T20588c.hs-boot:7:1: error:
    Class ‘C’ has conflicting definitions in the module
    and its hs-boot file
    Main module: type C :: * -> Constraint
                 class C a where
                   meth :: a
                   default meth :: Monoid a => a
    Boot file:   type C :: * -> Constraint
                 class C a where
                   meth :: a
                   {-# MINIMAL meth #-}
    The methods do not match:
      The default methods associated with ‘meth’ are different