Generic mapped as clause regression in 4.7

[samhh]

Bug Report

🔎 Search Terms

mapped, generic, as clause

🕗 Version & Regression Information

• This changed between versions 4.6.4 and 4.7.2/4.7.3

⏯ Playground Link

Playground link with relevant code

💻 Code

The issue goes away if you remove the as Id<A> in Obj.

type Id<A> = A;

type Obj<A extends string> = {
  [K in A as Id<A>]: null
}

declare const getObj: <A extends string>() => Obj<A>

const f = <A extends 'a'>(): null => {
  const o = getObj<A>()
  return o['a' as A]
}

🙁 Actual behavior

1. Type 'Obj<A>[A]' is not assignable to type 'null'.
     Type 'Obj<A>["a"]' is not assignable to type 'null'.

🙂 Expected behavior

The compiler used to resolve o['a' as A] as null.

[nicolas377]

git bisecting locally off this repro landed the first bad commit at 8c060ee (#47889, #47794).

[nicolas377]

I also narrowed down to this line in the PR's diff. What to do with it, I'm unsure of, but I thought it might be of use.

[ahejlsberg]

This is working as intended. The commits above added checks that should have been present in the first place. The issue we have is that when a mapped type includes an as clause, we can't consistently reason about the constraint of that mapped type. In the particular example here it is obvious that the constraint is null because the as clause specifies an identity mapping and the template type null doesn't reference the iteration type K. However, in most cases the template type does reference the iteration type, and often it isn't even possible to construct a reverse mapping for the as clause type.

Also see comment here.

[typescript-bot]

This issue has been marked 'Working as Intended' and has seen no recent activity. It has been automatically closed for house-keeping purposes.