Before, GHC was extremely permissive about the form a default type
signature could take on in a class declaration. Notably, it would accept
garbage like this:

  class Monad m => MonadSupply m where
    fresh :: m Integer
    default fresh :: MonadTrans t => t m Integer
    fresh = lift fresh

And then give an extremely confusing error message when you actually
tried to declare an empty instance of MonadSupply. We now do extra
validity checking of default type signatures to ensure that they align
with their non-default type signature counterparts. That is, a default
type signature is allowed to differ from the non-default one only in its
context - they must otherwise be alpha-equivalent.

Fixes #12918.

Test Plan: ./validate

Reviewers: goldfire, simonpj, austin, bgamari

Reviewed By: bgamari

Subscribers: mpickering, dfeuer, thomie

Differential Revision: https://phabricator.haskell.org/D2983

GHC Trac Issues: #12918