mod40.hs:3:1: error: [GHC-29210]
    • Superclass cycle for ‘C1’
        one of whose superclasses is ‘C2’
        one of whose superclasses is ‘C1’
    • In the class declaration for ‘C1’
    Suggested fix: Perhaps you intended to use UndecidableSuperClasses

mod40.hs:4:1: error: [GHC-29210]
    • Superclass cycle for ‘C2’
        one of whose superclasses is ‘C1’
        one of whose superclasses is ‘C2’
    • In the class declaration for ‘C2’
    Suggested fix: Perhaps you intended to use UndecidableSuperClasses