Promise.then should use this return type

[saschanaz]

class Subclass<T> extends Promise<T> { }
new Subclass<void>((resolve, reject) => resolve()).then() instanceof Subclass
// true

[weswigham]

Problem: there's no way to rebind the generic parameters of the this type to match the return type of the function passed to then - and, conceptually, a given subclass may not even be generic.

[kitsonk]

I realised that this is directly related to #6223. In a theory, not being able to express some sort of generic resolution of polymorphic this will cause challenges.

[zpdDG4gta8XKpMCd]

#5999

[mhegazy]

see #6223 and #5999