Contraint error when "out" parameter has incorrect initial content

[pascal.malaise@gmail.com]

Hi,
I am wondering if GNAT is correct when raising Constraint_Error in the following case:
procedure Out_Discr is

  type T (B : Boolean := False) is record
    case B is
      when True => null;
      when False => null;
    end case;
  end record;
  subtype Tt is T(True);

  procedure P (V : out Tt) is
  begin
    V := (B => True);
  end P;

  M : T;

begin
  M := (B => False);
  P (M); -- <-- Here
end Out_Discr;

>> raised CONSTRAINT_ERROR : out_discr.adb:20 discriminant check failed
Indeed M is not Tt when calling P, but M is "out" parameter. Shouldn't it be overwritten without any constraint check?

Thank you.