https://gcc.gnu.org/bugzilla/show_bug.cgi?id=117936
--- Comment #3 from Saada Mehdi <00120260a at gmail dot com> --- Ok I got it, it's caused by pragma Unimplemented_Unit. It troubled me because the doc says "If this pragma occurs in a unit that is processed by the compiler, GNAT aborts with the message xxx not implemented, where xxx is the name of the current compilation unit." and that is not the message I received. So either change the message, or the doc.
