[GHC] #7019: Wrong error message when using forall in constraints

[GHC]

#7019: Wrong error message when using forall in constraints
------------------------------+---------------------------------------------
 Reporter:  sjoerd_visscher   |          Owner:                  
     Type:  bug               |         Status:  new             
 Priority:  normal            |      Component:  Compiler        
  Version:  7.4.1             |       Keywords:                  
       Os:  Unknown/Multiple  |   Architecture:  Unknown/Multiple
  Failure:  None/Unknown      |       Testcase:                  
Blockedby:                    |       Blocking:                  
  Related:                    |  
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 I tried this in GHC 7.4.1:

 {{{
 {-# LANGUAGE ConstraintKinds, RankNTypes, UndecidableInstances #-}

 newtype Free c a = Free { runFree :: forall r. c r => (a -> r) -> r }

 type C c = forall a. c (Free c a)
 instance C c => Monad (Free c) where
   return a = Free ($ a)
   Free f >>= g = f g
 }}}

 Which gives this error:

 {{{
     Could not deduce (c (Free c b)) arising from a use of `f'
     from the context (C c)
 }}}

 So apparently the forall is silently dropped.

 Without UndecidableInstances the error is:

 {{{
     Illegal irreducible constraint forall a. c (Free c a)
     in superclass/instance head context (Use -XUndecidableInstances to
 permit this)
 }}}

 giving the impression that it is allowed.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7019>
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