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

Calculating dependencies ... done!
Dependency resolution took 1.01 s (backtrack: 0/20).

[ebuild  N     ] dev-cpp/picojson-1.3.0::gentoo  15 KiB
[ebuild  N     ] dev-cpp/nlohmann_json-3.12.0-r1::gentoo  USE="-test" 9452 KiB
[ebuild  N     ] dev-cpp/jwt-cpp-0.7.1::gentoo  USE="picojson -doc -test" 157 KiB

Total: 3 packages (3 new), Size of downloads: 9623 KiB