[humanrebar]

"Head" at Google means "head of the google3 monorepo". I gather they stay close to upstream for GCC and LLVM. Alsonote that they aren't necessarily matching upstream offerings with respect to C++ standards flags. They aren't necessarily matching upstream for various third-party libraries either. "Head" for third-party libraries means "the latest vendored in from upstream", which doesn't have to be particularly new.

In contrast, those third-party projects like boost would probably consider the latest commit on their HEADs of their git repos to be "head". So "live at head" is a statement about how each organization should version its dependencies. It doesn't really make sense in the context of boost maintainers deciding their support surfaces since they have all the "heads" to worry about, inherently -- all the organizations using boost libraries.