Hidden class members still show up in instances

[harpocrates]

With haddock-ghc-8.10.1 (and tip of ghc-8.10 as of writing)

module ClassMember (Foo(a)) where

class Foo a where
  a :: a -> Int
  a x = b x

  b :: a -> Int
  b _ = 6

instance Foo Int where

produces

b has no business showing up in the instance! I expect this problem shows up for instance lists under data declarations. Although this example has an obviously correct case, it is not always clear what we should do. For instance: suppose a class is exported from two different locations with different methods visible, which do we show

• under the first place it is re-exported
• under the seconds place it is re-exported
• under data types elsewhere which declare instances