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.

added concepts feature labels

Why do we need to re-implement such basic thing from C++20? Does any compiler hold us back? Shouldn't we think about bumping the compiler requirements instead of re-implementing so much C++20?

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• 31baafd5 - Provide library utilities for the three way comparison operator <=>

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• 906822c7 - Provide library utilities for the three way comparison operator <=>

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• 7c6a8c8c - Move the functor compare_three_way into compare.hh

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mentioned in commit 20ae67fb