Error: allocation from empty storage pool

["Alejandro R. Mosteo" <alejandro@mosteo.com>]

In a library I'm trying to have all allocations done from user-specified 
storage pools. There is no restriction on using the heap, as long as it 
comes from a user pool (that can default to the regular heap, of course).

The idea was then to use "pragma Default_Storage_Pool (null)" at the 
library root to ensure no use of default allocators, and wherever needed 
provide facilities to get a pool from the user.

However, this test fails:

---8<---

pragma Restrictions (No_Secondary_Stack); -- Just to be sure in this ex.
pragma Default_Storage_Pool (null);

procedure Anon is

    type Holder is record
       I : aliased Integer;
    end record;

    type Ref (Elem : access constant Integer) is limited null record;

    function To_Ref (Hold : aliased Holder) return Ref is
      (Elem => Hold.I'Access); -- Error in subject here

begin
    null;
end Anon;

---8<---

There's actually no allocation being made, and I could have a Holder 
variable in the stack, and take a reference, and still no pool would be 
used at all.

So it seems this pragma is too naïve. To make it into questions:

Is this the pragma expected behavior or a particularity of gnat? Is the 
approach reasonable? This is my first attempt at working in a 
"restricted" Ada environment so I don't really have a clear idea of the 
preferred way to do what I want. Also, I'd like if possible to avoid 
making everything generic on the user pool.

Thanks,
Álex.