[libc++] Implement P2417R2 (A more constexpr bitset)

Reviewed By: ldionne, #libc

Spies: jloser, arichardson, libcxx-commits, arphaman

Differential Revision: https://reviews.llvm.org/D131218

Author: philnik777

libcxx/docs/FeatureTestMacroTable.rst

@@ -308,6 +308,8 @@ Status
     ------------------------------------------------- -----------------
     ``__cpp_lib_byteswap``                            ``202110L``
     ------------------------------------------------- -----------------
+    ``__cpp_lib_constexpr_bitset``                    ``202207L``
+    ------------------------------------------------- -----------------
     ``__cpp_lib_constexpr_cmath``                     *unimplemented*
     ------------------------------------------------- -----------------
     ``__cpp_lib_constexpr_typeinfo``                  *unimplemented*

libcxx/docs/ReleaseNotes.rst

@@ -38,6 +38,7 @@ What's New in Libc++ 16.0.0?
 Implemented Papers
 ------------------
 - P2499R0 - ``string_view`` range constructor should be ``explicit``
+- P2417R2 - A more constexpr bitset
 
 Improvements and New Features
 -----------------------------

libcxx/docs/Status/Cxx2bPapers.csv

@@ -69,7 +69,7 @@
 "`P2374R4 <https://wg21.link/P2374R4>`__","LWG","``views::cartesian_product``","July 2022","",""
 "`P2404R3 <https://wg21.link/P2404R3>`__","LWG","Move-only types for ``equality_comparable_with``, ``totally_ordered_with``, and ``three_way_comparable_with``","July 2022","",""
 "`P2408R5 <https://wg21.link/P2408R5>`__","LWG","Ranges iterators as inputs to non-Ranges algorithms","July 2022","",""
-"`P2417R2 <https://wg21.link/P2417R2>`__","LWG","A more ``constexpr`` ``bitset``","July 2022","",""
+"`P2417R2 <https://wg21.link/P2417R2>`__","LWG","A more ``constexpr`` ``bitset``","July 2022","|Complete|","16.0"
 "`P2419R2 <https://wg21.link/P2419R2>`__","LWG","Clarify handling of encodings in localized formatting of chrono types","July 2022","",""
 "`P2438R2 <https://wg21.link/P2438R2>`__","LWG","``std::string::substr() &&``","July 2022","",""
 "`P2445R1 <https://wg21.link/P2445R1>`__","LWG","``forward_like``","July 2022","",""

libcxx/include/__bit_reference

@@ -263,7 +263,7 @@ find(__bit_iterator<_Cp, _IsConst> __first, __bit_iterator<_Cp, _IsConst> __last
 // count
 
 template <class _Cp, bool _IsConst>
-_LIBCPP_HIDE_FROM_ABI typename __bit_iterator<_Cp, _IsConst>::difference_type
+_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_AFTER_CXX20 typename __bit_iterator<_Cp, _IsConst>::difference_type
 __count_bool_true(__bit_iterator<_Cp, _IsConst> __first, typename _Cp::size_type __n)
 {
     typedef __bit_iterator<_Cp, _IsConst> _It;
@@ -1344,6 +1344,7 @@ private:
     _LIBCPP_CONSTEXPR_AFTER_CXX17
     friend __bit_iterator<_Dp, _IC> __find_bool_false(__bit_iterator<_Dp, _IC>, typename _Dp::size_type);
     template <class _Dp, bool _IC> friend typename __bit_iterator<_Dp, _IC>::difference_type
+    _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_AFTER_CXX20
                    __count_bool_true(__bit_iterator<_Dp, _IC>, typename _Dp::size_type);
     template <class _Dp, bool _IC> friend typename __bit_iterator<_Dp, _IC>::difference_type
                    __count_bool_false(__bit_iterator<_Dp, _IC>, typename _Dp::size_type);

libcxx/include/__config

@@ -155,7 +155,7 @@
 #    define _LIBCPP_ABI_BAD_FUNCTION_CALL_KEY_FUNCTION
 #  endif
 
-#  define _LIBCPP_TOSTRING2(x) #  x
+#  define _LIBCPP_TOSTRING2(x) #x
 #  define _LIBCPP_TOSTRING(x) _LIBCPP_TOSTRING2(x)
 
 #  if __cplusplus < 201103L
@@ -842,6 +842,12 @@ _LIBCPP_BEGIN_NAMESPACE_STD _LIBCPP_END_NAMESPACE_STD
 #    define _LIBCPP_CONSTEXPR_AFTER_CXX17
 #  endif
 
+#  if _LIBCPP_STD_VER > 20
+#    define _LIBCPP_CONSTEXPR_AFTER_CXX20 constexpr
+#  else
+#    define _LIBCPP_CONSTEXPR_AFTER_CXX20
+#  endif
+
 #  if __has_cpp_attribute(nodiscard)
 #    define _LIBCPP_NODISCARD [[nodiscard]]
 #  else