both_libraries targets used to be considered as a shared library by default. There is now the default_both_libraries option to change this default.
When default_both_libraries is ‘auto’, [[both_libraries]] with dependencies that are [[@both_libs]] themselves will link with the same kind of library. For example, if libA is a [[@both_libs]] and libB is a [[@both_libs]] linked with libA (or with an internal dependency on libA), the static lib of libB will link with the static lib of libA, and the shared lib of libA will link with the shared lib of libB.