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

Calculating dependencies ... done!
Dependency resolution took 1.73 s (backtrack: 0/20).

[ebuild  N     ] dev-libs/libaio-0.3.113_p8::gentoo  USE="-static-libs -test" 74 KiB
[ebuild  N     ] dev-build/b2-5.5.3::gentoo  USE="-examples" 1198 KiB
[ebuild  N     ] sys-libs/liburing-2.15:0/2::gentoo  USE="-examples -static-libs -test" 538 KiB
[ebuild  N     ] dev-cpp/gflags-2.2.2-r1:0/2.2::gentoo  USE="-static-libs -test" 97 KiB
[ebuild     UD ] dev-libs/libfmt-10.2.1-r2:0/10.2.1::gentoo [12.1.0:0/12.1.0::gentoo] USE="-test" 835 KiB
[ebuild  N     ] app-arch/snappy-1.2.2-r1:0/1.1::gentoo  USE="-test" CPU_FLAGS_X86="avx avx2" 1083 KiB
[ebuild  N     ] dev-libs/double-conversion-3.4.0:0/3::gentoo  USE="-test" 6871 KiB
[ebuild  N     ] dev-cpp/glog-0.6.0-r1:0/1::gentoo  USE="gflags libunwind -llvm-libunwind -test" 189 KiB
[ebuild  N     ] dev-libs/boost-1.91.0-r2:0/1.91.0::gentoo  USE="bzip2 context nls python stacktrace zlib -debug -doc -icu -lzma -mpi -numpy -test -test-full -tools -zstd" PYTHON_TARGETS="python3_14 -python3_12 -python3_13" 199785 KiB
[ebuild  N     ] app-arch/lz4-1.10.0-r1:0/1.10.0-meson::gentoo  USE="-static-libs -test" 379 KiB
[ebuild  N     ] dev-cpp/folly-2023.06.19.00-r1:0/2023.06.19.00::gentoo  USE="-llvm-libunwind -test" 3761 KiB

Total: 11 packages (1 downgrade, 10 new), Size of downloads: 214805 KiB

!!! Multiple package instances within a single package slot have been pulled
!!! into the dependency graph, resulting in a slot conflict:

dev-libs/libfmt:0

  (dev-libs/libfmt-10.2.1-r2:0/10.2.1::gentoo, ebuild scheduled for merge) USE="-test" pulled in by
    <dev-libs/libfmt-11:= required by (dev-cpp/folly-2023.06.19.00-r1:0/2023.06.19.00::gentoo, ebuild scheduled for merge) USE="-llvm-libunwind -test"
    ^                ^^ ^                                                                                                                                                                                                                                             

  (dev-libs/libfmt-12.1.0:0/12.1.0::gentoo, installed) USE="-test" pulled in by
    >=dev-libs/libfmt-11.0.2:=[abi_x86_32(-)?,abi_x86_64(-)?,abi_x86_x32(-)?,abi_mips_n32(-)?,abi_mips_n64(-)?,abi_mips_o32(-)?,abi_s390_32(-)?,abi_s390_64(-)?] required by (dev-libs/spdlog-1.17.0:0/1.17::gentoo, installed) USE="-test"
    ^^                ^^^^^^                                                                                                                                                                                                                                                                                                            
    dev-libs/libfmt:0/12.1.0= required by (app-text/doxygen-1.16.1:0/0::gentoo, installed) USE="-clang -debug -doc -dot -doxysearch -gui -test" LLVM_SLOT="21 -18 -19 -20 -22"
                   ^^^^^^^^^^                                                                                                                                                                                                                                                                                                                                                                                                                         
    (and 1 more with the same problems)

NOTE: Use the '--verbose-conflicts' option to display parents omitted above

It may be possible to solve this problem by using package.mask to
prevent one of those packages from being selected. However, it is also
possible that conflicting dependencies exist such that they are
impossible to satisfy simultaneously.  If such a conflict exists in
the dependencies of two different packages, then those packages can
not be installed simultaneously.

For more information, see MASKED PACKAGES section in the emerge man
page or refer to the Gentoo Handbook.


The following USE changes are necessary to proceed:
 (see "package.use" in the portage(5) man page for more details)
# required by dev-cpp/folly-2023.06.19.00-r1::gentoo
# required by =dev-cpp/folly-2023.06.19.00-r1 (argument)
>=dev-cpp/glog-0.6.0-r1 gflags

 * 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.