root@lap:~$ emerge -a =gnat-gpl-2016
 * Last emerge --sync was 4y 322d 19h 57m 8s ago.

These are the packages that would be merged, in order:

Calculating dependencies... done!
[ebuild  N    ~] dev-lang/gnat-gpl-2016  USE="cxx hardened nptl openmp sanitize vtv (-altivec) -awt -bootstrap -cilk -debug -doc (-fixed-point) -fortran -gcj -go -graphite -libssp (-multilib)
-nls -nopie -nossp -objc -objc++ -objc-gc -pgo -regression-test -vanilla"
[blocks B      ] sys-devel/gcc:4.9.4 ("sys-devel/gcc:4.9.4" is blocking dev-lang/gnat-gpl-2016)

 * Error: The above package list contains packages which cannot be
 * installed at the same time on the same system.

  (dev-lang/gnat-gpl-2016:4.9.4/4.9.4::gentoo, ebuild scheduled for merge) pulled in by
    =gnat-gpl-2016

  (sys-devel/gcc-4.9.4:4.9.4/4.9.4::gentoo, installed) pulled in by
    sys-devel/gcc required by @system
    >=sys-devel/gcc-4.7 required by (sys-libs/glibc-2.25-r9:2.2/2.2::gentoo, installed)