argument of conversion cannot be an allocator

[Dale Stanbrough <dale@goanna.cs.rmit.EDU.AU>]

I came across the following problem...


	type a is tagged private;
	type a_ptr is access all a'class;
	
	type b is new a with private;
	type b_ptr is acces all b'class;
	
	...
	
	
	return a'(new b);

which results in the error message (from GNAT)

	"argument of conversion cannot be an allocator"

The work around is...

	declare
	   temp : b_Ptr := new b;
	begin
	   return a_ptr(temp);
	end;


I presume there is a good reason for this; can anyone provide enlightenment
on this point (or even a simpler way to accomplish the above)?

Dale