%3CA%3E%20%2F%2F%20%22a%22

Related Issues:

Could not find any

[jack-williams]

I think this has been fixed by #28718. Try the latest version of master and see if that works.

[ahejlsberg]

This is indeed the same issue as #28654 and has been fixed by #28718.

[typescript-bot]

This issue has been marked as a duplicate and has seen no activity in the last day. It has been closed automatic house-keeping purposes.