Recusive superclass epansion is over-keen (#14774) · Issues · Glasgow Haskell Compiler / GHC

Recusive superclass epansion is over-keen

Consider this, with RecursiveSuperClasses:

class C a => D a where
  cop :: a -> a

class D a => C a where
  dop :: a -> a

g :: C a => a -> Bool
g x = g x

f :: C a => a -> Bool
f x = g x

When we compile this we get

f = \ (@ a_aXf) ($dC_aXh :: C a_aXf) (eta_B1 :: a_aXf) ->
      g @ a_aXf (Foo.$p1D @ a_aXf (Foo.$p1C @ a_aXf $dC_aXh)) eta_B1

Notice the ridiculous double superclass selection from $dC_aXh. It's not wrong, just stupid!

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