T17817.hs:16:10: error:
    • Unsaturated use of a representation-polymorphic primitive function.
      The first argument of ‘mkWeak#’
      does not have a fixed runtime representation.
      Its type is:
        a0 :: TYPE ('BoxedRep c0)
      Cannot unify ‘l’ with the type variable ‘c0’
      because it is not a concrete ‘Levity’.
    • In the expression: mkWeak#
      In an equation for ‘primop’: primop = mkWeak#
    • Relevant bindings include
        primop :: a
                  -> b
                  -> (State# RealWorld -> (# State# RealWorld, c #))
                  -> State# RealWorld
                  -> (# State# RealWorld, Weak# b #)
          (bound at T17817.hs:16:1)