drvfail011.hs:8:1: error: [GHC-39999]
    • No instance for ‘Eq a’ arising from a use of ‘==’
      Possible fix: add (Eq a) to the context of the instance declaration
    • In the expression: a1 == b1
      In a case alternative: (T1 b1) -> ((a1 == b1))
      In the expression: case b of (T1 b1) -> ((a1 == b1))
      When typechecking the code for ‘==’
        in a derived instance for ‘Eq (T a)’:
        To see the code I am typechecking, use -ddump-deriv