mod39.hs:4:17: error: [GHC-25079]
    • Constraint ‘Eq a’ in the type of ‘f’
        constrains only the class type variables
    • When checking the class method: f :: forall a. (C a, Eq a) => a
      In the class declaration for ‘C’
    Suggested fix: Perhaps you intended to use ConstrainedClassMethods