[libc++][NFC] Mark LWG2731 as complete

[jloser]

Summary

Mark LWG2731 as complete. The type alias mutex_type is only provided if
scoped_lock is given one mutex type and it has been implemented that
way since the beginning of Clang 5 it seems. There already are tests for
verifying existence (and lack thereof) for mutex_type type alias
depending on the number of mutex types, so there is nothing to
do for this LWG issue.

[• Quuxplusone]

libcxx/docs/Status/Cxx2bIssues.csv

14

Despite the issue title, the issue is actually about scoped_lock, not lock_guard, and so I don't see how "5.0" could possibly be the right fix-version.
Other than that, LGTM, ship it.

[jloser]

libcxx/docs/Status/Cxx2bIssues.csv

14

Well, scoped_lock was introduced in https://github.com/llvm/llvm-project/commit/6015dd11c875b9467c93221f19e5d59b0522757a (March 23rd, 2017) and then llvm5 got released on September 7th, 2017. Given that the mutex_type type alias was never an issue from its origin, that's why I dated this resolution back to 5.0.

Would you prefer I just mark it as "Nothing to Do" and omit the version value? I don't feel strongly. I'd also be OK if you just want to label this as 14.0, but that doesn't feel quite right since there was no code change required in this patch.

[• Quuxplusone]

libcxx/docs/Status/Cxx2bIssues.csv

14

OK; I didn't guess either that scoped_lock was that old or (more to the point) that LLVM 5 was that recent. We've been through 10 major version numbers in 4 years, eh. 😛 Then I'm OK with 5.0 here.

[Mordante]

LGTM!

libcxx/docs/Status/Cxx2bIssues.csv

 "Issue #","Issue Name","Meeting","Status","First released version","Labels"
 "`2839 <https://wg21.link/LWG2839>`__","Self-move-assignment of library types, again","November 2020","",""
 "`3117 <https://wg21.link/LWG3117>`__","Missing ``packaged_task`` deduction guides","November 2020","",""
 "`3143 <https://wg21.link/LWG3143>`__","``monotonic_buffer_resource`` growth policy is unclear","November 2020","",""
 "`3195 <https://wg21.link/LWG3195>`__","What is the stored pointer value of an empty ``weak_ptr``?","November 2020","",""
 "`3211 <https://wg21.link/LWG3211>`__","``std::tuple<>`` should be trivially constructible","November 2020","",""
 "`3236 <https://wg21.link/LWG3236>`__","Random access iterator requirements lack limiting relational operators domain to comparing those from the same range","November 2020","","","|ranges|"
 "`3265 <https://wg21.link/LWG3265>`__","``move_iterator``'s conversions are more broken after P1207","November 2020","Fixed by `LWG3435 <https://wg21.link/LWG3435>`__",""
 "`3435 <https://wg21.link/LWG3435>`__","``three_way_comparable_with<reverse_iterator<int*>, reverse_iterator<const int*>>``","November 2020","|Complete|","13.0"
 "`3432 <https://wg21.link/LWG3432>`__","Missing requirement for ``comparison_category``","November 2020","","","|spaceship|"
 "`3447 <https://wg21.link/LWG3447>`__","Deduction guides for ``take_view`` and ``drop_view`` have different constraints","November 2020","|Complete|","14.0"
 "`3450 <https://wg21.link/LWG3450>`__","The const overloads of ``take_while_view::begin/end`` are underconstrained","November 2020","","","|ranges|"
 "`3464 <https://wg21.link/LWG3464>`__","``istream::gcount()`` can overflow","November 2020","",""
-"`2731 <https://wg21.link/LWG2731>`__","Existence of ``lock_guard<MutexTypes...>::mutex_type`` typedef unclear","November 2020","",""
+"`2731 <https://wg21.link/LWG2731>`__","Existence of ``lock_guard<MutexTypes...>::mutex_type`` typedef unclear","November 2020","|Complete|","5.0"
 "`2743 <https://wg21.link/LWG2743>`__","P0083R3 ``node_handle`` private members missing ""exposition only"" comment","November 2020","",""
 "`2820 <https://wg21.link/LWG2820>`__","Clarify ``<cstdint>`` macros","November 2020","",""
 "`3120 <https://wg21.link/LWG3120>`__","Unclear behavior of ``monotonic_buffer_resource::release()``","November 2020","",""
 "`3170 <https://wg21.link/LWG3170>`__","``is_always_equal`` added to ``std::allocator`` makes the standard library treat derived types as always equal","November 2020","",""
 "`3036 <https://wg21.link/LWG3036>`__","``polymorphic_allocator::destroy`` is extraneous","November 2020","",""
 "`3171 <https://wg21.link/LWG3171>`__","LWG2989 breaks ``directory_entry`` stream insertion","November 2020","",""
 "`3306 <https://wg21.link/LWG3306>`__","``ranges::advance`` violates its preconditions","November 2020","","","|ranges|"
 "`3403 <https://wg21.link/LWG3403>`__","Domain of ``ranges::ssize(E)`` doesn't ``match ranges::size(E)``","November 2020","","","|ranges|"

libcxx/include/mutex

public:
     lock_guard(lock_guard const&) = delete;
     lock_guard& operator=(lock_guard const&) = delete;
 };
 
 template <class... MutexTypes>
 class scoped_lock // C++17
 {
 public:
-    using mutex_type = Mutex;  // If MutexTypes... consists of the single type Mutex
+    using mutex_type = Mutex;  // Only if sizeof...(MutexTypes) == 1
 
     explicit scoped_lock(MutexTypes&... m);
     scoped_lock(adopt_lock_t, MutexTypes&... m);
     ~scoped_lock();
     scoped_lock(scoped_lock const&) = delete;
     scoped_lock& operator=(scoped_lock const&) = delete;
 private:
     tuple<MutexTypes&...> pm; // exposition only

libcxx/test/std/thread/thread.mutex/thread.lock/thread.lock.scoped/types.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
 // UNSUPPORTED: libcpp-has-no-threads
 // UNSUPPORTED: c++03, c++11, c++14
 
 // <mutex>
 
-// template <class Mutex...>
+// template <class... MutexTypes>
 // class scoped_lock
 // {
 // public:
-//     typedef Mutex mutex_type;  // only if sizeof...(Mutex) == 1
+//     typedef Mutex mutex_type;  // Only if sizeof...(MutexTypes) == 1
 //     ...
 // };
 
 #include <mutex>
 #include <type_traits>
 #include "test_macros.h"
 
 struct NAT {};