The C++ standards committee is working on a TS (Technical Specification) for Concepts extension: "Programming Languages - C++ Extensions for Concepts". N4377 is the latest version of this document. For inclusion into the C++ standard features are asked to be implemented, ideally for a publicly accessible system.
I'm aware of concept-gcc but the concepts proposal above (colloquially referred to as Concepts Lite) is different. I heard that there is a concepts branch and I have tried the origin/asutton/c++-concepts from gcc's git mirror but that didn't compile. How do I build and use a version of gcc supporting concepts as specified in the above [draft] TS?
According to Andrew Sutton the git mirror of the Concepts Lite work is entirely out of date and shouldn't be used. There is a concepts branch in the svn repository for gcc implementing "concepts-lite" (there was a pre-C++11 effort to add concepts to C++ which was abandoned; a new effort to add concepts to C++ was started using the name concepts lite). It is fairly straight forward to build (assuming you already have a recent gcc and the packages it depends on installed; see the gcc install instruction for other options you might want to pass: the instructions below worked for me except that I explicitly pointed configure at the locations where gmp, mpc, and mpfr are installed):
cd <suitable-directory>
svn checkout svn://gcc.gnu.org/svn/gcc/branches/c++-concepts
mkdir build-concepts
cd build-concepts
../c++-concepts/configure --prefix=<concept-gcc-root> --disable-bootstrap --enable-languages=c,c++
make -j8
make install
What isn't entirely obvious is how to actually use concepts: the option -fconcepts does exist but seems to unconditionally do something different than enabling concepts-lite. Instead, you'd use the -std=c++1z option when compiling with concepts enabled.