[GHC] #3227: Can't specify type signature of value typed with type families

[GHC]

#3227: Can't specify type signature of value typed with type families
---------------------------+------------------------------------------------
Reporter:  Ashley Yakeley  |          Owner:                
    Type:  bug             |         Status:  new           
Priority:  normal          |      Component:  Compiler      
 Version:  6.10.3          |       Severity:  normal        
Keywords:                  |       Testcase:                
      Os:  Linux           |   Architecture:  x86_64 (amd64)
---------------------------+------------------------------------------------
 Compiling this:
 {{{
 {-# LANGUAGE TypeFamilies #-}
 module Bug where
   class C a where
     type T a

   p :: T a
   p = undefined

   q :: T a
   q = p
 }}}
 give this:
 {{{
 $ ghc -c Bug.hs -o Bug.o

 Bug.hs:10:6:
     Couldn't match expected type `T a1' against inferred type `T a'
     In the expression: p
     In the definition of `q': q = p
 }}}
 It compiles successfully if the type signature for `q` is omitted.
 {{{
 $ ghc --version
 The Glorious Glasgow Haskell Compilation System, version 6.10.3
 $ uname -a
 Linux glastonbury 2.6.28-12-generic #43-Ubuntu SMP Fri May 1 19:31:32 UTC
 2009 x86_64 GNU/Linux
 }}}

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3227>
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