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

Calculating dependencies ... done!
Dependency resolution took 0.99 s (backtrack: 0/20).

[ebuild  N     ] dev-libs/libsigc++-3.8.0:3::gentoo  USE="-gtk-doc -test" 973 KiB
[ebuild  N     ] dev-cpp/cairomm-1.18.1:1.16::gentoo  USE="X -gtk-doc -test" 600 KiB

Total: 2 packages (2 new), Size of downloads: 1572 KiB