Edit: keyof works as I thought; mapped types are just broken in interfaces. I also clarified a few things.

Allowing negated types and F-bounded mapped type variables would solve several issues:

• Subtraction types: Subtraction types #4183 - The proposed A - B could be encoded as A & !B as per this proposal.
• Mapped conditional types: Mapped conditional types #12424 - For key-based conditions, this + F-bounded mapped types would allow that to be solved without explicit support.
• Promises - Having negated types makes it possible to prevent thenables from being used as generic Promise parameters.

Here's my proposal:

1. Allow negated types, using !T to represent the set of all types that do not conform to T (i.e. not assignable to T). This is pretty simple to digest.

2. Expand mapped types to allow the key variable to be F-bounded, like in {[P in Keys<T, P>]: ...}.

The first is sufficient to cover subtraction types and Promise type issues, and the other two make mapped conditional types possible.