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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.