[GHC] #11608: Possible type-checker regression in GHC 8.0 when compiling `microlens`

[GHC]

#11608: Possible type-checker regression in GHC 8.0 when compiling `microlens`
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        Reporter:  hvr               |                Owner:
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.0.1
       Component:  Compiler (Type    |              Version:
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 Specifically, the issue seems to pertain to type equalities in default
 signatures in typeclasses. Here is the failing code, reduced to
 reproducible examples:

 From [http://hackage.haskell.org/package/microlens-0.4.2.0/docs/src/Lens-
 Micro-Internal.html#line-184 microlens]:

 {{{#!hs
 {-# LANGUAGE DefaultSignatures #-}
 {-# LANGUAGE FunctionalDependencies #-}
 {-# LANGUAGE MultiParamTypeClasses #-}
 {-# LANGUAGE RankNTypes #-}
 {-# LANGUAGE TypeFamilies #-}

 type Traversal s t a b = forall f. Applicative f => (a -> f b) -> s -> f t

 class Each s t a b | s -> a, t -> b, s b -> t, t a -> s where
   each :: Traversal s t a b
   default each :: (Traversable g, s ~ g a, t ~ g b) => Traversal s t a b
   each = traverse
 }}}

 From [http://hackage.haskell.org/package/free-4.12.4/docs/src/Control-
 Monad-Free-Class.html#line-106 free]:

 {{{#!hs
 {-# LANGUAGE DefaultSignatures #-}
 {-# LANGUAGE FunctionalDependencies #-}
 {-# LANGUAGE MultiParamTypeClasses #-}
 {-# LANGUAGE TypeFamilies #-}

 class MonadTrans t where
   lift :: Monad m => m a -> t m a

 class Monad m => MonadFree f m | m -> f where
   wrap :: f (m a) -> m a
   default wrap :: (m ~ t n, MonadTrans t, MonadFree f n, Functor f) => f
 (m a) -> m a
   wrap = join . lift . wrap . fmap return
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11608#comment:2>
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