Gentoo's Bugzilla –
Attachment 357090 Details for
Bug 482576dev-db/mongodb USE=(embedded-v8|-spidermonkey) with sys-devel/gcc-4.8 - src/third_party/v8/src/checks.h:251:22: error: typedef '__StaticAssertTypedef__128' locally defined but not used [-Werror=unused-local-typedefs]