Compilers which do not support namespaces at all will be sunk. GCC has
supported namespaces for quite a while now (since the early egcs days)
but since libstdc++ implementation lagged, it defaulted to making std::
the same as global scope. That means your code would compile.
I have had C++ open source software deployed since 1998 and the only
problems I encountered with namespaces is with accidentially writing
code which did not respect them (due to primarily using GCC).
So, in my opinion, it is safe to use these standard C++ features.