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

Calculating dependencies ... done!
Dependency resolution took 3.22 s (backtrack: 5/20).

[ebuild  r  UD ] dev-libs/spdlog-1.15.3-r1:0/1.15::gentoo [1.17.0:0/1.17::gentoo] USE="-test" 272 KiB
[ebuild  r  U  ] app-text/doxygen-1.18.0-r1::gentoo [1.16.1::gentoo] USE="-clang -debug -doc -dot -doxysearch -gui -test" LLVM_SLOT="22* -18 -19 -20 -21*" 9674 KiB

Total: 2 packages (1 upgrade, 1 downgrade), Size of downloads: 9945 KiB

WARNING: One or more updates/rebuilds have been skipped due to a dependency conflict:

dev-libs/spdlog:0

  (dev-libs/spdlog-1.17.0:0/1.17::gentoo, ebuild scheduled for merge) USE="-test" conflicts with
    =dev-libs/spdlog-1.15.3-r1


The following packages are causing rebuilds:

  (dev-libs/spdlog-1.15.3-r1:0/1.15::gentoo, ebuild scheduled for merge) causes rebuilds for:
    (app-text/doxygen-1.18.0-r1:0/0::gentoo, ebuild scheduled for merge)