
<interactive>:5:1: error:
    Illegal generalised algebraic data declaration for ‘T’
      (Use GADTs to allow GADTs)
    In the data declaration for ‘T’

ghci057.hs:3:3: error:
    Data constructor ‘C’ has existential type variables, a context, or a specialised result type
      C :: T Int
      (Use ExistentialQuantification or GADTs to allow this)
    In the definition of data constructor ‘C’
    In the data type declaration for ‘T’

ghci057.hs:3:3: error:
    Data constructor ‘C’ has existential type variables, a context, or a specialised result type
      C :: T Int
      (Use ExistentialQuantification or GADTs to allow this)
    In the definition of data constructor ‘C’
    In the data type declaration for ‘T’