[pbsd]

Concepts will certainly be nice once they arrive in the standard, but it is already possible today with some trickery. Example checking whether an std::vector<T>'s type is comparable with `operator<`: http://coliru.stacked-crooked.com/a/2ea46e0afd894d3d

The error you get if you try to sort a vector with an unordered type is relatively easy to read:

    main.cpp: In instantiation of 'void merge_sort(std::vector<_RealType>&) [with T = type]':
    main.cpp:45:17:   required from here
    main.cpp:30:5: error: static assertion failed: Element type is not sortable
         static_assert( concept::is_ord<T>(), "Element type is not sortable" );
         ^