Release MSVC Build Tools 14.50 · microsoft/STL

This shipped in VS 2026 18.0.

The STL no longer supports targeting Windows 7 / Server 2008 R2, Windows 8 / Server 2012, and Windows 8.1 / Server 2012 R2:
- Improved the performance of system_clock::now() and atomic::wait() by directly calling APIs that were added in Windows 8. #5432 #5496
- Improved the performance of STL Hardening by using the MSVC __fastfail intrinsic that was added in Windows 8, and the Clang __builtin_verbose_trap intrinsic. #5433 #5458
- Simplified the <filesystem> implementation by unconditionally calling APIs that were added in Windows 8. #5434
- Windows 10 / Server 2016 are our minimum supported operating systems. #5510
The STL no longer supports targeting 32-bit ARM. #5594
Merged C++26 features:
- P0472R3 #5382 Put monostate In <utility>
- P3223R2 #5604 Making istream::ignore() Less Surprising
Merged partial C++26 features:
- P3697R1 #5650 Minor Additions To Standard Library Hardening
Merged LWG issue resolutions:
- LWG-2503 #5535 multiline option should be added to syntax_option_type
  - For us, this is a regex behavioral change, so we added an escape hatch. By default, we define _REGEX_LEGACY_MULTILINE_MODE to 0, which requests Standard behavior:
    - For ECMAScript, matching is non-multiline by default, but regex_constants::multiline can be requested.
    - For POSIX grammars, matching is non-multiline, and regex_constants::multiline is ignored; see N5008 [tab:re.synopt].
  - Defining _REGEX_LEGACY_MULTILINE_MODE to 1 requests legacy behavior:
    - For all grammars, matching is multiline, and regex_constants::multiline is redundant.
- LWG-4186 #5444 regex_traits::transform_primary mistakenly detects typeid of a function
- LWG-4222 #5602 expected constructor from a single value missing a constraint
- LWG-4242 #5603 ranges::distance does not work with volatile iterators
Merged proposed resolutions for LWG issues (not yet accepted for the C++ Working Draft, but we've chosen to implement this speculatively):
- LWG-4270 #5572 Diagnose misuse of std::projected::operator*
- LWG-4273 #5578 Standard execution policy types should be conventional tag class types
- LWG-4303 #5603 std::decay_t in the specification of ranges::distance is problematic
Fixed bugs:
- Fixed regex's behavior:
  - For the basic and grep grammars to properly handle ^ caret characters. #5165
  - For the basic and grep grammars to properly handle $ dollar characters. #5362
  - For the basic and grep grammars to make backreferences to unmatched capture groups fail to match anything, instead of matching the empty string "". #5376
  - For the basic and grep grammars to properly handle backslashes inside square-bracket character classes. #5523
  - For POSIX grammars to allow a ] right square bracket at the beginning of a character range. #5364
  - For POSIX grammars to avoid resetting matched capture groups. #5377
  - For the ECMAScript grammar's (?!pattern) negative lookahead assertions to make capture groups within them always match nothing after the assertions succeed. #5366
  - For the ECMAScript grammar's \b word boundary assertion to not match the empty string "" (and inversely for the \B negative word boundary assertion). #5375
  - For the ECMAScript grammar's \c escapes to require the following ControlLetter to be an ASCII alphabetic character. #5524
    - That is, only \ca through \cz and \cA through \cZ are allowed.
  - For the ECMAScript grammar to reset all capture groups to "unmatched" at the start of each repetition of a loop. #5456
  - For the ECMAScript grammar to reject bogus groups by throwing a regex_error with the code regex_constants::error_badrepeat, removing the non-Standard code regex_constants::error_syntax that was previously used. #5542
    - That is, only non-capture groups (?:meow) and positive/negative lookahead assertions (?=meow)/(?!meow) are allowed; other characters following (? are bogus. Notably, the C++ Standard doesn't support the lookbehind assertions (?<=meow)/(?<!meow) that were added in later versions of ECMAScript.
  - For the ECMAScript, basic, grep, and awk grammars to properly handle escape sequences. #5380
    - POSIX grammars now accept \] as an identity escape for a right square bracket. #5399
  - For capture groups to follow ECMAScript's depth-first and POSIX's leftmost-longest matching rules. #5218
    - Removed unnecessary code for POSIX's leftmost-longest rule, reducing stack space consumption. #5405
  - For empty matches to follow ECMAScript's and POSIX's slightly different rules. #5494
  - For negated character class escapes (\D for non-digits, \S for non-whitespace, \W for non-words) inside square-bracket character classes when matching against Unicode characters. #5403 #5487
    - This completes a series of fixes started by #5160 and #5214 in VS 2022 17.14.
  - For combinations of character class escapes inside square-bracket character classes (like [\w\s] for words and whitespace) when matching against Unicode characters. #5438
  - For character ranges with regex_constants::collate to follow the locale's collation order. #5238
  - For collating symbols and equivalence classes. #5392 #5444
  - For small character ranges containing U+00FF and U+0100. #5437
  - For regex_constants::icase and regex_constants::collate to translate each character exactly once before any comparisons. #5553
  - For extremely unusual user-defined character types. #5592 #5675
  - To avoid requiring non-Standard typedefs in custom regex traits classes. #5671
- Fixed collate<wchar_t> to follow the locale's collation order when compiling with /Zc:wchar_t- (making wchar_t a non-Standard typedef for unsigned short) and linking to the STL dynamically (with /MD or /MDd). #5361
- Fixed how collate::do_transform() handles wrongly encoded input. #5431
- Fixed collate::do_hash() to return equal hashes for strings that collate as equivalent. #5469
- Fixed ranges::enable_view to correctly report false for ranges::view_interface itself. #5369 #5407
- Fixed <filesystem> status functions to avoid failing for invalid paths when the current directory is a network path. #5381
- Fixed compiler errors in repeat_view's piecewise constructor for certain scenarios. #5388
- Fixed compiler errors in constexpr copy() for vector<bool>. #5347
  - This fixed a regression that was introduced by #3353 in VS 2022 17.9.
- Fixed the barrier constructor's exception specification to be conditionally "strengthened", instead of unconditionally. #5398
- Fixed heap-use-after-free bugs in our undocumented/quasi-supported _HAS_EXCEPTIONS=0 mode when constructing system_error, chrono::ambiguous_local_time, and chrono::nonexistent_local_time. #5406
- Fixed compiler errors when passing volatile ranges to basic_string's from_range constructor and append_range(), assign_range(), insert_range(), and replace_with_range() member functions. #5409
- Fixed nth_element() and ranges::nth_element to have worst-case linear complexity, instead of quadratic. #5100
- Fixed compiler errors when passing iterators with unusual integer-class difference types to vectorized algorithms. #5471
- Fixed destroy(), destroy_at(), destroy_n(), ranges::destroy, ranges::destroy_at, and ranges::destroy_n to destroy objects during constant evaluation even when they're trivially destructible. #5449
- Fixed num_get::do_get() for bool to properly handle bad digit groupings. #5476
- Fixed thread::hardware_concurrency() to correctly report the total number of logical processors when more than 64 are available, for both single-socket and multi-socket machines. #5459
- Fixed <mdspan>'s layout_stride::mapping<E>::is_exhaustive() to correctly handle unusual extents. #5477
- Fixed reverse_copy() and ranges::reverse_copy being improperly vectorized for pair<T&, U&> on x86 (where such pairs are 8 bytes). #5528
  - This fixed a regression that was introduced by #804 in VS 2019 16.8.
- Fixed chrono::zoned_time to provide a fallback time zone abbreviation (of the form "-05" or "+0530") when the system's code page can't represent the localized abbreviation that would normally be returned. #5558
- Fixed counting_semaphore::try_acquire_for() to consistently use steady_clock. #5575
- Silenced a static analysis warning C6510 emitted by the extremely unusual scenario of basic_string<char_like_struct>. #5563
- Fixed compiler errors involving incomplete types in <chrono>. #5629
- Fixed occurrences of Clang 21's new -Wcharacter-conversion warning in <locale> by avoiding implicit conversions between charN_t types. #5653
- Fixed integer overflow in stable_sort() and ranges::stable_sort when sorting huge inputs on x86. #5677
  - This affected 32-bit architectures with over a billion 1-byte elements. 64-bit architectures were immune.
  - This fix was backported to VS 2022 17.14.20.
- Fixed filesystem::temp_directory_path(error_code&) to return an empty filesystem::path when an error is encountered. #5570
Improved performance:
- Improved performance for regex matching:
  - When a regex starts with a ? question mark quantifier or several alternatives, by avoiding quadratic complexity. #5457
  - When a regex starts with a + plus quantifier (or {min,} or {min,max} quantifiers requiring at least 1 repetition). #5509
  - When a regex contains no capture groups, by avoiding unnecessary allocations. #5518
  - For all regex patterns, by not generating unnecessary "if" nodes with a single branch. #5539
  - When a regex starts with positive/negative word boundary assertions \b/\B and lookahead assertions (?=meow)/(?!meow). #5576
  - By calling std::search() internally. #5586
  - By avoiding unnecessary work in regex_constants::collate mode. #5672
- Started using [[msvc::no_unique_address]] as a space optimization in several C++23 components. #4960
- Added x64/x86 vectorized implementations of:
  - basic_string::find_first_not_of() and basic_string::find_last_not_of() for one character. #5102
  - basic_string::find_first_not_of() and basic_string::find_last_not_of() for multiple characters. #5206
  - adjacent_find() and ranges::adjacent_find. #5331
  - unique() and ranges::unique. #5092 #5363
  - search_n() and ranges::search_n for small values of n. #5352
  - remove_copy(), ranges::remove_copy, unique_copy(), and ranges::unique_copy. #5355
  - is_sorted_until() and ranges::is_sorted_until. #5420 #5540
  - search(), ranges::search, default_searcher, find_end(), and ranges::find_end, for 4-byte and 8-byte elements. #5484 #5519
    - They were vectorized for 1-byte and 2-byte elements in VS 2022 17.13.
  - rotate() and ranges::rotate. #5502 #5525
- Improved the x64/x86 vectorized implementations of:
  - reverse_copy() and ranges::reverse_copy. #5493
- Improved the ARM64EC vectorized implementations of:
  - find() and ranges::find. #5597
- Improved the performance of ranges::find for 2-byte elements on ARM64, matching find()'s behavior. #5628
- Used Clang's builtin __is_trivially_equality_comparable to improve the performance of equal(), ranges::equal, and many vectorized algorithms for more types. #5527
- Optimized search_n() and ranges::search_n for n=1 to internally call find() and ranges::find. #5346
- Improved codegen for has_single_bit(), especially when popcount() intrinsics are unconditionally available. #5367 #5534
  - For example, x86/x64 with /arch:AVX and ARM64 can unconditionally use popcount() intrinsics.
- Optimized poisson_distribution's constructor by avoiding unnecessary work. #5411
- Implemented an exponential speedup (yes, literally) for minstd_rand and minstd_rand0's discard() member function. #5412
- Improved linear_congruential_engine performance by using our modern internal implementation of 128-bit integers. #5436 #5473
- Significantly improved performance for chrono::tzdb::locate_zone() by using binary searches for time zone names. #5548
- Improved std::includes() to have the same performance as ranges::includes. #5543 #5595
- Improved the performance of count() for vector<bool>. #5640
Enhanced behavior:
- Used a new Clang compiler intrinsic to implement is_scoped_enum. #5358
- Added debug checks for overlapping ranges in replace_copy(), replace_copy_if(), reverse_copy(), rotate_copy(), and their ranges counterparts. #5495
- Improved <mdspan>'s debug checks in layout_stride::mapping<E>'s constructor. #5505
- Avoided compiler warnings in vector<bool>::max_size() with the compiler's upcoming implementation of Defect Report P2280R4. #5550 #5566
- Updated several checks for precondition violations and impossible situations to use the STL's usual error reporting mechanism. #5560
- Improved regex parsing to detect excessive recursion (caused by extreme numbers of non-capturing groups or lookahead assertions) and throw a regex_error with the code regex_constants::error_stack. #5588
- Changed <regex> matching to use heap allocations for saved match state, slightly reducing stack pressure. #5682
- Silenced CodeQL warnings. #5625
- Silenced a new compiler warning C5291 throughout the STL. #5645
Improved debugger visualization:
- Fixed an off-by-one mistake in the displayed year for system_clock::time_point. #5389
- Updated the visualizer for system_clock::time_point to recognize the synonymous but slightly different type name emitted by Clang. #5390
Improved test coverage:
- Updated our LLVM submodule, including new tests. #5349 #5467 #5688
- Changed the benchmark build from /O2 /Ob1 to /O2 /Ob2, producing more realistic and useful results. #5370
- Added benchmarks for:
  - fill() and fill_n(). #5400
  - reverse() and reverse_copy(). #5493
  - The count() optimization for vector<bool>. #5684
- Improved the consistency of how normal_distribution is used in benchmarks and tests. #5404
- Increased the consistency of several benchmarks by adding allocators to control alignment. #5443
- Updated the benchmarks to support being built with Clang. #5533
- Updated an iter_rvalue_reference_t test in response to a compiler change. #5378
- Fixed sporadic test failures by avoiding excessive compiler memory consumption. #5383
- Improved the runtime performance of the major test for vectorized algorithms. #5425
- Added more test coverage for search_n() and ranges::search_n. #5439 #5440
- Updated the test harness to properly detect the locale name "Czech_Czechia.1250". #5480
- Updated source_location test coverage to handle upcoming changes in the EDG front-end used for IntelliSense. #5551
- Categorized the remaining <regex> test failures in the libcxx suite. #5587
- Avoided/suppressed compiler warnings in tests. #5654 #5655 #5656 #5657 #5676
- Worked around assertions in the lerp() test on ARM64. #5687
Improved documentation:
- Clarified our machine-searchable SPDX-License-Identifier comments to properly describe how we used Ryu to implement <charconv>. #5401
- Fixed confusing comments around the control macro for vectorizing algorithms with floating-point types. #5417
Code cleanups:
- Simplified the control flow in ranges::search_n. #5343
- Simplified the implementation of regex quantifiers. #5253
- Removed comments that were citing proposed resolutions for LWG issues, now that those issues have been officially resolved by the November 2024 and June 2025 meetings. #5360 #5584
- Removed compiler bug workarounds. #5410 #5580 #5634 #5661 #5674
- Significantly cleaned up the implementation of x64/x86 vectorized algorithms. #5429 #5450 #5485 #5532
- Removed an unnecessarily verbose internal macro. #5475
- Removed unnecessary code for constant evaluation in char_traits::assign(). #5488
- Various cleanups (described in detail in the PRs, not repeated here). #5498 #5500 #5541 #5565 #5636
- Restructured code to if constexpr (condition) { ... } else { ... }, improving clarity and avoiding unreachable code. #5643
- Simplified how <chrono> initializes a tm struct. #5662
Infrastructure improvements:
- Added ARM64EC stages to Azure Pipelines. #5492 #5517 #5521
  - Including an Early Build, benchmarks, and tests.
- Improved code format validation to report line numbers in error messages. #5559
- Updated dependencies. #5370 #5410 #5478 #5512 #5530 #5580 #5641 #5659 #5674
  - Updated build compiler to VS 2022 17.14.12 Preview 1.
  - Updated CMake to 3.31 (now required).
  - Updated Google Benchmark to 1.9.4.
  - Updated Python to 3.13.6.
  - Updated to Windows 11 SDK 26100. This is now required for building and testing the STL, but not for using it.
Updated _MSVC_STL_UPDATE and _MSVC_STL_VERSION. #5385 #5464 #5568 #5633 #5664

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