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Ryan Scott authored
Summary: After 8136a5cb (#11450), if you have a class with an associated type: ``` class C a where type T a b ``` And you try to create an instance of `C` like this: ``` instance C Int where type T Int Char = Bool ``` Then it is rejected, since you're trying to instantiate the variable ``b`` with something other than a type variable. But this restriction proves quite onerous in practice, as it prevents you from doing things like this: ``` class C a where type T a (b :: Identity c) :: c instance C Int where type T Int ('Identity x) = x ``` You have to resort to an auxiliary type family in order to define this now, which becomes extremely tiring. This lifts this restriction and fixes #13398, in which it was discovered that adding this restriction broke code in the wild. Test Plan: ./validate Reviewers: simonpj, bgamari, austin Reviewed By: simonpj Subscribers: rwbarton, thomie Differential Revision: https://phabricator.haskell.org/D3302
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