GNAT bug with assertions

[AdaMagica <christ-usch.grein@t-online.de>]

I would have expected that predicates are also checked on return values as they are checked on parameters.
In the following test, no exception is raised on call of funcction Wrong although it returns a value not fulfilling the predicate.

Is this a GNAT bug or my wrong expectation?

package Pack is

  type Priv is private;
  C: constant Priv;

  function Test (X: Priv) return Boolean;
  subtype Subt is Priv with Dynamic_Predicate => (Test (Subt));

  function Wrong return Subt;
  function Good (X: Subt) return Boolean;

private

  type Priv is new Integer;
  C: constant Priv := -1;

  function Test (X: Priv) return Boolean is (X > 0);

  function Wrong return Subt is (-1);
  function Good (X: Subt) return Boolean is (True);

end Pack; 

with Ada.Assertions, Ada.Text_IO;
use  Ada.Assertions, Ada.Text_IO;

with Pack;
use  Pack;

procedure Test_Pack is
begin

  begin
    Put_Line (Good (C)'Image);
  exception
    when Assertion_Error =>
      Put_Line ("Good (C) raises Assertion_Error => OK");
  end;

  declare
    X: Priv;
  begin
    X := Wrong;
    Put_Line ("Assertion_Error expected by calling Wrong => KO");
  exception
    when Assertion_Error =>
      Put_Line ("Wrong raises Assertion_Error => OK");
  end;

end Test_Pack;