Unclear what is T[keyof T] for a generic T assignable to

[rkirov]

Search Terms:
T[keyof T]

Code

class Unbounded<T> {
  foo(x: T[keyof T]) {
    let y: {}|undefined|null = x;
  }
}

Expected behavior:
Not absolutely sure, but I would like to be able to reason about T[keyof T] in terms of the types I already known - either it should behave like {} | null | undefined or like never. Or maybe I shouldn't be allowed to write that type.

Actual behavior:
Type 'T[keyof T]' is not assignable to type '{} | null | undefined'.

We noticed this during upgrading TS2.6 to TS2.7. It appears in TS2.6 this assignment was allowed for if one wrote Bounded<T extends {}>, where as in 2.7 and 2.8 that makes no difference. As expected when then lower bound has actual keys, they are used.

P.S. I miss the TS spec.

[mhegazy]

It should be assignable to the top type, everything should (void excluded).

[rkirov]

github search for T[keyof T] has failed me, but accidentally I stumbled on #21368 which seems like a duplicate of this.

The issue #21368 is closed, but I feel it didn't reach its full resolution, so I am keeping this open until further clarification from TS team.