Sync/Async mix return types with Generics not allowed about typescript HOT 1 OPEN

Comments from @gabritto and @weswigham respectively:

probably we mess up inference when unifying the two signatures before comparing them

yes, that's the problem: we infer type argument string for the derived T, so the return types we compare are string and T | Promise

Specifically: we infer T_sub = T_super | Promise<T_super> for T_sub when instantiating the subclass signature in the context of the superclass signature, and this result fails, because T_super | Promise<T_super> doesn't extend string (because of the Promise bit), so we fall back to the constraint of string. Which is actually entirely unhelpful here - this was only ever going to work to unify the type parameters between the signatures if inference succeeded.

This failure mode maybe suggests a method of improvement, though - if inference would fail when you try to infer a union, you can try each member of the union and pick one of those, instead - probably still not perfect, but could fix this case

from typescript.