tcfail154.hs 264 Bytes
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{-# OPTIONS -fglasgow-exts #-}

module ShouldFail where

data T a = MkT

class C a b where
  op :: T a -> T b -> Bool

-- Repeated type variable in an instance constraint
-- should require -fallow-undecidable-instances
instance C a a => Eq (T a) where
  (==) = op