access-to-classwide has non-static size?

[Simon Wright <simon@pushface.org>]

I'm creating a PO which has a member which is a bounded vector of
pointer-to-classwide,

   type Event_Base is abstract tagged limited private;
   type Event_P is access all Event_Base'Class;

...

   package Event_Queues is new Ada.Containers.Bounded_Vectors
     (Index_Type   => Positive,
      Element_Type => Event_P);

...

   protected type Dispatchable_Events is
      ...
   private
      ...
      The_Events : Event_Queues.Vector (Capacity => Capacity);

and when compiling with the Ravenscar profile (GNAT GPL 2014, GCC 4.9.1,
GCC 5.0.0 20150208) the compiler says

   warning: component has non-static size
   warning: creation of protected object of type "Dispatchable_Events"
    will violate restriction No_Implicit_Heap_Allocations

Is this (the non-static size part!) right?

I tried declaring Event_P "with Size => 64"; no difference.

I think, studying the output of -gnatR, a pointer-to-classwide is a thin
pointer, i.e. 4 bytes on arm-eabi).