setting 'Size and unchecked conversion safe here?

[Marius Amado Alves <amado.alves@netcabo.pt>]

I need to store access-to-subprogram values in a 64-bit modular type. I 
write

    for My_Access_To_Subprogram_Type'Size use 64;

and GNAT warns me about 32 bits being unused. But Unchecked_Conversion 
between the access and the modular works fine. Now, is it working fine 
by chance or by language definition?

(I know how I can write it differently, but if the above is safe I'd 
rather leave it as it is.)

Thanks a lot.

(This happens in both Mac OS X and Windows XP.)