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The following code does not compile in GHC 7.4, unless line (2) is uncommented. I thought line (1) would remove need for (2). If it does not, probably declaration (1) should cause an error.

{-# LANGUAGE TypeFamilies #-}
class R m where
  type D m a :: *
  type D m a = ()                -- (1)
  f :: m a -> D m a -> ()

instance R Maybe where
  -- type D Maybe a = ()         -- (2)
  f = undefined

x = f (Nothing :: Maybe Int) ()

Trac metadata

Trac field	Value
Version	7.4.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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