either bug or confusing error message mixing PolyKinds and TypeFamilies

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.

Trac metadata

Trac field	Value
Version	7.8.1-rc2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	gregmainland@gmail.com
Operating system
Architecture