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

Calculating dependencies ... done!
Dependency resolution took 0.97 s (backtrack: 0/20).

[ebuild  N     ] dev-libs/msgpack-7.0.1:0/2-c::gentoo  USE="-doc -examples -test" 69 KiB
[ebuild  N     ] dev-libs/mmtf-cpp-1.0.0-r1::gentoo  58 KiB

Total: 2 packages (2 new), Size of downloads: 127 KiB