T12709.hs:28:13: error:
    • The argument ‘1 + 2 + 3’ of ‘(+)’
      does not have a fixed runtime representation.
      Its type is:
        a :: TYPE rep
    • In the expression: 1 + 2 + 3 + 4
      In an equation for ‘u’: u = 1 + 2 + 3 + 4
      In the expression:
        let
          u :: Num (a :: TYPE rep) => a
          u = 1 + 2 + 3 + 4
        in BUB u u