{-# LANGUAGE DefaultSignatures #-}

class C a where
  reflexive :: a -> Bool
  default reflexive :: Eq a => a -> Bool
  reflexive x = x == x

data D

instance C D where

-- /home/abel/play/haskell/bugs/DefaultSig.hs:10:10:
--     No instance for (Eq D) arising from a use of ‘Main.$gdmreflexive’
--     In the expression: Main.$gdmreflexive
--     In an equation for ‘reflexive’: reflexive = Main.$gdmreflexive
--     In the instance declaration for ‘C D’

Error looks odd: The user has not written $gdmreflexive in his code.

TODO: Better error message.

Maybe this should just trigger a warning that method reflexive is undefined for instance D of C. Like when I remove the default method.

/home/abel/play/haskell/bugs/DefaultSig.hs:10:10: Warning:
    No explicit implementation for
      ‘reflexive’
    In the instance declaration for ‘C D’

It seems the semantics of a default signature is that each instance *must* implement this method.