rnfail053.hs:6:10: error: [GHC-25709]
    • Data constructor ‘MkT’ has existential type variables, a context, or a specialised result type
        MkT :: forall a. a -> T
    • In the definition of data constructor ‘MkT’
      In the data type declaration for ‘T’
    Suggested fixes:
       Perhaps you intended to use ExistentialQuantification
       Perhaps you intended to use GADTs