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

Calculating dependencies ... done!
Dependency resolution took 1.02 s (backtrack: 0/20).

[ebuild  N     ] dev-libs/userspace-rcu-0.15.6:0/8::gentoo  USE="-static-libs -test -verify-sig" 670 KiB
[ebuild  N     ] dev-util/lttng-ust-2.13.10:0/2.13::gentoo  USE="-examples -numa -test -verify-sig" 1328 KiB
[ebuild  N     ] dev-util/lttng-tools-2.13.15-r2:0/2.13::gentoo  USE="ust -kmod -test -verify-sig" 1890 KiB

Total: 3 packages (3 new), Size of downloads: 3888 KiB