Implement variadic lock_guard.

[EricWF]

Summary

This patch implements the variadic lock_guard paper.

Making lock_guard variadic is a ABI breaking change because the specialization lock_guard<_Mutex> mangles differently then when it was the primary template. This change only provides variadic lock_guard in ABI V2 or when _LIBCPP_ABI_VARIADIC_LOCK_GUARD is defined.

Note that in ABI V2 lock_guard must always be declared as a variadic template, even in C++03, in order to keep the ABI consistent. For this reason lock_guard is forward declared as a variadic template in all standard dialects and therefore depends on variadic templates being provided as an extension in C++03. All supported versions of Clang and GCC provide this extension.

[EricWF]

Address K-ballo's comments:

• Remove mutex_type typedef when sizeof...(MutexTypes) > 1.
• Move the variadic lock_guard implementation into <mutex> instead of <__mutex_base>. This prevents a lot of code reshuffling.
• Fix accidental explicit on deleted copy constructor.
• Mark non-variadic lock_guard copy ctor and assignment as deleted using _LIBCPP_EQUAL_DELETE.
• Fix mismatched struct and class tags.

[EricWF]

Add a variadic lock_guard test that compiles in C++03. This tests the C++11 behavior of lock_guard but using the new implementation.

[EricWF]

• Add missing _LIBCPP_ABI_VARIADIC_LOCK_GUARD macro in __config.
• Update cxx1z_status.html.

[mclow.lists]

You should only get the multi-mutex version in C++17.

include/mutex

613–670

You need to update the synopsis at the top of the file.

[EricWF]

Address review comments:

• Update synopsis it <mutex>
• Add missing // UNSUPPORTED: libcpp-has-no-threads in tests.

[K-ballo]

LGTM

[EricWF]

include/mutex

613–671

Done, but you might want to check that change. I tried to do it so that it's clear that the variadic lock_guard is only available in ABI V2.

[EricWF]

Add a __unlock helper function.

[EricWF]

Use primary template lock_guard<_MArgs...> for the variadic implementation instead of specialization lock_guard<_M1, _M2, _MRest...>.

[mclow.lists]

LGTM. Thanks.

include/__config

 // improving its alignment in some cases.
 #define _LIBCPP_ABI_ALTERNATE_STRING_LAYOUT
 // Fix deque iterator type in order to support incomplete types.
 #define _LIBCPP_ABI_INCOMPLETE_TYPES_IN_DEQUE
 // Fix undefined behavior in how std::list stores it's linked nodes.
 #define _LIBCPP_ABI_LIST_REMOVE_NODE_POINTER_UB
 #define _LIBCPP_ABI_FORWARD_LIST_REMOVE_NODE_POINTER_UB
 #define _LIBCPP_ABI_FIX_UNORDERED_CONTAINER_SIZE_TYPE
+#define _LIBCPP_ABI_VARIADIC_LOCK_GUARD
 #endif
 
 #define _LIBCPP_CONCAT1(_LIBCPP_X,_LIBCPP_Y) _LIBCPP_X##_LIBCPP_Y
 #define _LIBCPP_CONCAT(_LIBCPP_X,_LIBCPP_Y) _LIBCPP_CONCAT1(_LIBCPP_X,_LIBCPP_Y)
 
 #define _LIBCPP_NAMESPACE _LIBCPP_CONCAT(__,_LIBCPP_ABI_VERSION)
 
 

include/__mutex_base

 #else
 
 constexpr defer_lock_t  defer_lock  = defer_lock_t();
 constexpr try_to_lock_t try_to_lock = try_to_lock_t();
 constexpr adopt_lock_t  adopt_lock  = adopt_lock_t();
 
 #endif
 
+
+// Forward declare lock_guard as a variadic template even in C++03 to keep
+// the mangling consistent between dialects.
+#if defined(_LIBCPP_ABI_VARIADIC_LOCK_GUARD)
+template <class ..._Mutexes>
+class _LIBCPP_TYPE_VIS_ONLY lock_guard;
+#endif
+
 template <class _Mutex>
-class _LIBCPP_TYPE_VIS_ONLY _LIBCPP_THREAD_SAFETY_ANNOTATION(scoped_lockable) lock_guard
+class _LIBCPP_TYPE_VIS_ONLY _LIBCPP_THREAD_SAFETY_ANNOTATION(scoped_lockable)
+#if !defined(_LIBCPP_ABI_VARIADIC_LOCK_GUARD)
+lock_guard
+#else
+lock_guard<_Mutex>
+#endif
 {
 public:
     typedef _Mutex mutex_type;
 
 private:
     mutex_type& __m_;
 public:
 
     _LIBCPP_INLINE_VISIBILITY
     explicit lock_guard(mutex_type& __m) _LIBCPP_THREAD_SAFETY_ANNOTATION(acquire_capability(__m))
         : __m_(__m) {__m_.lock();}
     _LIBCPP_INLINE_VISIBILITY
     lock_guard(mutex_type& __m, adopt_lock_t) _LIBCPP_THREAD_SAFETY_ANNOTATION(requires_capability(__m))
         : __m_(__m) {}
     _LIBCPP_INLINE_VISIBILITY
     ~lock_guard() _LIBCPP_THREAD_SAFETY_ANNOTATION(release_capability()) {__m_.unlock();}
 
 private:
-    lock_guard(lock_guard const&);// = delete;
-    lock_guard& operator=(lock_guard const&);// = delete;
+    lock_guard(lock_guard const&) _LIBCPP_EQUAL_DELETE;
+    lock_guard& operator=(lock_guard const&) _LIBCPP_EQUAL_DELETE;
 };
 
 template <class _Mutex>
 class _LIBCPP_TYPE_VIS_ONLY unique_lock
 {
 public:
     typedef _Mutex mutex_type;
 

include/mutex

public:
     explicit lock_guard(mutex_type& m);
     lock_guard(mutex_type& m, adopt_lock_t);
     ~lock_guard();
 
     lock_guard(lock_guard const&) = delete;
     lock_guard& operator=(lock_guard const&) = delete;
 };
 
+template <class... MutexTypes> // Variadic lock_guard only provided in ABI V2.
+class lock_guard
+{
+public:
+    explicit lock_guard(MutexTypes&... m);
+    lock_guard(MutexTypes&... m, adopt_lock_t);
+    ~lock_guard();
+    lock_guard(lock_guard const&) = delete;
+    lock_guard& operator=(lock_guard const&) = delete;
+private:
+    tuple<MutexTypes&...> pm; // exposition only
+};
+
 template <class Mutex>
 class unique_lock
 {
 public:
     typedef Mutex mutex_type;
     unique_lock() noexcept;
     explicit unique_lock(mutex_type& m);
     unique_lock(mutex_type& m, defer_lock_t) noexcept;

 template <class _L0, class _L1, class _L2, class ..._L3>
 inline _LIBCPP_INLINE_VISIBILITY
 void
 lock(_L0& __l0, _L1& __l1, _L2& __l2, _L3& ...__l3)
 {
     __lock_first(0, __l0, __l1, __l2, __l3...);
 }
 
+template <class _L0>
+inline _LIBCPP_INLINE_VISIBILITY
+void __unlock(_L0& __l0) {
+    __l0.unlock();
+}
+
+template <class _L0, class _L1>
+inline _LIBCPP_INLINE_VISIBILITY
+void __unlock(_L0& __l0, _L1& __l1) {
+    __l0.unlock();
+    __l1.unlock();
+}
+
+template <class _L0, class _L1, class _L2, class ..._L3>
+inline _LIBCPP_INLINE_VISIBILITY
+void __unlock(_L0& __l0, _L1& __l1, _L2& __l2, _L3&... __l3) {
+    __l0.unlock();
+    __l1.unlock();
+    _VSTD::__unlock(__l2, __l3...);
+}
+
 #endif  // _LIBCPP_HAS_NO_VARIADICS
 
 #endif // !_LIBCPP_HAS_NO_THREADS
 
 struct _LIBCPP_TYPE_VIS_ONLY once_flag;
 
 #ifndef _LIBCPP_HAS_NO_VARIADICS
 
call_once(once_flag& __flag, const _Callable& __func)
     if (__flag.__state_ != ~0ul)
     {
         __call_once_param<const _Callable> __p(__func);
         __call_once(__flag.__state_, &__p, &__call_once_proxy<const _Callable>);
     }
 }
 
 #endif  // _LIBCPP_HAS_NO_VARIADICS
 
+
+#if defined(_LIBCPP_ABI_VARIADIC_LOCK_GUARD) \
+    && !defined(_LIBCPP_CXX03_LANG)
+template <>
+class _LIBCPP_TYPE_VIS_ONLY lock_guard<> {
+public:
+    explicit lock_guard() = default;
+    ~lock_guard() = default;
+
+    _LIBCPP_INLINE_VISIBILITY
+    explicit lock_guard(adopt_lock_t) {}
+
+    lock_guard(lock_guard const&) = delete;
+    lock_guard& operator=(lock_guard const&) = delete;
+};
+
+template <class ..._MArgs>
+class _LIBCPP_TYPE_VIS_ONLY lock_guard
+{
+    static_assert(sizeof...(_MArgs) >= 2, "At least 2 lock types required");
+    typedef tuple<_MArgs&...> _MutexTuple;
+
+public:
+    _LIBCPP_INLINE_VISIBILITY
+    explicit lock_guard(_MArgs&... __margs)
+      : __t_(__margs...)
+    {
+        _VSTD::lock(__margs...);
+    }
+
+    _LIBCPP_INLINE_VISIBILITY
+    lock_guard(_MArgs&... __margs, adopt_lock_t)
+        : __t_(__margs...)
+    {
+    }
+
+    _LIBCPP_INLINE_VISIBILITY
+    ~lock_guard() {
+        typedef typename __make_tuple_indices<sizeof...(_MArgs)>::type _Indices;
+        __unlock_unpack(_Indices{}, __t_);
+    }
+
+    lock_guard(lock_guard const&) = delete;
+    lock_guard& operator=(lock_guard const&) = delete;
+
+private:
+    template <size_t ..._Indx>
+    _LIBCPP_INLINE_VISIBILITY
+    static void __unlock_unpack(__tuple_indices<_Indx...>, _MutexTuple& __mt) {
+        _VSTD::__unlock(_VSTD::get<_Indx>(__mt)...);
+    }
+
+    _MutexTuple __t_;
+};
+
+#endif // _LIBCPP_ABI_VARIADIC_LOCK_GUARD
+
 _LIBCPP_END_NAMESPACE_STD
 
 #endif  // _LIBCPP_MUTEX

test/libcxx/thread/thread.mutex/thread.lock/thread.lock.guard/variadic_mutex_mangling.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// UNSUPPORTED: libcpp-has-no-threads
+
+// THIS TESTS C++03 EXTENSIONS.
+
+// <mutex>
+
+// template <class ...Mutex> class lock_guard;
+
+// Test that the the variadic lock guard implementation mangles the same in
+// C++11 and C++03. This is important since the mangling of `lock_guard` depends
+// on it being declared as a variadic template, even in C++03.
+
+#define _LIBCPP_ABI_VARIADIC_LOCK_GUARD
+#include <mutex>
+#include <typeinfo>
+#include <cassert>
+#include <iostream>
+
+int main() {
+    const std::string expect = "NSt3__110lock_guardIJNS_5mutexEEEE";
+    assert(typeid(std::lock_guard<std::mutex>).name() == expect);
+}

test/std/thread/thread.mutex/thread.lock/thread.lock.guard/mutex.fail.cpp

 //===----------------------------------------------------------------------===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is dual licensed under the MIT and the University of Illinois Open
 // Source Licenses. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 
+// UNSUPPORTED: libcpp-has-no-threads
+
 // <mutex>
 
 // template <class Mutex> class lock_guard;
 
 // explicit lock_guard(mutex_type& m);
 
 #include <mutex>
-#include <thread>
-#include <cstdlib>
-#include <cassert>
-
-std::mutex m;
-
-typedef std::chrono::system_clock Clock;
-typedef Clock::time_point time_point;
-typedef Clock::duration duration;
-typedef std::chrono::milliseconds ms;
-typedef std::chrono::nanoseconds ns;
-
-void f()
-{
-    time_point t0 = Clock::now();
-    time_point t1;
-    {
-    std::lock_guard<std::mutex> lg = m;
-    t1 = Clock::now();
-    }
-    ns d = t1 - t0 - ms(250);
-    assert(d < ns(2500000));  // within 2.5ms
-}
 
 int main()
 {
-    m.lock();
-    std::thread t(f);
-    std::this_thread::sleep_for(ms(250));
-    m.unlock();
-    t.join();
+    std::mutex m;
+    std::lock_guard<std::mutex> lg = m; // expected-error{{no viable conversion}}
 }

test/std/thread/thread.mutex/thread.lock/thread.lock.guard/variadic_adopt_lock.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// UNSUPPORTED: libcpp-has-no-threads
+// UNSUPPORTED: c++98, c++03
+
+// <mutex>
+
+// template <class ...Mutex> class lock_guard;
+
+// lock_guard(Mutex&..., adopt_lock_t);
+
+#define _LIBCPP_ABI_VARIADIC_LOCK_GUARD
+#include <mutex>
+#include <cassert>
+
+struct TestMutex {
+    bool locked = false;
+    TestMutex() = default;
+
+    void lock() { assert(!locked); locked = true; }
+    bool try_lock() { if (locked) return false; return locked = true; }
+    void unlock() { assert(locked); locked = false; }
+
+    TestMutex(TestMutex const&) = delete;
+    TestMutex& operator=(TestMutex const&) = delete;
+};
+
+int main()
+{
+    {
+        using LG = std::lock_guard<>;
+        LG lg(std::adopt_lock);
+    }
+    {
+        TestMutex m1, m2;
+        using LG = std::lock_guard<TestMutex, TestMutex>;
+        m1.lock(); m2.lock();
+        {
+            LG lg(m1, m2, std::adopt_lock);
+            assert(m1.locked && m2.locked);
+        }
+        assert(!m1.locked && !m2.locked);
+    }
+    {
+        TestMutex m1, m2, m3;
+        using LG = std::lock_guard<TestMutex, TestMutex, TestMutex>;
+        m1.lock(); m2.lock(); m3.lock();
+        {
+            LG lg(m1, m2, m3, std::adopt_lock);
+            assert(m1.locked && m2.locked && m3.locked);
+        }
+        assert(!m1.locked && !m2.locked && !m3.locked);
+    }
+
+}

test/std/thread/thread.mutex/thread.lock/thread.lock.guard/variadic_assign.fail.cpp

+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: libcpp-has-no-threads
+// UNSUPPORTED: c++98, c++03
+
+// <mutex>
+
+// template <class ...Mutex> class lock_guard;
+
+// lock_guard& operator=(lock_guard const&) = delete;
+
+#define _LIBCPP_ABI_VARIADIC_LOCK_GUARD
+#include <mutex>
+
+int main()
+{
+    using M = std::mutex;
+    M m0, m1, m2;
+    M om0, om1, om2;
+    {
+        using LG = std::lock_guard<>;
+        LG lg1, lg2;
+        lg1 = lg2; // expected-error{{overload resolution selected deleted operator '='}}
+    }
+    {
+        using LG = std::lock_guard<M, M>;
+        LG lg1(m0, m1);
+        LG lg2(om0, om1);
+        lg1 = lg2; // expected-error{{overload resolution selected deleted operator '='}}
+    }
+    {
+        using LG = std::lock_guard<M, M, M>;
+        LG lg1(m0, m1, m2);
+        LG lg2(om0, om1, om2);
+        lg1 = lg2; // expected-error{{overload resolution selected deleted operator '='}}
+    }
+}

test/std/thread/thread.mutex/thread.lock/thread.lock.guard/variadic_copy.fail.cpp

+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: libcpp-has-no-threads
+// UNSUPPORTED: c++98, c++03
+
+// <mutex>
+
+// template <class ...Mutex> class lock_guard;
+
+// lock_guard(lock_guard const&) = delete;
+
+#define _LIBCPP_ABI_VARIADIC_LOCK_GUARD
+#include <mutex>
+
+int main()
+{
+    using M = std::mutex;
+    M m0, m1, m2;
+    {
+        using LG = std::lock_guard<>;
+        const LG Orig;
+        LG Copy(Orig); // expected-error{{call to deleted constructor of 'LG'}}
+    }
+    {
+        using LG = std::lock_guard<M, M>;
+        const LG Orig(m0, m1);
+        LG Copy(Orig); // expected-error{{call to deleted constructor of 'LG'}}
+    }
+    {
+        using LG = std::lock_guard<M, M, M>;
+        const LG Orig(m0, m1, m2);
+        LG Copy(Orig); // expected-error{{call to deleted constructor of 'LG'}}
+    }
+}

test/std/thread/thread.mutex/thread.lock/thread.lock.guard/variadic_mutex.fail.cpp

+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: libcpp-has-no-threads
+// UNSUPPORTED: c++98, c++03
+
+// <mutex>
+
+// template <class ...Mutex> class lock_guard;
+
+// explicit lock_guard(Mutex&...);
+
+#define _LIBCPP_ABI_VARIADIC_LOCK_GUARD
+#include <mutex>
+
+template <class LG>
+void test_conversion(LG) {}
+
+int main()
+{
+    using M = std::mutex;
+    M m0, m1, m2;
+    M n0, n1, n2;
+    {
+        using LG = std::lock_guard<>;
+        LG lg = {}; // expected-error{{chosen constructor is explicit in copy-initialization}}
+        test_conversion<LG>({}); // expected-error{{no matching function for call}}
+        ((void)lg);
+    }
+    {
+        using LG = std::lock_guard<M, M>;
+        LG lg = {m0, m1}; // expected-error{{chosen constructor is explicit in copy-initialization}}
+        test_conversion<LG>({n0, n1}); // expected-error{{no matching function for call}}
+        ((void)lg);
+    }
+    {
+        using LG = std::lock_guard<M, M, M>;
+        LG lg = {m0, m1, m2}; // expected-error{{chosen constructor is explicit in copy-initialization}}
+        test_conversion<LG>({n0, n1, n2}); // expected-error{{no matching function for call}}
+    }
+}

test/std/thread/thread.mutex/thread.lock/thread.lock.guard/variadic_mutex.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// UNSUPPORTED: libcpp-has-no-threads
+// UNSUPPORTED: c++98, c++03
+
+// <mutex>
+
+// template <class ...Mutex> class lock_guard;
+
+// explicit lock_guard(mutex_type& m);
+
+#define _LIBCPP_ABI_VARIADIC_LOCK_GUARD
+#include <mutex>
+#include <cassert>
+
+struct TestMutex {
+    bool locked = false;
+    TestMutex() = default;
+    ~TestMutex() { assert(!locked); }
+
+    void lock() { assert(!locked); locked = true; }
+    bool try_lock() { if (locked) return false; return locked = true; }
+    void unlock() { assert(locked); locked = false; }
+
+    TestMutex(TestMutex const&) = delete;
+    TestMutex& operator=(TestMutex const&) = delete;
+};
+
+#if !defined(TEST_HAS_NO_EXCEPTIONS)
+struct TestMutexThrows {
+    bool locked = false;
+    bool throws_on_lock = false;
+
+    TestMutexThrows() = default;
+    ~TestMutexThrows() { assert(!locked); }
+
+    void lock() {
+        assert(!locked);
+        if (throws_on_lock) {
+            throw 42;
+        }
+        locked = true;
+    }
+
+    bool try_lock() {
+        if (locked) return false;
+        lock();
+        return true;
+    }
+
+    void unlock() { assert(locked); locked = false; }
+
+    TestMutexThrows(TestMutexThrows const&) = delete;
+    TestMutexThrows& operator=(TestMutexThrows const&) = delete;
+};
+#endif // !defined(TEST_HAS_NO_EXCEPTIONS)
+
+int main()
+{
+    {
+        using LG = std::lock_guard<>;
+        LG lg;
+    }
+    {
+        using LG = std::lock_guard<TestMutex, TestMutex>;
+        TestMutex m1, m2;
+        {
+            LG lg(m1, m2);
+            assert(m1.locked && m2.locked);
+        }
+        assert(!m1.locked && !m2.locked);
+    }
+    {
+        using LG = std::lock_guard<TestMutex, TestMutex, TestMutex>;
+        TestMutex m1, m2, m3;
+        {
+            LG lg(m1, m2, m3);
+            assert(m1.locked && m2.locked && m3.locked);
+        }
+        assert(!m1.locked && !m2.locked && !m3.locked);
+    }
+#if !defined(TEST_HAS_NO_EXCEPTIONS)
+    {
+        using MT = TestMutexThrows;
+        using LG = std::lock_guard<MT, MT>;
+        MT m1, m2;
+        m1.throws_on_lock = true;
+        try {
+            LG lg(m1, m2);
+            assert(false);
+        } catch (int) {}
+        assert(!m1.locked && !m2.locked);
+    }
+    {
+        using MT = TestMutexThrows;
+        using LG = std::lock_guard<MT, MT, MT>;
+        MT m1, m2, m3;
+        m2.throws_on_lock = true;
+        try {
+            LG lg(m1, m2, m3);
+            assert(false);
+        } catch (int) {}
+        assert(!m1.locked && !m2.locked && !m3.locked);
+    }
+#endif
+}

test/std/thread/thread.mutex/thread.lock/thread.lock.guard/variadic_mutex_cxx03.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// UNSUPPORTED: libcpp-has-no-threads
+
+// <mutex>
+
+// template <class ...Mutex> class lock_guard;
+
+// Test that the variadic lock guard implementation compiles in all standard
+// dialects, including C++03, even though it is forward declared using
+// variadic templates.
+
+#define _LIBCPP_ABI_VARIADIC_LOCK_GUARD
+#include "mutex.pass.cpp" // Use the existing non-variadic test

test/std/thread/thread.mutex/thread.lock/thread.lock.guard/variadic_types.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// UNSUPPORTED: libcpp-has-no-threads
+// UNSUPPORTED: c++98, c++03
+
+// <mutex>
+
+// template <class Mutex>
+// class lock_guard
+// {
+// public:
+//     typedef Mutex mutex_type;
+//     ...
+// };
+
+#define _LIBCPP_ABI_VARIADIC_LOCK_GUARD
+#include <mutex>
+#include <type_traits>
+
+struct NAT {};
+
+template <class LG>
+auto test_typedef(int) -> typename LG::mutex_type;
+
+template <class LG>
+auto test_typedef(...) -> NAT;
+
+template <class LG>
+constexpr bool has_mutex_type() {
+    return !std::is_same<decltype(test_typedef<LG>(0)), NAT>::value;
+}
+
+int main()
+{
+    {
+        using T = std::lock_guard<>;
+        static_assert(!has_mutex_type<T>(), "");
+    }
+    {
+        using M1 = std::mutex;
+        using T = std::lock_guard<M1>;
+        static_assert(std::is_same<T::mutex_type, M1>::value, "");
+    }
+    {
+        using M1 = std::recursive_mutex;
+        using T = std::lock_guard<M1>;
+        static_assert(std::is_same<T::mutex_type, M1>::value, "");
+    }
+    {
+        using M1 = std::mutex;
+        using M2 = std::recursive_mutex;
+        using T = std::lock_guard<M1, M2>;
+        static_assert(!has_mutex_type<T>(), "");
+    }
+    {
+        using M1 = std::mutex;
+        using M2 = std::recursive_mutex;
+        using T = std::lock_guard<M1, M1, M2>;
+        static_assert(!has_mutex_type<T>(), "");
+    }
+    {
+        using M1 = std::mutex;
+        using T = std::lock_guard<M1, M1>;
+        static_assert(!has_mutex_type<T>(), "");
+    }
+    {
+        using M1 = std::recursive_mutex;
+        using T = std::lock_guard<M1, M1, M1>;
+        static_assert(!has_mutex_type<T>(), "");
+    }
+}

www/cxx1z_status.html

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   <tr><td><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4508">N4508</a></td><td>LWG</td></td><td>shared_mutex for C++17</td><td>Lenexa</td><td>Complete</td><td>3.7</td></tr>
   <tr><td><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4366">N4366</a></td><td>LWG</td></td><td>LWG 2228 missing SFINAE rule</td><td>Lenexa</td><td>Complete</td><td>3.1</td></tr>
   <tr><td><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4510">N4510</a></td><td>LWG</td></td><td>Minimal incomplete type support for standard containers, revision 4</td><td>Lenexa</td><td>Complete</td><td>3.6</td></tr>
     <tr><td></td><td></td><td></td><td></td><td></td><td></td></tr>
   <tr><td><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/P0004R1.html">P0004R1</a></td><td>LWG</td><td>Remove Deprecated iostreams aliases.</td><td>Kona</td><td>Complete</td><td>3.8</td></tr>
   <tr><td><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/P0006R0.html">P0006R0</a></td><td>LWG</td><td>Adopt Type Traits Variable Templates for C++17.</td><td>Kona</td><td>Complete</td><td>3.8</td></tr>
   <tr><td><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/P0092R1.html">P0092R1</a></td><td>LWG</td><td>Polishing &lt;chrono&gt;</td><td>Kona</td><td>Complete</td><td>3.8</td></tr>
   <tr><td><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/P0007R1.html">P0007R1</a></td><td>LWG</td><td>Constant View: A proposal for a <tt>std::as_const</tt> helper function template.</td><td>Kona</td><td>Complete</td><td>3.8</td></tr>
-  <tr><td><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/P0156R0.htm" >P0156R0</a></td><td>LWG</td><td>Variadic lock_guard(rev 3).</td><td>Kona</td><td></td><td></td></tr>
+  <tr><td><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/p0156r0.html" >P0156R0</a></td><td>LWG</td><td>Variadic lock_guard(rev 3).</td><td>Kona</td><td>Complete (ABI V2 Only)</td><td>3.9</td></tr>
   <tr><td><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/P0074R0.html">P0074R0</a></td><td>LWG</td><td>Making <tt>std::owner_less</tt> more flexible</td><td>Kona</td><td>Complete</td><td>3.8</td></tr>
   <tr><td><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/P0013R1.html">P0013R1</a></td><td>LWG</td><td>Logical type traits rev 2</td><td>Kona</td><td>Complete</td><td>3.8</td></tr>
     <tr><td></td><td></td><td></td><td></td><td></td><td></td></tr>
   <tr><td><a href="http://wg21.link/P0024R2">P0024R2</a></td><td>LWG</td><td>The Parallelism TS Should be Standardized</td><td>Jacksonville</td><td></td><td></td></tr>
   <tr><td><a href="http://wg21.link/P0226R1">P0226R1</a></td><td>LWG</td><td>Mathematical Special Functions for C++17</td><td>Jacksonville</td><td></td><td></td></tr>
   <tr><td><a href="http://wg21.link/P0220R1">P0220R1</a></td><td>LWG</td><td>Adopt Library Fundamentals V1 TS Components for C++17</td><td>Jacksonville</td><td></td><td></td></tr>
   <tr><td><a href="http://wg21.link/P0218R1">P0218R1</a></td><td>LWG</td><td>Adopt the File System TS for C++17</td><td>Jacksonville</td><td></td><td></td></tr>
   <tr><td><a href="http://wg21.link/P0033R1">P0033R1</a></td><td>LWG</td><td>Re-enabling shared_from_this</td><td>Jacksonville</td><td>Complete</td><td>3.9</td></tr>