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

Calculating dependencies ... done!
Dependency resolution took 1.05 s (backtrack: 0/20).

[ebuild  N     ] dev-libs/libsigc++-3.8.0:3::gentoo  USE="-gtk-doc -test" 973 KiB
[ebuild  N     ] dev-cpp/glibmm-2.84.0:2.68::gentoo  USE="-debug -gtk-doc -test" 9372 KiB

Total: 2 packages (2 new), Size of downloads: 10345 KiB