testsuite/tests/typecheck/should_fail/tcfail154.hs


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{-# LANGUAGE MultiParamTypeClasses #-}

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 UndecidableInstances
instance C a a => Eq (T a) where
  (==) = op