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{-# LANGUAGE CPP,FlexibleInstances #-}
class C a where
  f :: a -> String
instance {-# OVERLAPPABLE #-} C a where
  f x = "a"
-- #if 0 で通る
#if 1
instance {-# OVERLAPPABLE #-} C () where
  f x = "()"
#endif
main = print $ f undefined -- (undefined :: ()) で通る

[1 of 1] Compiling Main             ( prog.hs, prog.o )

prog.hs:11:16: error:
    • Overlapping instances for C a0 arising from a use of ‘f’
      Matching instances:
        instance [overlappable] C a -- Defined at prog.hs:4:31
        instance [overlappable] C () -- Defined at prog.hs:8:31
      (The choice depends on the instantiation of ‘a0’
       To pick the first instance above, use IncoherentInstances
       when compiling the other instance declarations)
    • In the second argument of ‘($)’, namely ‘f undefined’
      In the expression: print $ f undefined
      In an equation for ‘main’: main = print $ f undefined

Standard output is empty