[GHC] #12784: Typechecker regression in GHC 8.0.2 involving DefaultSignatures

[GHC]

#12784: Typechecker regression in GHC 8.0.2 involving DefaultSignatures
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        Reporter:  RyanGlScott       |                Owner:
            Type:  bug               |               Status:  merge
        Priority:  highest           |            Milestone:  8.0.2
       Component:  Compiler          |              Version:  8.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D2682
       Wiki Page:                    |
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Comment (by rwbarton):

 And the relevant code (excerpted from
 https://github.com/purescript/purescript/blob/master/src/Control/Monad/Supply/Class.hs):
 {{{#!hs
 class Monad m => MonadSupply m where
   fresh :: m Integer
   peek :: m Integer
   default fresh :: MonadTrans t => t m Integer
   fresh = lift fresh
   default peek :: MonadTrans t => t m Integer
   peek = lift peek

 instance MonadSupply m => MonadSupply (StateT s m)
 instance (Monoid w, MonadSupply m) => MonadSupply (WriterT w m)
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12784#comment:11>
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