An implementation of "C++ Extensions for Ranges" P0021 (was N4382). There are still quite a few rough edges, but the library is now feature-complete.
Compilation requires GCC trunk with the -std=c++1z command line option. Compile times are currently on the slow side, even for C++. The implementation of Concepts in GCC is still brand-spanking-new and hasn't yet had proper performance tuning.