boost: Update to 1.75.0

Changelog:
1.75.0
New Libraries

  * JSON: JSON parsing, serialization, and DOM in C++11, from Vinnie Falco and
    Krystian Stasiowski.
      + Fast compilation requiring only C++11
      + Easy and safe modern API with allocator support
      + Compile without Boost, define BOOST_JSON_STANDALONE
      + Optional header-only, without linking to a library
  * LEAF: A lightweight error-handling library for C++11, from Emil Dotchevski.
      + Small single-header format, no dependencies.
      + Designed for maximum efficiency ("happy" path and "sad" path).
      + No dynamic memory allocations, even with heavy payloads.
      + O(1) transport of arbitrary error types (independent of call stack
        depth).
      + Can be used with or without exception handling.
  * PFR: Basic reflection without macro or boilerplate code for user defined
    types, from Antony Polukhin.

Updated Libraries

  * Asio:
      + Enabled support for UNIX domain sockets on Windows.
      + Added executor-converting construction and assignment to ip::
        basic_resolver.
      + Added compatibility between polymorphic executors and the (deprecated)
        handler invocation hook.
      + Added the experimental::as_single completion token adapter.
      + Added support for MSG_NOSIGNAL on more platforms by using
        _POSIX_VERSION to detect whether it is supported.
      + Added the ability to compile using libpthread on Windows.
      + Added workarounds for the Intel C++ compiler.
      + Added more support for detecting and optimising for handlers that have
        no custom executor.
      + Reduced lock contention for timer cancellation on Windows.
      + Reinstated a previously removed null-pointer check, as it had a
        measurable impact on performance.
      + Fixed the executor concept to test for a const-qualified execute().
      + Fixed any_executor support for builds without RTTI support.
      + Fixed the thread_pool unit test to work without RTTI support.
      + Fixed C++20 coroutines compatibility with clang on Windows.
      + Fixed some compatibility issues with Windows Runtime.
      + Fixed shadow name warnings caused by addition of asio::query.
      + Fixed a "logical ‘or’ of equal expressions" warning on linux.
      + Fixed a benign switch fallthrough warning.
      + Added missing push/pop_options.hpp includes.
      + Suppressed zero-as-null-pointer-constant warnings.
      + Fixed a comma-operator warning.
      + Updated the documentation to clarify when the select reactor is used on
        Windows.
      + Fixed potential ambiguity caused by any_executor comparisons and
        conversion.
      + Added detection of non-experimental C++20 coroutines on MSVC 19.8.
      + Fixed compatibility with uClibc.
      + Fixed strand<> adaptation of Networking TS executors when targeting
        older C++ versions or less conformant compilers.
      + Consult the Revision History for further details.
  * Atomic:
      + Implemented SSE2 and SSE4.1 versions of address lookup algorithm, which
        is used in the internal lock pool implementation. This may improve
        performance of waiting and notifying operations in heavily contended
        cases.
      + Fixed a possible compilation error on AArch64 targets caused by
        incorrect instructions generated for bitwise (logical) operations with
        immediate constants. (#41)
  * Beast:
      + This update brings bug fixes and support for the
        BOOST_ASIO_ENBALE_HANDLER_TRACKING compile flag from Boost.Asio:
      + We'd love to know how you or your company use Beast, consider adding an
        entry to the Companies and Individuals Using Beast list.
      + See the full Release Notes for a complete list of changes.
  * Container:
      + New devector container.
      + Fixed bugs/issues:
          o #152 Tree-based containers have troubles with move-only types.
          o #156 Compile error with vector.
          o PR#157 Add missing include.
          o #159: pmr::monotonic_buffer_resource crashes on large single
            allocations.
          o #160: Usage of uses_allocator needs a remove_cvref_t.
          o #162: small_vector on MSVC x86 call-by-value crash.
          o #161: polymorphic_allocator(memory_resource*) non-standard
            extension causes headache.
          o PR#163: container_rebind for small_vector with options.
          o #165: Link error with shared library and memory_resource inline
            members.
          o PR#166: Fix encoding error in copyright headers.
          o PR#167: error: the address of 'msg' will always evaluate as 'true'
            warning with GCC 4.4.
          o #169: flood of warnings building dlmalloc_ext_2_8_6.c on clang11.
  * Endian:
      + endian_arithmetic no longer inherits from endian_buffer
      + When BOOST_ENDIAN_NO_CTORS is defined, the unaligned endian_buffer and
        endian_arithmetic are C++03 PODs, to enable use of __attribute__((
        packed))
  * Filesystem:
      + New: Added creation_time operation, which allows to obtain file
        creation time. (Inspired by PR#134)
      + The returned value of last_write_time(p, ec) operation in case of
        failure has been changed to a minimal value representable by std::
        time_t instead of -1.
      + The returned value of hard_link_count(p, ec) operation in case of
        failure has been changed to static_cast<uintmax_t>(-1) instead of 0.
      + On POSIX systems, file_size will now indicate error code errc::
        function_not_supported if the path resolves to a non-regular file.
        Previously, errc::operation_not_permitted was reported.
      + On Linux, many operations now use statx system call internally, when
        possible, which allows to reduce the amount of information queried from
        the filesystem and potentially improve performance. The statx system
        call was introduced in Linux kernel 4.11.
      + Removed const-qualification from return types of some path methods.
        This could prevent move construction and move assignment at the call
        site in some cases. (#160)
      + On OpenBSD 4.4 and newer, use statvfs system call to obtain filesystem
        space information. (Inspired by PR#162)
      + On Windows, space now returns with an error if the provided path does
        not idendify an existing file. (#167)
  * GIL:
      + BREAKING: In next release, we are going to drop support for GCC 5. We
        may also change the required minimum C++ version from C++11 to C++14.
  * Histogram:
      + This update brings
          o Bug-fixes for corner-cases
          o Small documentation improvements
          o Fixes for new warnings from latest compilers and when compiling
            against the C++20 standard
      + See the full Release Notes for a complete list of changes.
  * Interprocess:
      + Fixed bugs:
          o #127: static assertion failure with boost interprocess 1.74 and
            basic_managed_shared_memory.
  * Intrusive:
      + Fixed bugs:
          o PR#48: MSVC "assignment within conditional" warning fix.
          o PR#49: Fix empty control statement warnings.
          o #52: Invalid casting in BOOST_INTRUSIVE_BSR_INTRINSIC.
  * Log:
      + Bug fixes:
          o Corrected the file counter that would be used in text_file_backend
            when generating the target file name (based on the pattern set by
            set_target_file_name_pattern method) when the log file is rotated.
            (#125)
          o Replaced a volatile version counter in basic_sink_frontend with an
            atomic. (#128)
          o In the asynchronous_sink frontend, resolved a possible conflict
            between flush and run methods, if run is called from a user's
            thread instead of the internal dedicated thread spawned by the
            frontend. (#131)
      + See changelog for more details.
  * Move:
      + Fixed bugs:
          o #30: (void) C-cast is a non-portable way of suppressing compiler
            warnings.
  * Mp11:
      + Added mp_pairwise_fold (suggested by Barry Revzin)
      + Removed mp_invoke (use mp_invoke_q)
  * Optional:
      + boost::none is constexpr-declared.
      + Fixed issue #78.
  * Outcome:
      + Announcements:
          o After a year and three major Boost releases announcing this
            upcoming change, this is the FINAL RELEASE of the v2.1 branch. From
            Boost 1.76 onwards, the v2.2 branch becomes the default. This
            branch has a number of major breaking changes to Outcome v2.1, see
            the documentation for details.
      + Enhancements:
          o The ADL discovered event hooks have been replaced with
            policy-specified event hooks instead. This is due to brittleness
            (where hooks would quietly self-disable if somebody changed
            something), compiler bugs (a difference in compiler settings causes
            the wrong hooks, or some but not all hooks, to get discovered), and
            end user difficulty in using them at all. The policy-specified
            event hooks can be told to default to ADL discovered hooks for
            backwards compatibility: set OUTCOME_ENABLE_LEGACY_SUPPORT_FOR to
            less than 220 to enable emulation.
          o Improve configuring OUTCOME_GCC6_CONCEPT_BOOL. Older GCCs had
            boolean based concepts syntax, whereas newer GCCs are standards
            conforming. However the precise logic of when to use legacy and
            conforming syntax was not well understood, which caused Outcome to
            fail to compile depending on what options you pass to GCC. The new
            logic always uses the legacy syntax if on GCC 8 or older, otherwise
            we use conforming syntax if and only if GCC is in C++ 20 mode or
            later. This hopefully will resolve the corner case build failures
            on GCC.
      + Bug fixes:
          o Boost.Outcome should now compile with BOOST_NO_EXCEPTIONS defined.
            Thanks to Emil, maintainer of Boost.Exception, making a change for
            me, Boost.Outcome should now compile with C++ exceptions globally
            disabled. You won't be able to use boost::exception_ptr as it can't
            be included if C++ exceptions are globally disabled.
          o #236 In the Coroutine support the final_suspend() was not noexcept,
            despite being required to be so in the C++ 20 standard. This has
            been fixed, but only if your compiler implements noop_coroutine.
            Additionally, if noop_coroutine is available, we use the much more
            efficient coroutine handle returning variant of await_suspend()
            which should significantly improve codegen and context switching
            performance.
  * Polygon:
      + C++20 fixes for event_comparison_type, vertex_equality_predicate_type,
        and voronoi_predicates. (Glen Fernandes)
  * Preprocessor:
      + When variadic data is empty in C++20 mode with __VA_OPT__ support the
        variadic size has been corrected to be 0. This also means that in this
        C++20 mode it is now valid to convert to and from empty arrays and
        lists and variadic data. The end-user can read the "C++20 Support For
        Variadic Macros" part of the "variadic macros" topic for more
        information about empty variadic data in the library.
      + The macro BOOST_PP_IS_STANDARD() has been added for identifying if the
        currently used preprocessor is a C++ standard conforming preprocessor.
        A number of preprocessors which generally work correctly with the
        library but need various internal workarounds, including the currently
        default VC++ preprocessor, are not considered C++ standard conforming
        preprocessors. However most preprocessors, including among others gcc,
        clang, and the new but currently non-default VC++ preprocessor in
        VS2019, are C++ standard conforming preprocessors.
      + For C++ standard conforming preprocessors a number of the limits
        defined in the config/limits.hpp can now be changed to higher amounts
        for a TU. The end-user should read the "limitations" topic to
        understand how and which limits can be changed.
      + For C++ standard conforming preprocessors, in order to allow the
        maximum number of FOR and WHILE iterations, the beginning 'r' and 'd'
        iteration numbers in the user-defined macros start at 1 and not 2, as
        it did in previous releases. This could be a breaking change if these
        iteration numbers are used in the user-defined macros ( they probably
        would not be ), but the change was necessary to fix some arcane bugs
        when dealing with numerical/logical operations with maximum numbers as
        well to allow the user-defined macros to be called the correct possible
        maximum number of times. For non-C++ conforming preprocessors, this
        change was not made because those non-conforming C++ preprocessors
        generally have limitations which disallow the maximum number of looping
        constructs to be run, and it was felt not to introduce a possible
        breaking change to those more fragile preprocessors would be better. It
        was also felt that besides fixing some arcane preprocessor bugs and
        providing the possible maximum number of user-defined macro
        invocations, this change could be made because it has never been
        documented what the starting 'r' and 'd' iteration numbers actually are
        but only that these numbers are incremented for each iteration.
      + The library has been upgraded to assume variadic macro support for any
        compiler working with the library. Ostensibly this means that the
        library is now a C++11 on up library, yet most of the major compilers,
        including gcc, clang, and VC++, also support variadic macros in C++98/
        C++03 mode as long as strict compliance to C++98/C++03 is not turned on
        when using one of those compilers.
  * Rational:
      + Fix Rational operators to not break under new C++20 operator==
        rewriting rules. (Glen Fernandes)
  * Signals2:
      + Correct C++ allocator model support to fix compilation in C++20
        standards mode. (Glen Fernandes)
  * System:
      + The platform-specific headers windows_error.hpp, linux_error.hpp, and
        cygwin_error.hpp emit deprecation messages and are slated for removal.
      + The old names for generic_category() and system_category() emit
        deprecation messages and are slated for removal.
      + error_condition::failed is deprecated and is slated for removal.
        operator bool() for error_condition has been reverted to its old
        meaning of value() != 0. This is done for compatibility with std::
        error_condition as the next release is expected to improve
        interoperability with <system_error> even further. Note that this does
        not affect error_code::failed, which is still alive and well.
      + The overload of error_condition::message that takes a buffer is
        deprecated and is slated for removal, for the same reasons. Note that
        this does not affect error_code::message.
  * uBLAS:
      + Correct C++ allocator model support to fix compilation in C++20
        standards mode. (Glen Fernandes and Conrad Poelman)
  * VMD:
      + The VMD number parsing has been upgraded to support the ability for the
        end-user to change the number limits in the Preprocessor library.
      + The macro BOOST_VMD_IS_GENERAL_IDENTIFIER has been added to support the
        parsing of input that represents a preprocessor token which matches the
        VMD identifier syntax, without having to register the identifier as a
        specific identifier.
  * Wave:
      + Added new C++20 tokens, including the spaceship operator <=>
      + Fixed bugs:
          o #94: fix incorrect behavior of __LINE__ and __FILE__ under
            rescanning

1.74.0
New Libraries

  * STLInterfaces: A library of CRTP bases to ease the writing of STL views,
    iterators, and sequence containers, from Zach Laine.

Updated Libraries

  * Asio:
      + Added an implementation of the proposed standard executors (P0443r13,
        P1348r0, and P1393r0).
      + Added support for the proposed standard executors to Asio's I/O
        facilities.
          o The supplied executors now meet the requirements for the proposed
            standard executors. These classes also continue to meet the
            existing requirements for the Networking TS model of executors.
          o All I/O objects, asynchronous operations, and utilities will
            interoperate with both new proposed standard executors, and with
            existing Networking TS executors.
          o The any_io_executor type alias has been introduced as the default
            runtime-polymorphic executor for all I/O objects. This defaults to
            the execution::any_executor<> template. If required for backward
            compatibility, BOOST_ASIO_USE_TS_EXECUTOR_AS_DEFAULT can be defined
            to use the old asio::executor polymorphic wrapper instead.
          o Support for the existing Networking TS model of executors can be
            disabled by defining BOOST_ASIO_NO_TS_EXECUTORS.
      + Added converting move construction and assignment to
        basic_waitable_timer.
      + Enabled C++20 coroutine support when using gcc 10.
      + Added overloads of co_spawn that launch an awaitable.
      + Added a new constructor overload to use_awaitable_t's default executor
        adapter, to enable conversion between executor types.
      + Added support for using detached_t as a default completion token, by
        adding members as_default_on() and as_default_on_t<>.
      + Added a move constructor to ssl::stream<>.
      + Changed ssl::stream<> write operations to linearise gather-write buffer
        sequences.
      + Added compile-time detection of the deprecated asio_handler_invoke,
        asio_handler_allocate, and asio_handler_deallocate hooks, when
        BOOST_ASIO_NO_DEPRECATED is defined.
      + Implemented a number of performance optimisations.
      + Added source location support to handler tracking.
      + Implemented various improvements to the handlerviz.pl tool.
      + Added the handlerlive.pl tool, which processes handler tracking output
        to produce a list of "live" handlers.
      + Added the handlertree.pl tool, which filters handler tracking output to
        include only those events in the tree that produced the nominated
        handlers.
      + Added changes for clang-based Embarcadero C++ compilers.
      + Fixed a deadlock that can occur when multiple threads concurrently
        initialise the Windows I/O completion port backend.
      + Fixed async_compose to work with copyable handlers when passed by
        lvalue.
      + Fixed completion signature deduction in co_spawn.
      + Removed a spurious Executor base class from the executor_binder
        implementation.
      + Various fixes and improvements in the documentation and examples.
      + Consult the Revision History for further details.
  * Atomic:
      + Added missing const qualifiers to some operations in atomic_ref.
      + Added support for yield instruction on ARMv8-A. The instruction is used
        internally in spin loops to reduce CPU power consumption.
      + Added support for C++20 waiting and notifying operations. The
        implementation includes generic backend that involves the internal lock
        pool, as well as specialized backends for Windows, Linux, FreeBSD,
        DragonFly BSD and NetBSD. Atomic types provide a new method
        has_native_wait_notify, a static boolean constant
        always_has_native_wait_notify and a set of capability macros that allow
        to detect if the implementation supports native waiting and notifying
        operations for a given type.
      + Changed internal representation of atomic_flag to use 32-bit storage.
        This allows for more efficient waiting and notifying operations on
        atomic_flag on some platforms.
      + Added support for build-time configuration of the internal lock pool
        size. The user can define the BOOST_ATOMIC_LOCK_POOL_SIZE_LOG2 macro to
        specify binary logarithm of the size of the lock pool. The default
        value is 8, meaning that the size of the lock pool is 256, up from 64
        used in the previous release.
      + Added support for a new set of atomic types dedicated for inter-process
        communication: ipc_atomic_flag, ipc_atomic and ipc_atomic_ref. Users
        are recommended to port their code using non-IPC types for
        inter-process communication to the new types. The new types provide the
        same set of operations as their non-IPC counterparts, with the
        following differences:
          o Most operations have an added precondition that is_lock_free
            returns true for the given atomic object. The library will issue a
            compile time error if this precondition is known to be not
            satisfied at compile time.
          o All provided operations are address-free, meaning that the atomic
            object (in case of ipc_atomic_ref - the referenced object) may be
            located in process-shared memory or mapped into the same process at
            multiple different addresses.
          o The new has_native_wait_notify operation and
            always_has_native_wait_notify constant indicate support for native
            inter-process waiting and notifying operations. When that support
            is not present, the operations are implemented with a busy loop,
            which is less efficient, but still is address-free. A separate set
            of capability macros is also provided to indicate this support.
      + Added new atomic_unsigned_lock_free and atomic_signed_lock_free types
        introduced in C++20. The types indicate the atomic object type for an
        unsigned or signed integer, respectively, that is lock-free and
        preferably has native support for waiting and notifying operations.
      + Added new gcc assembler backends for ARMv8-A (for both AArch32 and
        AArch64). The new backends are used to implement operations not
        supported by compiler intrinsics (including 128-bit operations on
        AArch64) and can also be used when compiler intrinsics are not
        available. Both little and big endian targets are supported. AArch64
        backend supports extensions defined in ARMv8.1 and ARMv8.3.
      + Added support for big endian targets in the legacy ARM backend based on
        gcc assembler blocks (this backend is used on ARMv7 and older targets).
        Previously, the backend assumed little endian memory layout, which is
        significant for 64-bit operations.
      + Improved performance of seq_cst stores and thread fences on x86 by
        using lock-prefixed instructions instead of mfence. This means that the
        operations no longer affect non-temporal stores, which was also not
        guaranteed before. Use specialized instructions and intrinsics to order
        non-temporal memory accesses.
      + Fixed capability macros for 80-bit long double on x86 targets not
        indicating lock-free operations even if 128-bit atomic operations were
        available.
      + Fixed compilation of gcc asm blocks on Alpha targets.
      + In the gcc __sync* intrinsics backend, fixed that store and load
        operations of large objects (larger than a pointer size) could be
        non-atomic. The implementation currently assumes that small objects can
        be stored with a single instruction atomically on all modern
        architectures.
  * Beast:
      + This update brings bug fixes and support for the following changes
        changes in Boost.Asio:
      + Beast supports BOOST_ASIO_NO_DEPRECATED. Define this to help identify
        areas of your Beast and Asio code which use deprecated Asio interfaces.
      + Beast also supports BOOST_ASIO_NO_TS_EXECUTORS. Define this to identify
        uses of executors from the now potentially outdated Networking TS
      + Asio will use the Standard Executors model by default. You can prevent
        this behaviour by defining BOOST_ASIO_USE_TS_EXECUTOR_AS_DEFAULT in
        which the Networking TS model will be used by default. Setting this
        flag does not prevent a program from using executors from the Standard
        Executors model explicitly.
      + We'd love to know how you or your company use Beast, consider adding an
        entry to the Companies and Individuals Using Beast list.
      + See the full Release Notes for a complete list of changes.
  * Bimap:
      + Correct allocator usage (fixes C++20 compilation). (Glen Fernandes)
  * Config:
      + Implement BOOST_NO_CXX11_OVERRIDE and BOOST_OVERRIDE. (Glen Fernandes)
  * Core:
      + Implemented the allocator access utilities which provide a replacement
        for allocator_traits with individual traits and functions for each
        facility. They support the C++11 allocator model when possible and
        provide a fallback for C++98 compatibility. These are now used in
        Circular_Buffer, Wave, Lockfree, Heap, Smart_Ptr, Dynamic_Bitset,
        Format, Bimap and more. (Glen Fernandes)
  * DLL:
      + Multiple fixes for the library_info work on empty shared objects.
      + Compilation fixes for C++98 and C++11 modes (#28).
      + Fixes for smart_library manglings (thanks to XiaLiChao82 #37).
  * Endian:
      + Enabled scoped enumeration types in endian_reverse.
      + Enabled bool, enum, float, double in endian_reverse_inplace.
      + Added an overload of endian_reverse_inplace for arrays.
  * Filesystem:
      + Removed compile-time checks for support for symlinks and hardlink on
        Windows. Instead, a runtime check is used. (PR#142)
      + Fixed handling of reparse points in canonical and read_symlink on
        Windows. This also affects other algorithms that involve canonical and
        read_symlink in their implementation. (PR#100, #85, #99, #123, #125)
      + Fixed that read_symlink on Windows could potentially fail or cause
        failures elsewhere with a sharing violation error, if the same symlink
        was opened concurrently. (#138)
      + Fixed that is_symlink(directory_entry) would always return false, even
        if the directory entry actually referred to a symlink. (PR#148)
      + Added missing status inspection operation overloads for directory_entry
        and error_code (e.g. is_directory(directory_entry, error_code&)).
        Removed incorrect noexcept specifications for the overloads not taking
        the error_code arguments.
      + copy_file implementation has been updated to perform checks on the
        source and target files, as required by C++20 ([fs.op.copy.file]/4.1).
        In particular, the operation will fail if the source or target file is
        not a regular file or the source and target paths identify the same
        file.
      + copy_file on POSIX systems will now also copy the source file
        permissions to the target file, if the target file is overwritten.
      + New: Added copy_file implementations based on sendfile and
        copy_file_range system calls on Linux, which may improve file copying
        performance, especially on network filesystems.
      + Deprecated: The copy_option enumeration that is used with the copy_file
        operation is deprecated. As a replacement, the new enum copy_options
        (note the trailing 's') has been added. The new enum contains values
        similar to the copy_options enum from C++20. The old enum values are
        mapped onto the new enum. The old enum will be removed in a future
        release.
      + New: Added copy_options::skip_existing option, which allows copy_file
        operation to succeed without overwriting the target file, if it exists.
      + New: Added copy_options::update_existing option, which allows copy_file
        operation to conditionally overwrite the target file, if it exists, if
        its last write time is older than that of the replacement file.
      + New: copy_file now returns bool, which indicates whether the file was
        copied.
      + New, breaking change: copy operation has been extended and reworked to
        implement behavior specified in C++20 [fs.op.copy]. This includes
        support for copy_options::recursive, copy_options::copy_symlinks,
        copy_options::skip_symlinks, copy_options::directories_only,
        copy_options::create_symlinks and copy_options::create_hard_links
        options. The operation performs additional checks based on the
        specified options. Applying copy to a directory with default
        copy_options will now also copy files residing in that directory (but
        not nested directories or files in those directories).
      + New: Added create_directory overload taking two paths. The second path
        is a path to an existing directory, which is used as a source of
        permission attributes to use in the directory to create.
      + Deprecated: copy_directory operation has been deprecated in favor of
        the new create_directory overload. Note that the two operations have
        reversed order of the path arguments.
      + equivalent on POSIX systems now returns the actual error code from the
        OS if one of the paths does not resolve to a file. Previously the
        function would return an error code of 1. (#141)
      + equivalent no longer considers file size and last modification time in
        order to test whether the two paths refer to the same file. These
        checks could result in a false negative if the file was modified during
        the equivalent call.
      + New: Added absolute overloads taking error_code argument.
      + Operations that have current_path() as the default value of their
        arguments and also have an error_code argument will use the
        current_path(error_code& ec) overload to obtain the current path, so
        that its failure is reported via the error_code argument instead of an
        exception.
      + space now initializes the space_info structure members to -1 values on
        error, as required by C++20 ([fs.op.space]/1).
      + space on Windows now accepts paths referring to arbitrary files, not
        only directories. This is similar to POSIX systems and corresponds to
        the operation description in C++20. (#73)
      + New: Added implementation of temp_directory_path for Windows CE. (PR#25
        )
      + New: Improved compatibility with WASI platform. (PR#144)
      + New: Improved support for Embarcadero compilers. (PR#130)
      + New: Added implementations of unique_path operation based on getrandom
        (Linux), arc4random_buf (OpenBSD/FreeBSD/CloudABI) and BCrypt (Windows)
        system APIs.
      + Deprecated: Auto-linking against system libraries on Windows with
        MSVC-compatible compilers is deprecated and will be removed in a future
        release. This affects users linking against static library of
        Boost.Filesystem. Users are advised to update their project build
        systems to either use a shared library of Boost.Filesystem, or
        explicitly specify the dependencies of Boost.Filesystem in the linker
        command line. Users of shared library of Boost.Filesystem are not
        affected.
  * Flyweight:
      + Maintenance work.
  * Format:
      + Correct allocator usage (fixes C++20 compilation). (Glen Fernandes)
  * Geometry:
      + Improvements
          o PR#720 Additional R-tree constructors (thanks to Caian Benedicto).
          o Various improvements in buffer, set and relational operations.
      + Solved issues
          o #709 memcpy called for object with no trivial copy-assignment.
          o #721 Compilation error in bgi::detail::rtree::visitors::insert.
          o #727 MSVC warning: conditional expression is constant.
      + Bugfixes
          o PR#700 Missing cases for default strategies in distance algorithm.
          o PR#738 Longitudes out of range in direct geodesic formulas.
  * GIL:
      + Added
          o Added new constructor initializing any_image from r-value reference
            to any image (PR#486).
          o Implemented mechanism to reverse kernel_2d (PR#489).
      + Changed
          o BREAKING: Replace Boost.Variant with Boost.Variant2 (PR#474) which
            completes removal on uses of Boost.MPL (missing from Boost 1.72.0
            change added PR#274).
          o Use perfect forwarding from apply_operation to visit (PR#491).
      + Removed
          o BREAKING: Removed dependency on Boost.Variant
      + Fixed
          o Fixed invalid conversion from RGB8 to CMYK32 due to overflow (PR#
            470).
          o Fixed image constructor from other image (PR#477).
          o Fixed error plane_view_t is not a class or namespace name (PR#481).
          o Fixed interleaved_view factory using point<std::ptrdiff_t> for
            dimension (PR#487).
          o Fixed documentation replacing uses MPL with MP11 in tutorial (PR#
            494).
          o Fixed missing header in numeric/kernel.hpp to make it
            self-contained (PR#502).
      + Acknowledgements
          o Samuel Debionne, Pranam Lashkari, Mateusz Loskot, Debabrata Mandal
  * Heap:
      + Correct destruction of top node in skew_heap. (Glen Fernandes)
      + Correct and simplify allocator use. (Glen Fernandes)
  * Integer:
      + Fixed compilation of gcd in C++20 mode with clang 10.
      + Improved support for Embarcadero compilers. (PR#21)
  * Iterator:
      + boost/function_output_iterator.hpp header is now deprecated. Users
        should replace its inclusion with boost/iterator/
        function_output_iterator.hpp. (PR#51)
      + Improved support for Embarcadero compilers. (PR#55)
  * LexicalCast:
      + Fixed warnings on missing override (thanks to EugeneZelenko #35, #34).
      + Fixes for the the Embarcadero compilers (thanks to Edward Diener).
  * Log:
      + Bug fixes:
          o The syslog sink backend now verifies the IP version of the local
            and target addresses set by user. The addresses must have the same
            IP version as was specified in the ip_version named parameter on
            the sink backend construction (by default, IPv4 is assumed). When
            an address is obtained as a result of host name resolution, only
            addresses with matching IP version are considered. (#119)
      + New Features:
          o Move constructors and assignment operators of various components
            were marked noexcept.
          o Added a new range_manip stream manipulator that can be used for
            outputting elements of a range, optionally separated by a
            delimiter.
          o Added a new tuple_manip stream manipulator that can be used for
            outputting elements of a tuple or any other heterogeneous sequence,
            optionally separated by a delimiter.
          o Added a new optional_manip stream manipulator that can be used for
            outputting optionally present values.
      + See changelog for more details.
  * Mp11:
      + Improved compilation performance of mp_with_index<N> for large N.
      + Added tuple_transform (contributed by Hans Dembinski.)
  * Multi-index Containers:
      + Added node extraction and insertion following the analogous interface
        of associative containers as introduced in C++17. This feature has also
        been extended to non key-based indices, in contrast to C++ standard
        library sequence containers, which do not provide such functionality.
      + Clarified documentation on read/write key extractors (issue #32).
      + Maintenance work.
  * Nowide:
      + The library now requires a C++11-compliant compiler and stdlib
      + LFS: Add support for files > 2 GB where the underlying system supports
        it
      + Generic UTF conversion functions are now available in the boost::nowide
        ::utf namespace
      + Add support for stat with UTF-8 paths
  * Outcome:
      + Announcements:
          o The v2.1 branch is expected to be retired end of 2020, with the
            v2.2 branch becoming the default. You can use the future v2.2
            branch now using better_optimisation. This branch has a number of
            major breaking changes to Outcome v2.1, see the front page for
            details.
      + Enhancements:
          o BREAKING CHANGE void results and outcomes no longer default
            construct types during explicit construction. Previously if you
            explicitly constructed a result<T> from a non-errored result<void>,
            it default constructed T. This was found to cause unhelpful
            surprise, so it has been disabled.
          o New macro OUTCOME_ENABLE_LEGACY_SUPPORT_FOR. The macro
            OUTCOME_ENABLE_LEGACY_SUPPORT_FOR can be used to enable aliasing of
            older naming and features to newer naming and features when using a
            newer version of Outcome.
          o Concepts now have snake case style naming instead of camel case
            style. When Outcome was first implemented, it was thought that C++
            20 concepts were going to have camel case style. This was changed
            before the C++ 20 release, and Outcome's concepts have been renamed
            similarly. This won't break any code in Outcome v2.1, as
            compatibility aliases are provided. However code compiled against
            Outcome v2.2 will need to be upgraded, unless
            OUTCOME_ENABLE_LEGACY_SUPPORT_FOR is set to 210 or lower.
          o Concepts now live in OUTCOME_V2_NAMESPACE::concepts namespace.
            Previously concepts lived in the convert namespace, now they live
            in their own namespace.
          o New concepts basic_result<T> and basic_outcome<T> added. End users
            were finding an unhelpful gap in between is_basic_result<T> and
            value_or_error<T> where they wanted a concept that matched types
            which were basic_result, but not exactly one of those. Concepts
            filling that gap were added.
          o Operation TRY works differently from Outcome v2.2 onwards. This is
            a severely code breaking change which change the syntax of how one
            uses OUTCOME_TRY(). A regular expression suitable for upgrading
            code can be found in the list of changes between Outcome v2.1 and
            v2.2.
      + Bug fixes:
          o #224 The clang Apple ships in Xcode 11.4 (currently the latest) has
            not been patched with the fixes to LLVM clang that fix noexcept(std
            ::is_constructible<T, void>) failing to compile which I originally
            submitted years ago. So give up waiting on Apple to fix their
            clang, add a workaround to Outcome.
          o Spare storage could not be used from within no-value policy
            classes. Due to an obvious brain fart when writing the code at the
            time, the spare storage APIs had the wrong prototype which
            prevented them working from within policy classes. Sorry.
  * PolyCollection:
      + Fixed internal ambiguity problem between boost::type_erasure::any and
        boost::any (issue #17).
      + Maintenance work.
  * SmartPtr:
      + Added owner_equals to shared_ptr, weak_ptr, local_shared_ptr.
      + Added owner_hash_value to shared_ptr, weak_ptr.
      + Added owner_equal_to, owner_hash.
      + Added std::hash specializations for shared_ptr, local_shared_ptr.
      + Added boost::hash support to, and std::hash, std::equal_to
        specializations for, weak_ptr.
  * Stacktrace:
      + Fixed a build error when compiled with -fno-exceptions (thanks to
        Jeremiah Rodriguez #91).
  * System:
      + operator bool() now returns failed() instead of value() != 0.
  * Type_Traits:
      + Implemented conjunction, disjunction, negation, is_trivially_copyable,
        is_scoped_enum, and is_unscoped_enum. (Glen Fernandes)
  * Variant:
      + Fixed warnings on missing override (thanks to EugeneZelenko #78).
      + Fixes for the the Embarcadero compilers (thanks to Edward Diener #79).
      + Updated header locations to avoid warnings about using deprecated
        headers (thanks to Andrey Semashev #80)
  * Variant2:
      + Added support for derived types in visit.
      + Improved compilation performance for many (hundreds of) alternatives.
      + Added support for visit<R>.
  * Wave:
      + Implement C++20 features for variadics, including __VA_OPT__ (PR#75)
      + Implement __has_include (PR#102)
      + Introduce new sample: check_macro_naming, useful with Boost itself (PR#
        97)
      + Fix compilation issue caused by std::allocator member removal in C++20
        (PR#72)
      + Repair Xpressive lexer and token_statistics sample (PR#79)
      + Repair lexertl lexer (PR#78)
      + Ensure hooks are run on predefined macros as well (PR#87)
      + Various minor bug fixes
      + C++98/03 support is now deprecated and will be removed in 1.77
  * YAP:
      + Fixed compilation errors for placeholders; they now work in the general
        case, and in particular work with yap::print().
      + constexpr all the YAP.
      + Fix printing of correct value category in yap::print().
      + Doc clarification.

Updated Tools

  * Boostbook:
      + Change encoding of generated documentation from US-ASCII to UTF-8.
        (Glen Fernandes)

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