[GHC] #8913: either bug or confusing error message mixing PolyKinds and TypeFamilies

[GHC]

#8913: either bug or confusing error message mixing PolyKinds and TypeFamilies
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       Reporter:  ghorn                    |             Owner:
           Type:  bug                      |            Status:  new
       Priority:  normal                   |         Milestone:
      Component:  Compiler (Type checker)  |           Version:  7.8.1-rc2
       Keywords:  PolyKinds, TypeFamilies  |  Operating System:
   Architecture:  Unknown/Multiple         |  Unknown/Multiple
     Difficulty:  Unknown                  |   Type of failure:
     Blocked By:                           |  None/Unknown
Related Tickets:                           |         Test Case:
                                           |          Blocking:
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 I found this when using GHC.Generics, but it has to do with TypeFamilies
 so here is a stand-alone example:

 {{{
 {-# OPTIONS_GHC -Wall #-}
 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE FlexibleContexts #-}
 {-# LANGUAGE PolyKinds #-}

 module Test where

 class GCat f where
   gcat :: f p -> Int

 cat :: (GCat (MyRep a), MyGeneric a) => a -> Int
 cat x = gcat (from x)

 class MyGeneric a where
   type MyRep a :: * -> *
   from :: a -> (MyRep a) p
 }}}

 This code gives the error message
 {{{
 src/Dyno/Test.hs:12:9:
     Could not deduce (GCat (MyRep a)) arising from a use of ‘gcat’
     from the context (GCat (MyRep a), MyGeneric a)
       bound by the type signature for
                  cat :: (GCat (MyRep a), MyGeneric a) => a -> Int
       at src/Dyno/Test.hs:11:8-48
     In the expression: gcat (from x)
     In an equation for ‘cat’: cat x = gcat (from x)
 Failed, modules loaded: none.
 }}}

 If this is not a bug then error message is pretty confusing because it's
 saying "Can't deduce (C a) from (C a)", where the message I'm used to is
 "Can't derive (C a) from (C a0)" or something that indicates the mismatch.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8913>
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