Superclass expansion fails in instance declaration
I'm sure I'm doing something very silly.
{-# LANGUAGE FlexibleInstances, UndecidableInstances #-}
module Bug where
class A a
class A a => B a
class A a => C a
instance B a => C a
GHC rejects, saying it can't prove A a
, required in order to write the C a
instance. But shouldn't my B a
constraint imply A a
?
Help?