Release VS 2019 16.8 · microsoft/STL

Merged C++20 features:
- P0019R8 #843 atomic_ref
- P0528R3 #1029 Atomic Compare-And-Exchange With Padding Bits (for MSVC; Clang support is pending compiler intrinsics, see LLVM-46685)
- Completed P0553R4 and P0556R3 #795 by adding MSVC support for the functions in <bit> that were previously only supported when compiling with Clang (bit_ceil, bit_floor, bit_width, countl_one, countl_zero, countr_one, countr_zero, has_single_bit, popcount, rotl, and rotr).
- Completed P0811R3 #1048 midpoint(), lerp() by adding constexpr support and improving handling of infinite arguments.
- P0912R5 #894 #961 #1204 Library Support For Coroutines
- P1001R2 #1111 execution::unseq
- P1032R1 #491 #861 Miscellaneous constexpr
- P1065R2 #703 constexpr INVOKE
- Completed P1135R6 The C++20 Synchronization Library:
  - This feature adds a new satellite DLL "atomic_wait".
  - #593 #1143 #1179 std::atomic::wait
  - P1644R0 #1136 Adding wait/notify To atomic<shared_ptr<T>>, atomic<weak_ptr<T>>
  - #1057 <barrier>, <latch>, and <semaphore>
Merged partial C++20 features:
- P0896R4 Ranges:
  - #888 algorithm ranges::move
  - #892 algorithm ranges::is_permutation
  - #896 algorithm ranges::search
  - #904 algorithms ranges::fill and ranges::fill_n
  - #905 algorithms ranges::generate and ranges::generate_n
  - #914 algorithm ranges::search_n
  - #915 algorithms ranges::is_sorted and ranges::is_sorted_until
  - #916 algorithms ranges::min, ranges::max, ranges::minmax, ranges::clamp, ranges::min_element, ranges::max_element, and ranges::minmax_element
  - #917 algorithms ranges::lower_bound, ranges::upper_bound, ranges::equal_range, and ranges::binary_search
  - #918 algorithm ranges::swap_ranges
  - #925 concepts std::permutable and std::sortable
  - #930 algorithms ranges::is_heap, ranges::is_heap_until, ranges::make_heap, ranges::push_heap, ranges::pop_heap, and ranges::sort_heap
  - #941 algorithm ranges::transform
  - #976 algorithms ranges::is_partitioned, ranges::partition, ranges::partition_copy, and ranges::partition_point
  - #983 algorithms ranges::replace, ranges::replace_copy, and ranges::replace_copy_if
  - #984 algorithm ranges::replace_if
  - #1005 algorithms ranges::remove, ranges::remove_if, ranges::remove_copy, and ranges::remove_copy_if
  - #1026 algorithm ranges::copy_backward
  - #1028 algorithm ranges::reverse (vectorized like std::reverse)
  - #1039 algorithms ranges::unique and ranges::unique_copy
  - #1044 Concept mergeable, and algorithms ranges::includes, ranges::set_difference, ranges::set_intersection, ranges::set_symmetric_difference, and ranges::set_union
  - #1052 algorithms ranges::sample and ranges::shuffle
  - #1053 algorithm ranges::move_backward
  - #1063 algorithm ranges::nth_element
  - #1073 algorithms ranges::rotate and ranges::rotate_copy
  - #1076 range access function ranges::ssize
  - #1081 algorithm ranges::lexicographical_compare
  - #1083 iterator adaptor counted_iterator
  - #1095 algorithm ranges::reverse_copy (vectorized like std::reverse_copy)
  - #1099 algorithms ranges::next_permutation and ranges::prev_permutation
  - #1101 algorithm ranges::merge
  - #1127 algorithm ranges::sort
  - #1132 view ranges::ref_view
  - #1137 algorithm ranges::uninitialized_move
  - #1201 range factories views::empty and views::single
  - #1229 range adaptors views::all and views::reverse
  - #1231 algorithms ranges::partial_sort and ranges::partial_sort_copy
  - #1245 P1391R4 Range Constructor For string_view
  - #1252 range adaptor views::filter
  - #1253 range adaptors views::drop and views::take
  - #1258 range adaptor views::transform
Merged LWG issue resolutions:
- LWG-1203 #1225 More useful rvalue stream insertion
- LWG-2597 #884 std::log misspecified for complex numbers
- LWG-3070 #923 path::lexically_relative causes surprising results if a filename can also be a root-name
Fixed bugs:
- variant no longer emits warnings about narrowing/sign conversions when determining which alternative should be activated for a given initializer. (Warnings can still be emitted by the chosen initialization; that's intentional and desirable.) #858
- condition_variable_any now correctly unlocks-and-relocks the lock argument to timed wait functions when given a non-positive relative timeout or absolute time in the past. #685
- std::pow is now templated to handle integral and floating-point types as required by C++11 and later. For example, std::pow(float, int) previously returned float and now returns double. #903
- Removed a bogus overload of ranges::copy_n that wasn't supposed to exist and wouldn't have compiled. #957
- filesystem_error now transcodes wide characters with no equivalent in the current locale to replacement characters instead of failing and throwing system error. #1010
- Fixed 0 <=> partial_ordering::unordered to return the correct value, and improved <compare> codegen. #1049
- discrete_distribution no longer yields invalid results when its parameters sum very close to, but not equal to 1. #1025
- move_iterator::current is no longer inadvertently a protected data member - it has become properly private and been renamed. #1080
- Range algorithms no longer mishandle pointer-to-member predicates and projections when they are larger than void*. #1091
- The slice_array, gslice_array, mask_array, and indirect_array helpers defined in <valarray> now have proper copy constructors and copy assignment operators. #988
- Fixed a bug in ranges::iter_swap that would cause it to sometimes drop one of the "swapped" values on the floor. #1072
- weak_ptr conversions now preserve control blocks for expired objects. #1104
- Fixed <bit>'s countl_zero to correctly use the bsr instruction as a fallback for old CPUs that don't support the lzcnt instruction. #1108 #1166
- Fixed partial_sort_copy to perform only operations that are required by the Standard to compile. #1088
- Fixed deque::erase(iter, iter) to no longer perform self-move-assignments when called with an empty range. #1148
- Fixed coroutine_handle<>::resume and coroutine_handle<>::operator() to allow exceptions to be thrown from coroutines. #1182
- Fixed piecewise_constant_distribution and piecewise_linear_distribution to return correctly distributed values. #1032
- Fixed deque (and therefore queue and stack) compiler errors in unusual scenarios where iter + int and iter - int would invoke an unexpected operator overload. #1161
- Fixed any and atomic compiler errors when building with /Zc:noexceptTypes-. #1150
- Fixed <charconv> hexfloat from_chars() to correctly handle numbers like "0.fffffffffffff8p-1022" which are just below numeric_limits<Floating>::min(). #934
- Changed shared_ptr's atomic functions to use SRWLOCK instead of a naive spinlock, fixing priority inversion and improving performance. #1200
- Fixed incorrect constants that were producing incorrect results for cosh() and sinh(). #1156
- Fixed 3 bugs in how ostream << floating_point handles precision: #1173
  - Hexfloat output now ignores precision, as required by the Standard.
  - Implemented LWG-231 (resolved in C++11, but overlooked); now, 0 precision for scientific or general notation is directly passed to sprintf, instead of being replaced with 6 precision.
  - Negative precision no longer crashes.
- get_time() now ignores case when parsing day and month names, as required by the Standard. #1168 #1240
- <experimental/generator> now defines an unhandled_exception() member function, required for C++20 coroutine promise types, even when /EHs isn't being used. #1219
- Fixed compiler errors in the "iterator unwrapping" machinery for counted_iterator. #1213
- fill now works correctly with pointers-to-volatile integral types. (This change also applies our memset/memcmp optimizations for fill/equal a bit more aggressively; they now activate for filling contiguous ranges of bool or comparing such with contiguous ranges of single-byte integer types.) #1160
- valarray::operator[] now verifies its argument is in range when /D_CONTAINER_DEBUG_LEVEL=1. #1247
- Fixed a compiler error in <complex> (introduced in VS 2019 16.7 by #367) when compiling with the Intel C++ Compiler's 80-bit long double mode. #1316
  - Note: 80-bit long double remains unsupported by MSVC.
Improved performance:
- Extended equal()'s memcmp() optimization to ranges::equal_to and arbitrary contiguous iterators. #831
- optional no longer unnecessarily zero-fills its storage when default constructed. #878
- Improved span's debug codegen by simplifying its representation (as /std:c++latest isn't ABI-frozen). #877
- Use __iso_volatile_store64 for relaxed atomics on x86, and use __iso_volatile_store64 unconditionally. #694
- Enabled more constexpr in <system_error> using the [[msvc::noop_dtor]] attribute we requested from the MSVC compiler front-end. #1016
- Hand-vectorized std::reverse_copy. #804
- Refactored the comparison category types into aggregates so they can be passed in registers. #1065
- Optimized std::fill and std::fill_n for vector<bool>. #879
- Applied DisableThreadLibraryCalls to all "satellite" DLLs. #1142
- Improved debug codegen in <valarray> by extracting repeated calls to size(). Also improved readability by expanding macros, and avoided unnecessary use of magic statics for non-trivial types. #1165
- vector now avoids quadratic complexity when its size() is near max_size() (which is effectively impossible for the default allocator, making this an extremely obscure scenario). Now, if geometric growth would exceed max_size(), we clamp to max_size(). #1221
Enhanced behavior:
- Strengthened noexcept for all container iterators. #1227
- basic_string and basic_string_view now enforce the requirement that their character types must be non-array, trivial, standard-layout types. #1262
Improved throughput:
- Simplified is_function, is_object, is_member_function_pointer, is_member_object_pointer, and is_member_pointer. #460
Improved documentation:
- The README's IDE instructions now explain how to use the vcpkg submodule. #873
- The README's command prompt instructions now explicitly explain how to build both x86 and x64. #1226
Improved test coverage:
- Added test cases for the large integer fallback in <charconv> to_chars() plain shortest round-trip. #835
- Future-proofed test code for the upcoming addition of C++20 std::chrono::last. #837
- Enabled more libcxx test coverage for span. #839
- Added test coverage for the new /Zc:preprocessor compiler option. #677
- Added tests for LWG-3018 "shared_ptr of function type". #833
- Fixed "unresolved" tests by linking with /MANIFEST:EMBED. #862
- The test harness now emits a "note" for unsupported locales instead of a warning. #866
- Prevented failing tests from displaying assertion/crash dialog boxes. Also, the standard output streams of failing tests are no longer lost due to buffering. #906
- Increased test coverage in the ranges spaceship test by removing compiler bug workarounds. #852
- Improved ranges test machinery. #959
- Enabled "multiple translation unit" tests which hadn't been running in the new test infrastructure. These tests detect problems like forgetting to mark header-only non-templated functions as inline. #1109
- Added a test to verify that deque::erase(iter, iter) no longer performs self-move-assignments when called with an empty range. #1203
- Removed experimental /await from test configurations, and re-enabled various tests now that compiler bugs have been fixed. #1207
- Added a test for <coroutine> "end-to-end" behavior. #1215
Code cleanups:
- Updated comment to cite LWG-3446. #841
- Renamed identifiers like _R1 to _Rx1, avoiding any resemblance to macros. #865
- Added comments to <execution>, mentioning the subtle control flow. #849
- Avoided function-style casts in test code. #899
- Cleaned up stl/src, changing C casts to C++ casts, 0 to nullptr, and more. #900 #912
- Added ranges::_Ubegin and ranges::_Uend internal helper functions to simplify getting "unwrapped" iterators from a range. #901
- Renamed internal headers to .hpp and adjusted clang-format to sort them last. #902
- Simplified comments, as using _Pred and using operator< weren't necessary to mention. #977
- Removed compiler workarounds and used braces to construct objects. #978 #1020
- Improved ranges test machinery to better tolerate using-directives. #979
- Consistently formatted requires, preparing for when clang-format becomes aware of concepts. #999
- Provided comparison operators for map, multimap, set, and multiset instead of their internal base class _Tree. #1022
- Updated the implementations of some of the oldest Range algorithms to match our current style:
  - #956 ranges::copy
  - #958 ranges::copy_if
  - #970 ranges::move
  - #974 ranges::count
  - #975 ranges::count_if
  - #1058 the ranges::find family
  - #1060 the ranges::for_each family
  - #1061 the ranges::all_of family
  - #1062 ranges::swap_ranges and ranges::distance
- Took advantage of some of the clang-format changes in LLVM 10 to keep our codebase looking nice. #1075
- Removed unnecessary std:: qualification from product and test code. #1119
- Cleaned up compiler bug workarounds, which will be possible to remove in the near future. #1191
- Worked around some IntelliSense bugs in the concepts-enabled C++20 parts of the STL. #1216
- Removed an unnecessary compiler option /d2Zi+ from CMakeLists.txt. #1218
- Consistently use conjunction_v for short-circuiting in conditional explicit clauses. #1241
- Avoided null pointer constants masquerading as integers by replacing 0 with nullptr in a great many places. #1250
Infrastructure improvements:
- Updated dependencies.
  - Updated build compiler to VS 2019 16.8 Preview 1. #851 #911 #1015 #1153 #1185
  - Updated vcpkg to 2020.06. #1042
- CMakeLists.txt now defaults to using our vcpkg submodule, simplifying our build process. #1124
- Removed a timing assumption from test_atomic_wait.hpp that was causing intermittent test failures.
- Added /analyze to PR/CI builds, and fixed the resulting warnings. #1009
- Enhanced our Code Format Validation check to detect lines exceeding 120 columns, and cleaned up a few occurrences in areas where clang-format had been suppressed. #1214
Updated _MSVC_STL_UPDATE. #897 #989 #1144

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VS 2019 16.8