Gnat error msg meaning?

[tmoran@acm.org]

Perhaps I haven't recovered from the eggnog yet, but why does Gnat give
an error message, what does it mean, and what should I do?

unique_str.ads:11:23: Only static constraints allowed for parent
discriminants in the partial view

relevant source:

package root is
  type String_Counts is range 0 .. 50_000;

  type String_Character_Counts is range 0 .. 100_000_000;

  type Store_Type(Max_Strings : String_Counts;
                  Max_Characters : String_Character_Counts;
                  Task_Safe : Boolean)
    is new Ada.Finalization.Controlled with private;
  ...

package unique_str is
  type Store_Type(Max_Strings : root.String_Counts;
                  Max_Characters : root.String_Character_Counts;
                  Task_Safe : Boolean;
                  Comparison : Comparison_Map_Ptr)
           is new root.Store_Type(Max_Strings, Max_Characters, Task_Safe)
--                    ^ error location
           with private;