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

Calculating dependencies ... done!
Dependency resolution took 1.55 s (backtrack: 0/20).

[ebuild  N     ] dev-lang/luajit-2.1.1787165859:2/2.1.1787165859::gentoo  USE="-lua52compat -static-libs" 1078 KiB
[ebuild  N     ] dev-lua/lua-resty-string-0.16::gentoo  LUA_SINGLE_TARGET="luajit" 13 KiB
[ebuild  NS    ] dev-lang/lua-5.1.5-r200:5.1::gentoo [5.4.8:5.4::gentoo] USE="deprecated readline" 389 KiB
[ebuild  N     ] dev-lua/LuaBitOp-1.0.3::gentoo  LUA_TARGETS="lua5-1 luajit -lua5-3 -lua5-4" 20 KiB
[ebuild  N     ] dev-lua/lua-resty-mysql-0.27::gentoo  LUA_SINGLE_TARGET="luajit" 126 KiB

Total: 5 packages (4 new, 1 in new slot), Size of downloads: 1623 KiB

The following USE changes are necessary to proceed:
 (see "package.use" in the portage(5) man page for more details)
# required by dev-lua/lua-resty-mysql-0.27::gentoo[lua_single_target_luajit]
# required by =dev-lua/lua-resty-mysql-0.27 (argument)
>=dev-lua/LuaBitOp-1.0.3 lua_targets_luajit