private type discriminants ignored?

[Jay Sachs <sachs@cs.williams.edu>]

In a series of successive refinements of a stack package, I've
declared:

    package Stacks4 is
       type Stack(Size : Integer := 100) is private;
       -- this version:
       --      type Stack is private;
       -- also exhibits the same behavior

       procedure Push(S : in out Stack; X : Integer);
       procedure Pop(S : in out Stack; X : out Integer);
       function Empty(S : in Stack) return Boolean;
       Stack_Overflow : exception;
       Stack_Underflow : exception;

    private
       type stackrep is array(Integer range <>) of Integer;
       type Stack(Size : Integer := 100) is record 
	  Top : Natural := 0;
	  Rep : stackrep(1..Size);
       end record;
    end Stacks4;

I get warnings (from gnat 3.10) saying that I may get a storage_error
by creating a variable of type stack.  I do in fact get such an error
if I declare

  s : Stacks4.Stack;

and use it like:

  Stacks4.push(s,3);

However, supplying the initialization as in

  s2 : Stacks4.Stack(75);

allows

  Stacks4.push(s2,3);

with no error.  What is the reason that the default initialization
seems to be ignored in the discriminant for the private type?

-Jay