[libc++][ranges] P2542R8: Implement views::concat (#120920)

Closes #105419
Closes #105348
Closes #105349
Closes #171314

Assisted-by: Chatgpt. 
I use AI to help me write some tests, however. I have reviewed the code
I submit in the tests.

---------

Co-authored-by: A. Jiang <de34@live.cn>
Co-authored-by: Hristo Hristov <hghristov.rmm@gmail.com>
Co-authored-by: Hristo Hristov <zingam@outlook.com>

Author: 4 people

libcxx/docs/FeatureTestMacroTable.rst

@@ -506,7 +506,7 @@ Status
     ---------------------------------------------------------- -----------------
     ``__cpp_lib_philox_engine``                                *unimplemented*
     ---------------------------------------------------------- -----------------
-    ``__cpp_lib_ranges_concat``                                *unimplemented*
+    ``__cpp_lib_ranges_concat``                                ``202403L``
     ---------------------------------------------------------- -----------------
     ``__cpp_lib_ranges_indices``                               ``202506L``
     ---------------------------------------------------------- -----------------

libcxx/docs/ReleaseNotes/23.rst

@@ -48,6 +48,7 @@ Implemented Papers
 - P2164R9: ``views::enumerate`` (`Github <https://llvm.org/PR105251>`__)
 - P2322R6: ``ranges::fold`` (`Github <https://llvm.org/PR105208>`__)
 - P4144R1: Remove ``span``'s ``initializer_list`` constructor for C++26 (`Github <https://llvm.org/PR189612>`__)
+- P2542R8: ``views::concat`` (`Github <https://llvm.org/PR105419>`__)
 - P3383R3: ``mdspan.at()`` (`Github <https://llvm.org/PR175213>`__)
 - P3508R0: Wording for "constexpr for specialized memory algorithms" (`Github <https://llvm.org/PR118379>`__)
 

libcxx/docs/Status/Cxx2cIssues.csv

@@ -68,8 +68,8 @@
 "`LWG4071 <https://wg21.link/LWG4071>`__","``reference_wrapper`` comparisons are not SFINAE-friendly","2024-06 (St. Louis)","|Complete|","19","`#105345 <https://github.com/llvm/llvm-project/issues/105345>`__",""
 "`LWG4074 <https://wg21.link/LWG4074>`__","``compatible-joinable-ranges`` is underconstrained","2024-06 (St. Louis)","|Complete|","21","`#105346 <https://github.com/llvm/llvm-project/issues/105346>`__",""
 "`LWG4076 <https://wg21.link/LWG4076>`__","``concat_view`` should be freestanding","2024-06 (St. Louis)","","","`#105347 <https://github.com/llvm/llvm-project/issues/105347>`__",""
-"`LWG4079 <https://wg21.link/LWG4079>`__","Missing Preconditions in ``concat_view::iterator``\`s conversion constructor","2024-06 (St. Louis)","","","`#105348 <https://github.com/llvm/llvm-project/issues/105348>`__",""
-"`LWG4082 <https://wg21.link/LWG4082>`__","``views::concat(r)`` is well-formed when ``r`` is an ``output_range``","2024-06 (St. Louis)","","","`#105349 <https://github.com/llvm/llvm-project/issues/105349>`__",""
+"`LWG4079 <https://wg21.link/LWG4079>`__","Missing Preconditions in ``concat_view::iterator``\`s conversion constructor","2024-06 (St. Louis)","|Complete|","23","`#105348 <https://github.com/llvm/llvm-project/issues/105348>`__",""
+"`LWG4082 <https://wg21.link/LWG4082>`__","``views::concat(r)`` is well-formed when ``r`` is an ``output_range``","2024-06 (St. Louis)","|Complete|","23","`#105349 <https://github.com/llvm/llvm-project/issues/105349>`__",""
 "`LWG4083 <https://wg21.link/LWG4083>`__","``views::as_rvalue`` should reject non-input ranges","2024-06 (St. Louis)","|Complete|","22","`#105351 <https://github.com/llvm/llvm-project/issues/105351>`__",""
 "`LWG4096 <https://wg21.link/LWG4096>`__","``views::iota(views::iota(0))`` should be rejected","2024-06 (St. Louis)","|Complete|","22","`#105352 <https://github.com/llvm/llvm-project/issues/105352>`__",""
 "`LWG4098 <https://wg21.link/LWG4098>`__","``views::adjacent<0>`` should reject non-forward ranges","2024-06 (St. Louis)","","","`#105353 <https://github.com/llvm/llvm-project/issues/105353>`__",""
@@ -155,7 +155,7 @@
 "`LWG4020 <https://wg21.link/LWG4020>`__","``extents::index-cast`` weirdness","2025-11 (Kona)","|Complete|","23","`#171311 <https://github.com/llvm/llvm-project/issues/171311>`__",""
 "`LWG4136 <https://wg21.link/LWG4136>`__","Specify behavior of [linalg] Hermitian algorithms on diagonal with nonzero imaginary part","2025-11 (Kona)","","","`#171312 <https://github.com/llvm/llvm-project/issues/171312>`__",""
 "`LWG4137 <https://wg21.link/LWG4137>`__","Fix *Mandates*, *Preconditions*, and *Complexity* elements of [linalg] algorithms","2025-11 (Kona)","","","`#171313 <https://github.com/llvm/llvm-project/issues/171313>`__",""
-"`LWG4166 <https://wg21.link/LWG4166>`__","``concat_view::end()`` should be more constrained in order to support noncopyable iterators","2025-11 (Kona)","","","`#171314 <https://github.com/llvm/llvm-project/issues/171314>`__",""
+"`LWG4166 <https://wg21.link/LWG4166>`__","``concat_view::end()`` should be more constrained in order to support noncopyable iterators","2025-11 (Kona)","|Complete|","23","`#171314 <https://github.com/llvm/llvm-project/issues/171314>`__",""
 "`LWG4230 <https://wg21.link/LWG4230>`__","``simd<complex>::real/imag`` is overconstrained","2025-11 (Kona)","","","`#171316 <https://github.com/llvm/llvm-project/issues/171316>`__",""
 "`LWG4243 <https://wg21.link/LWG4243>`__","``as_bytes``/``as_writable_bytes`` is broken with ``span<volatile T>``","2025-11 (Kona)","","","`#171317 <https://github.com/llvm/llvm-project/issues/171317>`__",""
 "`LWG4251 <https://wg21.link/LWG4251>`__","Move assignment for ``indirect`` unnecessarily requires copy construction","2025-11 (Kona)","","","`#171318 <https://github.com/llvm/llvm-project/issues/171318>`__",""

libcxx/docs/Status/Cxx2cPapers.csv

@@ -54,7 +54,7 @@
 "`P3142R0 <https://wg21.link/P3142R0>`__","Printing Blank Lines with ``println``","2024-03 (Tokyo)","|Complete|","19","`#105415 <https://github.com/llvm/llvm-project/issues/105415>`__","Implemented as a DR against C++23. (MSVC STL and libstdc++ will do the same.)"
 "`P2845R8 <https://wg21.link/P2845R8>`__","Formatting of ``std::filesystem::path``","2024-03 (Tokyo)","","","`#105416 <https://github.com/llvm/llvm-project/issues/105416>`__",""
 "`P0493R5 <https://wg21.link/P0493R5>`__","Atomic minimum/maximum","2024-03 (Tokyo)","","","`#105418 <https://github.com/llvm/llvm-project/issues/105418>`__",""
-"`P2542R8 <https://wg21.link/P2542R8>`__","``views::concat``","2024-03 (Tokyo)","","","`#105419 <https://github.com/llvm/llvm-project/issues/105419>`__",""
+"`P2542R8 <https://wg21.link/P2542R8>`__","``views::concat``","2024-03 (Tokyo)","|Complete|","23","`#105419 <https://github.com/llvm/llvm-project/issues/105419>`__",""
 "`P2591R5 <https://wg21.link/P2591R5>`__","Concatenation of strings and string views","2024-03 (Tokyo)","|Complete|","19","`#105420 <https://github.com/llvm/llvm-project/issues/105420>`__",""
 "`P2248R8 <https://wg21.link/P2248R8>`__","Enabling list-initialization for algorithms","2024-03 (Tokyo)","","","`#105421 <https://github.com/llvm/llvm-project/issues/105421>`__",""
 "`P2810R4 <https://wg21.link/P2810R4>`__","``is_debugger_present`` ``is_replaceable``","2024-03 (Tokyo)","","","`#105422 <https://github.com/llvm/llvm-project/issues/105422>`__",""

libcxx/include/CMakeLists.txt

@@ -724,6 +724,7 @@ set(files
   __ranges/as_rvalue_view.h
   __ranges/chunk_by_view.h
   __ranges/common_view.h
+  __ranges/concat_view.h
   __ranges/concepts.h
   __ranges/container_compatible_range.h
   __ranges/counted.h