Implement syncstream (p0053)

This patch implements `std::basic_syncbuf` and `std::basic_osyncstream` as specified in paper p0053r7. ~~For ease of reviewing I am submitting this patch before submitting a patch for `std::basic_osyncstream`. ~~

~~Please note, this patch is not 100% complete. I plan on adding more tests (see comments), specifically I plan on adding tests for multithreading and synchronization.~~

Edit: I decided that it would be far easier for me to keep track of this and make changes that affect both `std::basic_syncbuf` and `std::basic_osyncstream` if both were in one patch.

The patch was originally written by @zoecarver

Implements
- P0053R7 - C++ Synchronized Buffered Ostream
- LWG-3127 basic_osyncstream::rdbuf needs a const_cast
- LWG-3334 basic_osyncstream move assignment and destruction calls basic_syncbuf::emit() twice
- LWG-3570 basic_osyncstream::emit should be an unformatted output function
- LWG-3867 Should std::basic_osyncstream's move assignment operator be noexcept?

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D67086

Author: mordante

libcxx/docs/FeatureTestMacroTable.rst

@@ -286,7 +286,7 @@ Status
     --------------------------------------------------- -----------------
     ``__cpp_lib_string_view``                           ``201803L``
     --------------------------------------------------- -----------------
-    ``__cpp_lib_syncbuf``                               *unimplemented*
+    ``__cpp_lib_syncbuf``                               ``201803L``
     --------------------------------------------------- -----------------
     ``__cpp_lib_three_way_comparison``                  *unimplemented*
     --------------------------------------------------- -----------------

libcxx/docs/ReleaseNotes/18.rst

@@ -47,6 +47,7 @@ Implemented Papers
 - P2443R1 - ``views::chunk_by``
 - P2538R1 - ADL-proof ``std::projected``
 - P2614R2 - Deprecate ``numeric_limits::has_denorm``
+- P0053R7 - C++ Synchronized Buffered Ostream (in the experimental library)
 
 
 Improvements and New Features

libcxx/docs/Status/Cxx20Issues.csv

@@ -114,7 +114,7 @@
 "`3096 <https://wg21.link/LWG3096>`__","``path::lexically_relative``\  is confused by trailing slashes","San Diego","|Complete|",""
 "`3116 <https://wg21.link/LWG3116>`__","``OUTERMOST_ALLOC_TRAITS``\  needs ``remove_reference_t``\ ","San Diego","",""
 "`3122 <https://wg21.link/LWG3122>`__","``__cpp_lib_chrono_udls``\  was accidentally dropped","San Diego","|Complete|",""
-"`3127 <https://wg21.link/LWG3127>`__","``basic_osyncstream::rdbuf``\  needs a ``const_cast``\ ","San Diego","",""
+"`3127 <https://wg21.link/LWG3127>`__","``basic_osyncstream::rdbuf``\ needs a ``const_cast``\ ","San Diego","|Complete|","18.0"
 "`3128 <https://wg21.link/LWG3128>`__","``strstream::rdbuf``\  needs a ``const_cast``\ ","San Diego","|Nothing To Do|",""
 "`3129 <https://wg21.link/LWG3129>`__","``regex_token_iterator``\  constructor uses wrong pointer arithmetic","San Diego","",""
 "`3130 <https://wg21.link/LWG3130>`__","|sect|\ [input.output] needs many ``addressof``\ ","San Diego","",""
@@ -254,7 +254,7 @@
 "`3330 <https://wg21.link/LWG3330>`__","Include ``<compare>``\  from most library headers","Prague","|Complete|","13.0","|spaceship|"
 "`3331 <https://wg21.link/LWG3331>`__","Define ``totally_ordered/_with``\  in terms of ``partially-ordered-with``\ ","Prague","|Complete|","13.0"
 "`3332 <https://wg21.link/LWG3332>`__","Issue in |sect|\ [time.format]","Prague","|Complete|","16.0","|chrono| |format|"
-"`3334 <https://wg21.link/LWG3334>`__","``basic_osyncstream``\  move assignment and destruction calls ``basic_syncbuf::emit()``\  twice","Prague","",""
+"`3334 <https://wg21.link/LWG3334>`__","``basic_osyncstream``\  move assignment and destruction calls ``basic_syncbuf::emit()``\ twice","Prague","|Complete|","18.0"
 "`3335 <https://wg21.link/LWG3335>`__","Resolve C++20 NB comments US 273 and GB 274","Prague","|Complete|","15.0","|ranges|"
 "`3338 <https://wg21.link/LWG3338>`__","Rename ``default_constructible``\  to ``default_initializable``\ ","Prague","|Complete|","13.0"
 "`3340 <https://wg21.link/LWG3340>`__","Formatting functions should throw on argument/format string mismatch in |sect|\ [format.functions]","Prague","|Complete|","14.0","|format|"

libcxx/docs/Status/Cxx20Papers.csv

@@ -3,7 +3,7 @@
 "`P0674R1 <https://wg21.link/P0674R1>`__","LWG","Extending make_shared to Support Arrays","Toronto","|Complete|","15.0"
 "","","","","","",""
 "`P0020R6 <https://wg21.link/P0020R6>`__","LWG","Floating Point Atomic","Albuquerque","",""
-"`P0053R7 <https://wg21.link/P0053R7>`__","LWG","C++ Synchronized Buffered Ostream","Albuquerque","",""
+"`P0053R7 <https://wg21.link/P0053R7>`__","LWG","C++ Synchronized Buffered Ostream","Albuquerque","|Complete|","18.0"
 "`P0202R3 <https://wg21.link/P0202R3>`__","LWG","Add constexpr modifiers to functions in <algorithm> and <utility> Headers","Albuquerque","|Complete|","12.0"
 "`P0415R1 <https://wg21.link/P0415R1>`__","LWG","Constexpr for ``std::complex``\ ","Albuquerque","|Complete|","16.0"
 "`P0439R0 <https://wg21.link/P0439R0>`__","LWG","Make ``std::memory_order``\  a scoped enumeration","Albuquerque","|Complete|",""

libcxx/docs/Status/Cxx23Issues.csv

@@ -122,7 +122,7 @@
 `3566 <https://wg21.link/LWG3566>`__,"Constraint recursion for ``operator<=>(optional<T>, U)``","October 2021","|Complete|","17.0","|spaceship|"
 `3567 <https://wg21.link/LWG3567>`__,"Formatting move-only iterators take two","October 2021","|Complete|","16.0","|format| |ranges|"
 `3568 <https://wg21.link/LWG3568>`__,"``basic_istream_view`` needs to initialize ``value_``","October 2021","|Complete|","16.0","|ranges|"
-`3570 <https://wg21.link/LWG3570>`__,"``basic_osyncstream::emit`` should be an unformatted output function","October 2021","",""
+`3570 <https://wg21.link/LWG3570>`__,"``basic_osyncstream::emit`` should be an unformatted output function","October 2021","|Complete|","18.0"
 `3571 <https://wg21.link/LWG3571>`__,"``flush_emit`` should set ``badbit`` if the ``emit`` call fails","October 2021","",""
 `3572 <https://wg21.link/LWG3572>`__,"``copyable-box`` should be fully ``constexpr``","October 2021","|Complete|","14.0","|ranges|"
 `3573 <https://wg21.link/LWG3573>`__,"Missing Throws element for ``basic_string_view(It begin, End end)``","October 2021","|Complete|","14.0"
@@ -275,7 +275,7 @@
 "`3857 <https://wg21.link/LWG3857>`__","``basic_string_view`` should allow explicit conversion when only traits vary","February 2023","|Complete|","17.0",""
 "`3860 <https://wg21.link/LWG3860>`__","``range_common_reference_t`` is missing","February 2023","|Complete|","17.0","|ranges|"
 "`3866 <https://wg21.link/LWG3866>`__","Bad Mandates for ``expected::transform_error`` overloads","February 2023","|Complete|","17.0",""
-"`3867 <https://wg21.link/LWG3867>`__","Should ``std::basic_osyncstream``'s move assignment operator be ``noexcept``?","February 2023","","",""
+"`3867 <https://wg21.link/LWG3867>`__","Should ``std::basic_osyncstream``'s move assignment operator be ``noexcept``?","February 2023","|Complete|","18.0",""
 "`3441 <https://wg21.link/LWG3441>`__","Misleading note about calls to customization points","February 2023","","",""
 "`3622 <https://wg21.link/LWG3622>`__","Misspecified transitivity of equivalence in §[unord.req.general]","February 2023","","",""
 "`3631 <https://wg21.link/LWG3631>`__","``basic_format_arg(T&&)`` should use ``remove_cvref_t<T>`` throughout","February 2023","|Complete|","17.0",""

libcxx/include/CMakeLists.txt

@@ -1000,6 +1000,7 @@ set(files
   string.h
   string_view
   strstream
+  syncstream
   system_error
   tgmath.h
   thread

libcxx/include/__config

@@ -452,6 +452,7 @@
 #    define _LIBCPP_HAS_NO_INCOMPLETE_PSTL
 #    define _LIBCPP_HAS_NO_EXPERIMENTAL_STOP_TOKEN
 #    define _LIBCPP_HAS_NO_INCOMPLETE_TZDB
+#    define _LIBCPP_HAS_NO_EXPERIMENTAL_SYNCSTREAM
 #  endif
 
 // Need to detect which libc we're using if we're on Linux.

libcxx/include/__std_clang_module

@@ -200,6 +200,9 @@
 #if !defined(_LIBCPP_HAS_NO_LOCALIZATION)
 #  include <strstream>
 #endif
+#if !defined(_LIBCPP_HAS_NO_LOCALIZATION)
+#  include <syncstream>
+#endif
 #include <system_error>
 #include <tgmath.h>
 #if !defined(_LIBCPP_HAS_NO_THREADS)

libcxx/include/iosfwd

@@ -90,6 +90,18 @@ using u8streampos = fpos<char_traits<char8_t>::state_type>; // C++20
 using u16streampos = fpos<char_traits<char16_t>::state_type>;
 using u32streampos = fpos<char_traits<char32_t>::state_type>;
 
+template <class charT, class traits = char_traits<charT>, class Allocator = allocator<charT>>
+    class basic_syncbuf;                          // C++20
+
+using syncbuf = basic_syncbuf<char>;              // C++20
+using wsyncbuf = basic_syncbuf<wchar_t>;          // C++20
+
+template <class charT, class traits = char_traits<charT>, class Allocator = allocator<charT>>
+    class basic_osyncstream;                      // C++20
+
+using osyncstream = basic_osyncstream<char>;      // C++20
+using wosyncstream = basic_osyncstream<wchar_t>;  // C++20
+
 }  // std
 
 */
@@ -130,6 +142,26 @@ typedef fpos<mbstate_t>    u8streampos;
 typedef fpos<mbstate_t>    u16streampos;
 typedef fpos<mbstate_t>    u32streampos;
 
+#if _LIBCPP_STD_VER >= 20
+
+template <class _CharT, class _Traits = char_traits<_CharT>, class _Allocator = allocator<_CharT>>
+class basic_syncbuf;
+
+using syncbuf  = basic_syncbuf<char>;
+#ifndef _LIBCPP_HAS_NO_WIDE_CHARACTERS
+using wsyncbuf = basic_syncbuf<wchar_t>;
+#endif
+
+template <class _CharT, class _Traits = char_traits<_CharT>, class _Allocator = allocator<_CharT>>
+class basic_osyncstream;
+
+using osyncstream  = basic_osyncstream<char>;
+#ifndef _LIBCPP_HAS_NO_WIDE_CHARACTERS
+using wosyncstream = basic_osyncstream<wchar_t>;
+#endif
+
+#endif // _LIBCPP_STD_VER >=20
+
 // Include other forward declarations here
 template <class _Tp, class _Alloc = allocator<_Tp> >
 class _LIBCPP_TEMPLATE_VIS vector;

libcxx/include/map

@@ -1700,6 +1700,8 @@ template <class _Key, class _Tp, class _Compare, class _Allocator>
 _Tp&
 map<_Key, _Tp, _Compare, _Allocator>::operator[](key_type&& __k)
 {
+    // TODO investigate this clang-tidy warning.
+    // NOLINTNEXTLINE(bugprone-use-after-move)
     return __tree_.__emplace_unique_key_args(__k,
         _VSTD::piecewise_construct,
         _VSTD::forward_as_tuple(_VSTD::move(__k)),

libcxx/include/streambuf

@@ -110,6 +110,7 @@ protected:
 #include <__assert> // all public C++ headers provide the assertion handler
 #include <__config>
 #include <__fwd/streambuf.h>
+#include <climits>
 #include <ios>
 #include <iosfwd>
 #include <version>