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

Calculating dependencies ...

!!! Problem resolving dependencies for =dev-ruby/mysql2-0.5.7
 done!
Dependency resolution took 0.83 s (backtrack: 0/20).


!!! The ebuild selected to satisfy "=dev-ruby/mysql2-0.5.7" has unmet requirements.
- dev-ruby/mysql2-0.5.7::gentoo USE="-mariadb -mysql -test" RUBY_TARGETS="ruby32 ruby33 -ruby34 -ruby40"

  The following REQUIRED_USE flag constraints are unsatisfied:
    exactly-one-of ( mariadb mysql )

  The above constraints are a subset of the following complete expression:
    exactly-one-of ( mariadb mysql ) any-of ( ruby_targets_ruby32 ruby_targets_ruby33 ruby_targets_ruby34 ruby_targets_ruby40 )