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