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

Calculating dependencies ... done!
Dependency resolution took 1.06 s (backtrack: 0/20).

[ebuild  N     ] dev-libs/libsigc++-2.12.1:2::gentoo  USE="-gtk-doc -test" 4924 KiB
[ebuild  N     ] dev-cpp/mm-common-1.0.8::gentoo  277 KiB
[ebuild  N     ] dev-cpp/glibmm-2.66.8-r1:2::gentoo  USE="-debug -gtk-doc -test" 8396 KiB

Total: 3 packages (3 new), Size of downloads: 13597 KiB