Backpack: poor error message implementing abstract data with a forall type

Backpack currently accepts an abstract data type to be implemented as a forall or quantified type:

unit p where
  signature H where
    data T1
    data T2
    data T3

unit q where
  module H where
    class C a where {}
    data S

    type T1 = forall a. a -> a
    type T2 = forall a. C a => a
    type T3 = C S => S -> S

unit r where
  dependency q
  dependency p[H=q:H]

We get errors like the following:

    * Type constructor `T3' has conflicting definitions in the module
      and its hsig file
      Main module: type T3 :: *
                   type T3 = C S => S -> S
      Hsig file:  type T3 :: *
                  data T3

It doesn't specify what the issue is (that a quantified type is not allowed in the RHS of the type synonym implementing the abstract datatype).