T16758.hs:14:8: error: [GHC-25897]
    • Couldn't match expected kind ‘Int’ with actual kind ‘a’
      ‘a’ is a rigid type variable bound by
        the class declaration for ‘C’
        at T16758.hs:13:9
    • In the type signature: f :: C a => a -> Int
      In the class declaration for ‘C’