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In rare cases, the static elaboration model fails to prevent
dispatching calls to not-yet-elaborated subprograms. In such cases, we
fall back to run-time checks; premature calls to any primitive
operation of a tagged type before the body of the operation has been
elaborated will raise Program_Error.
Access-to-subprogram types, however, are handled conservatively, and
do not require run-time checks. This was not true in earlier versions
of the compiler; you can use the -gnatd.U debug switch to
revert to the old behavior if the new conservative behavior causes
elaboration cycles. Here, “conservative” means that if you do
P'Access during elaboration, the compiler will assume that you
might call P indirectly during elaboration, so it adds an
implicit pragma Elaborate_All on the library unit containing
P. The -gnatd.U switch is safe if you know there are
no such calls. If the program worked before, it will continue to work
with -gnatd.U. But beware that code modifications such as
adding an indirect call can cause erroneous behavior in the presence
of -gnatd.U.