P2542R8 `views::concat`

[StephanTLavavej]

WG21-P2542R8 views::concat
LWG-4079 Missing Preconditions in concat_view::iterator's conversion constructor
LWG-4082 views::concat(r) is well-formed when r is an output_range
LWG-4166 concat_view::end() should be more constrained in order to support noncopyable iterators

Feature-test macro:

#define __cpp_lib_ranges_concat 202403L

Note: We're focused on implementing the remaining library-only features in C++23. Until that's done, we will NOT be accepting PRs for C++26 features.

[seishun]

I'm taking a stab at it in https://github.com/seishun/STL/tree/ranges. Tested only with clang-cl because cl doesn't support pack indexing yet.