[GHC] #7809: Silly error message suggesting LiberalTypeSynonyms

[GHC]

#7809: Silly error message suggesting LiberalTypeSynonyms
-----------------------------+----------------------------------------------
Reporter:  goldfire          |          Owner:                  
    Type:  bug               |         Status:  new             
Priority:  normal            |      Component:  Compiler        
 Version:  7.7               |       Keywords:                  
      Os:  Unknown/Multiple  |   Architecture:  Unknown/Multiple
 Failure:  None/Unknown      |      Blockedby:                  
Blocking:                    |        Related:                  
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 The following code (rightly) does not compile:

 {{{
 {-# LANGUAGE RankNTypes, TypeFamilies, LiberalTypeSynonyms #-}

 type PolyId = (forall a. a -> a)

 type family F a

 foo :: F PolyId
 foo = undefined
 }}}

 The error message is

 {{{
     Illegal polymorphic or qualified type: PolyId
     Perhaps you intended to use -XLiberalTypeSynonyms
     In the type signature for `foo': foo :: F PolyId
 }}}

 The code doesn't compile because we can't apply a type family to a
 polytype. The first line of the error message says this well enough,
 though more information might be helpful here. However, the second line of
 the error message is silly: I've turned on {{{LiberalTypeSynonyms}}}!

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7809>
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