Unexpected discriminant check failure involving access types

["Markus Schöpflin" <no.spam@spam.spam>]

Given the following piece of code:

---%<---
      1  procedure TEST
      2  is
      3     type T is (T1, T2);
      4
      5     type RECORD_T (X : T := T1) is record
      6        null;
      7     end record;
      8
      9     type PTR_T is access RECORD_T;
     10
     11     FOO : RECORD_T;
     12     FOO_PTR : constant PTR_T := new RECORD_T;
     13
     14     FOO1 : constant RECORD_T := (X => T1);
     15     FOO2 : constant RECORD_T := (X => T2);
     16  begin
     17     FOO := FOO1;
     18     FOO := FOO2;
     19
     20     FOO_PTR.all := FOO1;
     21     FOO_PTR.all := FOO2;
     22  end;
--->%---

When compiled and executed, I get:

 > ./test

raised CONSTRAINT_ERROR : test.adb:21 discriminant check failed

Can anyone please explain me why I get a discriminant check error when using 
access types? I would have expected it to work the same as for the non-access 
case.

TIA, Markus