conflicting multi-parameter family instance declarations

[Michal Konečný]

Dear all,

The following compiles with ghc 7.6 but fails with ghc 7.8:

-----
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE MultiParamTypeClasses #-}
module Test where

class M t s where
    type T t s
    
data I t = I t

instance M t t where
    type (T t t) = ()
    
instance M t (I t) where
    type (T t (I t)) = ()
-----

The error I get with ghc 7.8.3 and 7.8.4 is:

Test.hs:12:10:
    Conflicting family instance declarations:
      T t t -- Defined at Test.hs:12:10
      T t (I t) -- Defined at Test.hs:15:10


I am curious if this change is an improvement or a bug.  I would be grateful for help as this issue affects a fairly large library I develop.

Best regards,
Michal
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