Provide library utilities for the three way comparison operator <=>

Summary

Add concepts Std::three_way_comparable and Std::three_way_comparable_with as well as an algorithm Std::lexicographical_compare_three_way to provide library utilities for the <=> comparison operator.

While C++20 introduces the operator <=> into the language, not all standard libraries have implemented this comparison operator for std data structures, like tuple or array. Iterable containers could easily implement this operator using the lexicographic_compare_three_way algorithms that works with iterators. Since not all types can be compared with <=>, we also need to provide the concept definition that checks whether a type supports it. Also these concepts should be in the standard library, but are not yet fully implemented, especially in libc++-13.

Applications

This utility would allow to provide a three-way comparison operator for our Dune data structures FieldVector, DynamicVector and maybe also for the multi-dimensional array/span structures Std::mdarray and Std::mdspan. Especially for multi-indices a comparison and lexicographical ordering would be nice to have.

dune/common/std/test/CMakeLists.txt

@@ -7,6 +7,12 @@ link_libraries(Dune::Common)
 dune_add_test(SOURCES accessorstest.cc
               LABELS quick)
 
+dune_add_test(SOURCES algorithmtest.cc
+              LABELS quick)
+
+dune_add_test(SOURCES comparetest.cc
+              LABELS quick)
+
 dune_add_test(SOURCES extentstest.cc
               LABELS quick)