gprbuild's tests recently started failing  with
Error: no native compiler found for language 'ada', default runtime
p.gpr:2:09: no languages defined for this project
gnatvsn.gpr:24:09: no languages defined for this project
gprbuild: "p.gpr" processing failed
The same error can also be seen in gnat modules like
and presumably others. gprbuild itself did not change since the
failure started to happen, so I suppose some dependency changed.
According to  the most likely change is from gcc-6 6.2.0-4 , but
its changelog does not mention any dependency change.
Curiously I can make libgnatcoll's tests work again by additionally
installing gcc-6  as that makes gprconfig work again. But
gprbuild's own tests still already pull in gcc-6, so that cannot be
the complete explanation.
I'm afraid this should be investigated by someone who actually knows
the Ada toolchain now.
 autopkgtest libgnatcoll --setup-commands 'apt-get install -y
--no-install-recommends gcc' -- [...]
Martin Pitt | http://www.piware.de
Ubuntu Developer (www.ubuntu.com) | Debian Developer (www.debian.org)