[GHC] #11552: ScopedTypeVariables / InstanceSigs do not work in GHC 8.0

[GHC]

#11552: ScopedTypeVariables / InstanceSigs do not work in GHC 8.0
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        Reporter:  bitemyapp         |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.0.1
       Component:  Compiler          |              Version:  8.0.1-rc1
      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):
       Wiki Page:                    |
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Description changed by bitemyapp:

@@ -53,0 +53,5 @@
+
+ {{{
+ $ ghci --version
+ The Glorious Glasgow Haskell Compilation System, version 8.0.0.20160122
+ }}}

New description:

 {{{
 {-# LANGUAGE InstanceSigs        #-}
 {-# LANGUAGE ScopedTypeVariables #-}

 module MyMaybeT where

 newtype MaybeT m a =
   MaybeT { runMaybeT :: m (Maybe a) }

 instance (Functor m) => Functor (MaybeT m) where
   fmap f (MaybeT ma) =
     MaybeT $ (fmap . fmap) f ma

 instance forall f . (Applicative f) => Applicative (MaybeT f) where
   pure :: a -> MaybeT f a
   pure x = MaybeT (pure (pure x))

   (<*>) :: forall a b . Applicative f => MaybeT f (a -> b) -> MaybeT f a
 -> MaybeT f b
   (MaybeT fab) <*> (MaybeT mma) =
    let fab' :: f (Maybe (a -> b))
        fab' = fab
    in MaybeT $ undefined
 }}}

 Works fine in 7.10.3, breaks in GHC 8.0:

 {{{
 Prelude> :l code/myMaybeT1.hs
 [1 of 1] Compiling MyMaybeT         ( code/myMaybeT1.hs, interpreted )

 code/myMaybeT1.hs:13:10: error:
     Malformed instance: forall f.
                         (Applicative f) => Applicative (MaybeT f)

 code/myMaybeT1.hs:14:3: error:
     The class method signature for ‘pure’ lacks an accompanying binding
       (The class method signature must be given where ‘pure’ is declared)

 code/myMaybeT1.hs:17:3: error:
     The class method signature for ‘<*>’ lacks an accompanying binding
       (The class method signature must be given where ‘<*>’ is declared)

 code/myMaybeT1.hs:17:37: error: Not in scope: type variable ‘f’

 code/myMaybeT1.hs:17:49: error: Not in scope: type variable ‘f’

 code/myMaybeT1.hs:17:70: error: Not in scope: type variable ‘f’

 code/myMaybeT1.hs:17:84: error: Not in scope: type variable ‘f’
 Failed, modules loaded: none.
 }}}

 {{{
 $ ghci --version
 The Glorious Glasgow Haskell Compilation System, version 8.0.0.20160122
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11552#comment:1>
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