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

Calculating dependencies ... done!
Dependency resolution took 1.02 s (backtrack: 0/20).

[ebuild  N     ] dev-libs/libsigc++-2.12.1:2::gentoo  USE="-gtk-doc -test" 4924 KiB
[ebuild  N     ] dev-cpp/cairomm-1.14.5::gentoo  USE="X -gtk-doc -test" 638 KiB

Total: 2 packages (2 new), Size of downloads: 5562 KiB