tcfail019.hs:18:10: error: [GHC-39999]
    • No instance for ‘B [a]’
        arising from the superclasses of an instance declaration
    • In the instance declaration for ‘D [a]’

tcfail019.hs:18:10: error: [GHC-39999]
    • No instance for ‘C [a]’
        arising from the superclasses of an instance declaration
    • In the instance declaration for ‘D [a]’