Generic type parameter loses defined constraint

[dragomirtitian]

Bug Report

🔎 Search Terms

type parameter constraint

🕗 Version & Regression Information

• This changed between versions 4.9.0-dev.20220914 and 4.9.0-dev.20220915

⏯ Playground Link

Playground link with relevant code

💻 Code

export type TypeA = {
    A: 'A',
    B: 'B',
}
export type TypeB = {
    A: 'A',
    B: 'B',
    C: 'C',
}

// Fails to index TypeA with T
type R<T extends keyof TypeA> =
    T extends keyof TypeB ? [TypeA[T], TypeB[T]] : never

// This one works
type R2<T extends PropertyKey> =
    T extends keyof TypeA ?
    T extends keyof TypeB ?
        [TypeA[T], TypeB[T]] : never: never

🙁 Actual behavior

In R, TypeA[T] causes an error: Type 'T' cannot be used to index type 'TypeA'.

🙂 Expected behavior

TypeA[T] should work since T is by definition constrained to be a keyof TypeA

[RyanCavanaugh]

@typescript-bot bisect this

[RyanCavanaugh]

@typescript-bot bisect good v4.8.2 bad main

[typescript-bot]

The change between v4.8.2 and main occurred at 4110b80.

[Andarist]

This regression is specifically caused by including !(constraint.flags & TypeFlags.Never) at this line:
https://github.com/microsoft/TypeScript/pull/50735/files#diff-d9ab6589e714c71e657f601cf30ff51dfc607fc98419bf72e04f6b0fa92cc4b8R19523

In this very case, the source is "C" & T and the target is "A" and the constraint of T is keyof A. This all comes from comparing such source keyof TypeB & T and the target keyof TypeA. The problem is that in this instance the member of the union ("C" & T) is meant to be reduced to never and we want to leverage the fact that never is assignable to just anything. Effectively we want to eliminate this member from the union.

So it seems that the added condition should be only added conditionally - but I don't know what that conditions should be. I've poked around this and tried a couple of things but couldn't make it work.