Decide about naming of variables in TyCons

Consider

type T :: forall (x :: Type). x -> Type   -- Standalone kind signature
data T (a :: k) = MkT (a :: k)            -- Declaration

Now suppose, in GHCi, you say :kind T.

• What kind would you expect to see?
  • T :: forall x. x -> Type, taking the name of the kind variable from the kind signature.
  • T :: forall k. k -> Type, taking the name of the kind variable from the header of the declaration.
• Does it matter anyway?

The names used in the kind signature are fully alpha-renamable. The names used in the header of the declaration are not so renamable, because they scope over the body of the declaration, in this case the data constructors. (In a class declaration the type variables in the header scope over the method signatures and associated types.)

In HEAD we took the names from the kind signature, and used a variety of extra plumbing to make the declaration variables scope correctly. In !7105 (closed) I changed this to use the variable names from the declaration header, because the ones from the signature are fully alpha-renamable.

If we wanted to revert to the previous behaviour, we could do so by making the tyConKind field of a TyCon hold the kind from the signature, perhaps with different naming than the [TyConBinder] for the TyCon. (Currently the tyConKind field is just a cache for mkTyConKind tyConBinders tyConResKind.) Not difficult, but a bit of extra plumbing.

This ticket is to describe the issue and ask what choice we should make.