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

Calculating dependencies ... done!
Dependency resolution took 1.80 s (backtrack: 0/20).



[ebuild  N     ] dev-ada/gnatmem-2021-r6::gentoo  ADA_TARGET="gcc_15 -gcc_12 -gcc_13 -gcc_14 (-gcc_16)" 80 KiB
[ebuild  N     ]  dev-ada/gprbuild-26.0.0::gentoo  USE="-doc" ADA_TARGET="gcc_15 -gcc_12 -gcc_13 -gcc_14 (-gcc_16)" 1959 KiB
[ebuild     U  ]   sys-devel/gcc-15.3.1_p20261009:15::gentoo [15.2.1_p20260214:15::gentoo] USE="ada* cet (cxx) (default-stack-clash-protection) (default-znow) fortran graphite lto nls openmp pgo (pie) sanitize ssp zstd -cobol (-custom-cflags) -d -debug -doc (-fixed-point) -go -hardened (-ieee-long-double) -jit -libgdiagnostics (-libssp) -modula2 (-multilib) -objc -objc++ -objc-gc (-pch) -rust -systemtap -test (-time64) -valgrind -vanilla -vtv" 90378 KiB

Total: 3 packages (1 upgrade, 2 new), Size of downloads: 92416 KiB

 * Error: circular dependencies:

(sys-devel/gcc-15.3.1_p20261009:15/15::gentoo, ebuild scheduled for merge) depends on
 (sys-devel/gcc-15.3.1_p20261009:15/15::gentoo, ebuild scheduled for merge) (buildtime, USE=ada)

 * Note that circular dependencies can often be avoided by temporarily
 * disabling USE flags that trigger optional dependencies.

The following USE changes are necessary to proceed:
 (see "package.use" in the portage(5) man page for more details)
# required by dev-ada/gprbuild-26.0.0::gentoo[ada_target_gcc_15]
# required by dev-ada/gnatmem-2021-r6::gentoo
# required by =dev-ada/gnatmem-2021-r6 (argument)
>=sys-devel/gcc-15.3.1_p20261009:15 ada

 * In order to avoid wasting time, backtracking has terminated early
 * due to the above autounmask change(s). The --autounmask-backtrack=y
 * option can be used to force further backtracking, but there is no
 * guarantee that it will produce a solution.