[libc++] Implement LWG 2510

[ldionne]

Summary

LWG2510 makes tag types like allocator_arg_t explicitly default
constructible instead of implicitly default constructible. It also
makes the constructors for std::pair and std::tuple conditionally
explicit based on the explicit-ness of the default constructibility
for the pair/tuple's elements.

[ldionne]

Rebase onto master

[ldionne]

Add deduction guide for tuple<>. Otherwise, Clang seems to be unable to deduce
std::tuple t{} because the deduction guide leads to an explicit specialization.

[ldionne]

libcxx/include/tuple

975

@Quuxplusone Any idea why this is needed? Otherwise, I get:

<snip>/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/implicit_deduction_guides.pass.cpp:125:16: error: no viable constructor or deduction guide for deduction of template arguments of 'tuple'
    std::tuple t1{};
               ^
<snip>/libcxx/include/tuple:643:5: note: candidate template ignored: substitution failure [with _Tp = <>, _Dummy = true]: cannot reference member of primary template because deduced class template specialization 'tuple<>' is an explicit specialization
    tuple()
    ^
<snip>/libcxx/include/tuple:650:5: note: candidate template ignored: substitution failure [with _Tp = <>, _Dummy = true]: cannot reference member of primary template because deduced class template specialization 'tuple<>' is an explicit specialization
    tuple()
    ^
<snip>/libcxx/include/tuple:680:5: note: candidate template ignored: substitution failure [with _Tp = <>, _Dummy = true]: cannot reference member of primary template because deduced class template specialization 'tuple<>' is an explicit specialization
    tuple(const _Tp& ... __t) _NOEXCEPT_((__all<is_nothrow_copy_constructible<_Tp>::value...>::value))
    ^
<snip>/libcxx/include/tuple:698:14: note: candidate template ignored: substitution failure [with _Tp = <>, _Dummy = true]: cannot reference member of primary template because deduced class template specialization 'tuple<>' is an explicit specialization
    explicit tuple(const _Tp& ... __t) _NOEXCEPT_((__all<is_nothrow_copy_constructible<_Tp>::value...>::value))
             ^
<snip>/libcxx/include/tuple:762:9: note: candidate template ignored: substitution failure [with _Tp = <>, _Up = <>]: cannot reference member of primary template because deduced class template specialization 'tuple<>' is an explicit specialization
        tuple(_Up&&... __u)
        ^
<snip>/libcxx/include/tuple:795:9: note: candidate template ignored: substitution failure [with _Tp = <>, _Up = <>]: cannot reference member of primary template because deduced class template specialization 'tuple<>' is an explicit specialization
        tuple(_Up&&... __u)
        ^
<snip>/libcxx/include/__tuple:159:52: note: candidate function template not viable: requires 1 argument, but 0 were provided
template <class ..._Tp> class _LIBCPP_TEMPLATE_VIS tuple;
                                                   ^
<snip>/libcxx/include/tuple:653:5: note: candidate function template not viable: requires 1 argument, but 0 were provided
    tuple(tuple const&) = default;
    ^
<snip>/libcxx/include/tuple:654:5: note: candidate function template not viable: requires 1 argument, but 0 were provided
    tuple(tuple&&) = default;
    ^
<snip>/libcxx/include/tuple:664:5: note: candidate function template not viable: requires 2 arguments, but 0 were provided
    tuple(_AllocArgT, _Alloc const& __a)
    ^
<snip>/libcxx/include/tuple:716:7: note: candidate function template not viable: requires at least 2 arguments, but 0 were provided
      tuple(allocator_arg_t, const _Alloc& __a, const _Tp& ... __t)
      ^
<snip>/libcxx/include/tuple:736:7: note: candidate function template not viable: requires at least 2 arguments, but 0 were provided
      tuple(allocator_arg_t, const _Alloc& __a, const _Tp& ... __t)
      ^
<snip>/libcxx/include/tuple:822:9: note: candidate function template not viable: requires at least 2 arguments, but 0 were provided
        tuple(allocator_arg_t, const _Alloc& __a, _Up&&... __u)
        ^
<snip>/libcxx/include/tuple:842:9: note: candidate function template not viable: requires at least 2 arguments, but 0 were provided
        tuple(allocator_arg_t, const _Alloc& __a, _Up&&... __u)
        ^
<snip>/libcxx/include/tuple:852:9: note: candidate function template not viable: requires single argument '__t', but no arguments were provided
        tuple(_Tuple&& __t) _NOEXCEPT_((is_nothrow_constructible<_BaseT, _Tuple>::value))
        ^
<snip>/libcxx/include/tuple:857:9: note: candidate function template not viable: requires single argument '__t', but no arguments were provided
        tuple(const _Tuple& __t) _NOEXCEPT_((is_nothrow_constructible<_BaseT, const _Tuple&>

[...]

[ldionne]

Rebase on top of CTAD for tuple: https://reviews.llvm.org/D65225

[• Quuxplusone]

libcxx/include/tuple

975

I got it! :) https://godbolt.org/z/NGrj_6

template<class...> struct Tuple {
    static constexpr bool Example = true;
    template<bool = Example> Tuple();
};
template<> struct Tuple<> {};

Tuple t;

error: no viable constructor or deduction guide for deduction of template arguments of 'Tuple'
Tuple t;
      ^
note: candidate template ignored: substitution failure [with $0 = <>]: cannot reference member of primary template because deduced class template specialization 'Tuple<>' is an explicit specialization
    template<bool = Example> Tuple();
                    ~~~~~~~  ^
note: candidate function template not viable: requires 1 argument, but 0 were provided
template<class...> struct Tuple {
                          ^

Clang, probably rightly — although I'm not sure — considers only implicit deduction guides that are created from the actual constructors of the primary template. So, when it sees a constructor template, it needs to figure out whether that template corresponds to an actual constructor, or a SFINAE'd-away constructor (because if the latter, then it should not make a deduction guide for it). However, it needs to do that SFINAE check without actually instantiating the primary template (because it's simply not allowed to instantiate the primary template when a suitable specialization exists). So, if the SFINAE-space-validity of the constructor template is dependent on any member of the primary template — such as Tuple<Ts...>::Example in this example, or tuple<_Tp...>::_CheckArgsConstructor in libc++ — then the compiler can't safely figure out if the constructor template corresponds to an actual constructor or not, and so the compiler must throw it out. (Clang treats this as a "substitution failure," but intuitively I don't think that's exactly correct. It's more like, the compiler doesn't even know whether substitution is going to fail or not.)

Conclusion: You could solve the original cascade of errors by making all those helper structs (std::tuple<_Tp...>::_CheckArgsConstructor etc) into non-member struct templates (std::_TupleCheckArgsConstructor<_Tp> etc). Adding the deduction guide seems much simpler, and also follows the letter of the standard, which is a good thing.

[ldionne]

libcxx/include/tuple

975

Woah, thanks a lot for the analysis! Interesting, but indeed I think adding the deduction guides is cleaner.

[ldionne]

Rebase on top of master. This doesn't have a dependency anymore.

[zoecarver]

libcxx/include/type_traits

2819

You probably need an #if _LIBCPP_STD_VER > 03 here.

libcxx/test/std/utilities/memory/allocator.tag/allocator_arg.fail.cpp

12

// UNSUPPORTED: c++98, c++03

[ldionne]

libcxx/include/type_traits

2819

If you're thinking about the use of decltype, it's fine to assume that we have it in C++03 mode too. However, the implicit construction via {} is indeed a C++11 feature that's not supported in C++03, so I'll need to find another way to perform the check.

I think we want to find a way to do the check in C++03 instead of always having __is_implicitly_default_constructible<T>::value == false because that will change the explicit-ness of things like std::pair in C++03 vs C++11, which is a notable change. Also, the intent of a DR is to have it back-applied so it would be nice if we could implement the issue resolution even in C++03.

I'll admit I can't think of any way to implement the check in C++03 though.

libcxx/test/std/utilities/memory/allocator.tag/allocator_arg.fail.cpp

12

I think we should do our best to support this test (and the similar ones below) in C++03, since this is a DR.

[zoecarver]

libcxx/test/std/utilities/memory/allocator.tag/allocator_arg.fail.cpp

12

That's probably a good idea. See my comment below. There are a few other tests, maybe we should update/enable those too?

21

If we are keeping this test we should update this function to return std:: allocator_arg_t ().

[ldionne]

Handle C++03 and C++98 gracefully

[mclow.lists]

this looks fine to me now (with a nit).

libcxx/include/type_traits

2823

Nit: This comment doesn't really match the #ifndef _LIBCPP_CXX03_LANG that starts the block.

[zoecarver]

LGTM.

[ldionne]

This was reverted in r372832. I'm waiting to see the resolution of https://bugs.llvm.org/show_bug.cgi?id=43454 before I know what to do.

libcxx/test/std/utilities/memory/allocator.tag/allocator_arg.fail.cpp

21

The whole point of this test is to exercise the error caused by the non-explicit copy-initialization. I'm not sure I understand your comment.

[zoecarver]

libcxx/test/std/utilities/memory/allocator.tag/allocator_arg.fail.cpp

21

I think I was worried about C++03 mode. Now that this has been updated to be unsupported in C++03, don't worry about it.

[ldionne]

Workaround PR43454, which previously broke the LLVM build

libcxx/include/__functional_base

 template <class _Tp, class _Up>
 struct __is_transparent<_Tp, _Up,
                         typename __void_t<typename _Tp::is_transparent>::type>
    : true_type {};
 #endif
 
 // allocator_arg_t
 
-struct _LIBCPP_TEMPLATE_VIS allocator_arg_t { };
+struct _LIBCPP_TEMPLATE_VIS allocator_arg_t { explicit allocator_arg_t() = default; };
 
 #if defined(_LIBCPP_CXX03_LANG) || defined(_LIBCPP_BUILDING_LIBRARY)
 extern _LIBCPP_EXPORTED_FROM_ABI const allocator_arg_t allocator_arg;
 #else
 /* _LIBCPP_INLINE_VAR */ constexpr allocator_arg_t allocator_arg = allocator_arg_t();
 #endif
 
 // uses_allocator

libcxx/include/__mutex_base

#endif
 
     typedef __libcpp_mutex_t* native_handle_type;
     _LIBCPP_INLINE_VISIBILITY native_handle_type native_handle() {return &__m_;}
 };
 
 static_assert(is_nothrow_default_constructible<mutex>::value,
               "the default constructor for std::mutex must be nothrow");
 
-struct _LIBCPP_TYPE_VIS defer_lock_t {};
-struct _LIBCPP_TYPE_VIS try_to_lock_t {};
-struct _LIBCPP_TYPE_VIS adopt_lock_t {};
+struct _LIBCPP_TYPE_VIS defer_lock_t { explicit defer_lock_t() = default; };
+struct _LIBCPP_TYPE_VIS try_to_lock_t { explicit try_to_lock_t() = default; };
+struct _LIBCPP_TYPE_VIS adopt_lock_t { explicit adopt_lock_t() = default; };
 
 #if defined(_LIBCPP_CXX03_LANG) || defined(_LIBCPP_BUILDING_LIBRARY)
 
 extern _LIBCPP_EXPORTED_FROM_ABI const defer_lock_t  defer_lock;
 extern _LIBCPP_EXPORTED_FROM_ABI const try_to_lock_t try_to_lock;
 extern _LIBCPP_EXPORTED_FROM_ABI const adopt_lock_t  adopt_lock;
 
 #else

libcxx/include/mutex

public:
     bool try_lock() noexcept;
     template <class Rep, class Period>
         bool try_lock_for(const chrono::duration<Rep, Period>& rel_time);
     template <class Clock, class Duration>
         bool try_lock_until(const chrono::time_point<Clock, Duration>& abs_time);
     void unlock();
 };
 
-struct defer_lock_t {};
-struct try_to_lock_t {};
-struct adopt_lock_t {};
+struct defer_lock_t { explicit defer_lock_t() = default; };
+struct try_to_lock_t { explicit try_to_lock_t() = default; };
+struct adopt_lock_t { explicit adopt_lock_t() = default; };
 
 inline constexpr defer_lock_t  defer_lock{};
 inline constexpr try_to_lock_t try_to_lock{};
 inline constexpr adopt_lock_t  adopt_lock{};
 
 template <class Mutex>
 class lock_guard
 {

libcxx/include/new

 
 enum class align_val_t : size_t {}; // C++17
 
 struct destroying_delete_t { // C++20
   explicit destroying_delete_t() = default;
 };
 inline constexpr destroying_delete_t destroying_delete{}; // C++20
 
-struct nothrow_t {};
+struct nothrow_t { explicit nothrow_t() = default; };
 extern const nothrow_t nothrow;
 typedef void (*new_handler)();
 new_handler set_new_handler(new_handler new_p) noexcept;
 new_handler get_new_handler() noexcept;
 
 // 21.6.4, pointer optimization barrier
 template <class T> constexpr T* launder(T* p) noexcept; // C++17
 }  // std
#if !__has_builtin(__builtin_operator_new) || \
    __has_builtin(__builtin_operator_new) < 201802L
 #define _LIBCPP_HAS_NO_BUILTIN_OVERLOADED_OPERATOR_NEW_DELETE
 #endif
 
 namespace std  // purposefully not using versioning namespace
 {
 
 #if !defined(_LIBCPP_ABI_VCRUNTIME)
-struct _LIBCPP_TYPE_VIS nothrow_t {};
+struct _LIBCPP_TYPE_VIS nothrow_t { explicit nothrow_t() = default; };
 extern _LIBCPP_FUNC_VIS const nothrow_t nothrow;
 
 class _LIBCPP_EXCEPTION_ABI bad_alloc
     : public exception
 {
 public:
     bad_alloc() _NOEXCEPT;
     virtual ~bad_alloc() _NOEXCEPT;

libcxx/include/tuple

/*
     tuple synopsis
 
 namespace std
 {
 
 template <class... T>
 class tuple {
 public:
-    constexpr tuple();
+    explicit(see-below) constexpr tuple();
     explicit(see-below) tuple(const T&...);  // constexpr in C++14
     template <class... U>
         explicit(see-below) tuple(U&&...);  // constexpr in C++14
     tuple(const tuple&) = default;
     tuple(tuple&&) = default;
     template <class... U>
         explicit(see-below) tuple(const tuple<U...>&);  // constexpr in C++14
     template <class... U>
#endif
 
     template <bool _MaybeEnable, class _Dummy = void>
     struct _CheckArgsConstructor : __check_tuple_constructor_fail {};
 
     template <class _Dummy>
     struct _CheckArgsConstructor<true, _Dummy>
     {
         template <class ..._Args>
-        static constexpr bool __enable_default() {
-            return __all<is_default_constructible<_Args>::value...>::value;
+        static constexpr bool __enable_implicit_default() {
+            // In C++03, there's no way to implement the resolution of LWG2510.
+#ifdef _LIBCPP_CXX03_LANG
+            return true;
+#else
+            return __all<__is_implicitly_default_constructible<_Args>::value...>::value;
+#endif
+        }
+
+        template <class ..._Args>
+        static constexpr bool __enable_explicit_default() {
+            return __all<is_default_constructible<_Args>::value...>::value
+                && !__enable_implicit_default<_Args...>();
         }
 
         template <class ..._Args>
         static constexpr bool __enable_explicit() {
             return
                 __tuple_constructible<
                     tuple<_Args...>,
                     typename __make_tuple_types<tuple,
#endif
     template <size_t _Jp, class ..._Up> friend _LIBCPP_CONSTEXPR_AFTER_CXX11
         const typename tuple_element<_Jp, tuple<_Up...> >::type& get(const tuple<_Up...>&) _NOEXCEPT;
     template <size_t _Jp, class ..._Up> friend _LIBCPP_CONSTEXPR_AFTER_CXX11
         typename tuple_element<_Jp, tuple<_Up...> >::type&& get(tuple<_Up...>&&) _NOEXCEPT;
     template <size_t _Jp, class ..._Up> friend _LIBCPP_CONSTEXPR_AFTER_CXX11
         const typename tuple_element<_Jp, tuple<_Up...> >::type&& get(const tuple<_Up...>&&) _NOEXCEPT;
 public:
 
-    template <bool _Dummy = true, class = typename enable_if<
-        _CheckArgsConstructor<_Dummy>::template __enable_default<_Tp...>()
-    >::type>
-    _LIBCPP_INLINE_VISIBILITY
-    _LIBCPP_CONSTEXPR tuple()
+    template <bool _Dummy = true, _EnableIf<
+        _CheckArgsConstructor<_Dummy>::template __enable_implicit_default<_Tp...>()
+    , void*> = nullptr>
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR
+    tuple()
+        _NOEXCEPT_(__all<is_nothrow_default_constructible<_Tp>::value...>::value) {}
+
+    template <bool _Dummy = true, _EnableIf<
+        _CheckArgsConstructor<_Dummy>::template __enable_explicit_default<_Tp...>()
+    , void*> = nullptr>
+    explicit _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR
+    tuple()
         _NOEXCEPT_(__all<is_nothrow_default_constructible<_Tp>::value...>::value) {}
 
     tuple(tuple const&) = default;
     tuple(tuple&&) = default;
 
     template <class _AllocArgT, class _Alloc, bool _Dummy = true, class = _EnableIf<
         _And<
             _IsSame<allocator_arg_t, _AllocArgT>,
public:
     template <class _Alloc, class _Up>
     _LIBCPP_INLINE_VISIBILITY
         tuple(allocator_arg_t, const _Alloc&, array<_Up, 0>) _NOEXCEPT {}
     _LIBCPP_INLINE_VISIBILITY
     void swap(tuple&) _NOEXCEPT {}
 };
 
 #ifndef _LIBCPP_HAS_NO_DEDUCTION_GUIDES
 template <class ..._Tp>
 tuple(_Tp...) -> tuple<_Tp...>;
 template <class _Tp1, class _Tp2>
 tuple(pair<_Tp1, _Tp2>) -> tuple<_Tp1, _Tp2>;
 template <class _Alloc, class ..._Tp>
 tuple(allocator_arg_t, _Alloc, _Tp...) -> tuple<_Tp...>;
 template <class _Alloc, class _Tp1, class _Tp2>
 tuple(allocator_arg_t, _Alloc, pair<_Tp1, _Tp2>) -> tuple<_Tp1, _Tp2>;
 template <class _Alloc, class ..._Tp>

libcxx/include/type_traits

struct _LIBCPP_TEMPLATE_VIS is_default_constructible
     {};
 
 #if _LIBCPP_STD_VER > 14 && !defined(_LIBCPP_HAS_NO_VARIABLE_TEMPLATES)
 template <class _Tp>
 _LIBCPP_INLINE_VAR _LIBCPP_CONSTEXPR bool is_default_constructible_v
     = is_default_constructible<_Tp>::value;
 #endif
 
+#ifndef _LIBCPP_CXX03_LANG
+template <class _Tp>
+void __test_implicit_default_constructible(_Tp);
+
+template <class _Tp, class = void>
+struct __is_implicitly_default_constructible
+    : false_type
+{ };
+
+template <class _Tp>
+struct __is_implicitly_default_constructible<_Tp, decltype(__test_implicit_default_constructible<_Tp const&>({}))>
+    : is_default_constructible<_Tp>
+{ };
+#endif // >= C++11
+
 // is_copy_constructible
 
 template <class _Tp>
 struct _LIBCPP_TEMPLATE_VIS is_copy_constructible
     : public is_constructible<_Tp,
                   typename add_lvalue_reference<typename add_const<_Tp>::type>::type> {};
 
 #if _LIBCPP_STD_VER > 14 && !defined(_LIBCPP_HAS_NO_VARIABLE_TEMPLATES)

libcxx/include/utility

struct pair
     typedef T1 first_type;
     typedef T2 second_type;
 
     T1 first;
     T2 second;
 
     pair(const pair&) = default;
     pair(pair&&) = default;
-    constexpr pair();
+    explicit(see-below) constexpr pair();
     explicit(see-below) pair(const T1& x, const T2& y);                          // constexpr in C++14
     template <class U, class V> explicit(see-below) pair(U&& x, V&& y);          // constexpr in C++14
     template <class U, class V> explicit(see-below) pair(const pair<U, V>& p);   // constexpr in C++14
     template <class U, class V> explicit(see-below) pair(pair<U, V>&& p);        // constexpr in C++14
     template <class... Args1, class... Args2>
         pair(piecewise_construct_t, tuple<Args1...> first_args,
              tuple<Args2...> second_args);
 

 template <class T1, class T2> bool operator>=(const pair<T1,T2>&, const pair<T1,T2>&); // constexpr in C++14
 template <class T1, class T2> bool operator<=(const pair<T1,T2>&, const pair<T1,T2>&); // constexpr in C++14
 
 template <class T1, class T2> pair<V1, V2> make_pair(T1&&, T2&&);   // constexpr in C++14
 template <class T1, class T2>
 void
 swap(pair<T1, T2>& x, pair<T1, T2>& y) noexcept(noexcept(x.swap(y)));
 
-struct piecewise_construct_t { };
+struct piecewise_construct_t { explicit piecewise_construct_t() = default; };
 inline constexpr piecewise_construct_t piecewise_construct = piecewise_construct_t();
 
 template <class T> struct tuple_size;
 template <size_t I, class T> struct tuple_element;
 
 template <class T1, class T2> struct tuple_size<pair<T1, T2> >;
 template <class T1, class T2> struct tuple_element<0, pair<T1, T2> >;
 template <class T1, class T2> struct tuple_element<1, pair<T1, T2> >;
move_if_noexcept(_Tp& __x) _NOEXCEPT
     return _VSTD::move(__x);
 }
 
 #if _LIBCPP_STD_VER > 14
 template <class _Tp> constexpr add_const_t<_Tp>& as_const(_Tp& __t) noexcept { return __t; }
 template <class _Tp>                        void as_const(const _Tp&&) = delete;
 #endif
 
-struct _LIBCPP_TEMPLATE_VIS piecewise_construct_t { };
+struct _LIBCPP_TEMPLATE_VIS piecewise_construct_t { explicit piecewise_construct_t() = default; };
 #if defined(_LIBCPP_CXX03_LANG) || defined(_LIBCPP_BUILDING_LIBRARY)
 extern _LIBCPP_EXPORTED_FROM_ABI const piecewise_construct_t piecewise_construct;// = piecewise_construct_t();
 #else
 /* _LIBCPP_INLINE_VAR */ constexpr piecewise_construct_t piecewise_construct = piecewise_construct_t();
 #endif
 
 #if defined(_LIBCPP_DEPRECATED_ABI_DISABLE_PAIR_TRIVIAL_COPY_CTOR)
 template <class, class>
pair& operator=(pair const& __p) {
         return *this;
     }
 #else
     template <bool _Val>
     using _EnableB _LIBCPP_NODEBUG_TYPE = typename enable_if<_Val, bool>::type;
 
     struct _CheckArgs {
       template <class _U1, class _U2>
-      static constexpr bool __enable_default() {
+      static constexpr bool __enable_explicit_default() {
           return is_default_constructible<_U1>::value
-              && is_default_constructible<_U2>::value;
+              && is_default_constructible<_U2>::value
+              && !__enable_implicit_default<_U1, _U2>();
+      }
+
+      template <class _U1, class _U2>
+      static constexpr bool __enable_implicit_default() {
+          // In C++03, there's no way to implement the resolution of LWG2510.
+#ifdef _LIBCPP_CXX03_LANG
+          return true;
+#else
+          return __is_implicitly_default_constructible<_U1>::value
+              && __is_implicitly_default_constructible<_U2>::value;
+#endif
       }
 
       template <class _U1, class _U2>
       static constexpr bool __enable_explicit() {
           return is_constructible<first_type, _U1>::value
               && is_constructible<second_type, _U2>::value
               && (!is_convertible<_U1, first_type>::value
                   || !is_convertible<_U2, second_type>::value);
#endif
     using _CheckTLC _LIBCPP_NODEBUG_TYPE = typename conditional<
         __tuple_like_with_size<_Tuple, 2>::value
             && !is_same<typename decay<_Tuple>::type, pair>::value,
         _CheckTupleLikeConstructor,
         __check_tuple_constructor_fail
     >::type;
 
     template<bool _Dummy = true, _EnableB<
-            _CheckArgsDep<_Dummy>::template __enable_default<_T1, _T2>()
+            _CheckArgsDep<_Dummy>::template __enable_explicit_default<_T1, _T2>()
+    > = false>
+    explicit _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR
+    pair() _NOEXCEPT_(is_nothrow_default_constructible<first_type>::value &&
+                      is_nothrow_default_constructible<second_type>::value)
+        : first(), second() {}
+
+    template<bool _Dummy = true, _EnableB<
+            _CheckArgsDep<_Dummy>::template __enable_implicit_default<_T1, _T2>()
     > = false>
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR
     pair() _NOEXCEPT_(is_nothrow_default_constructible<first_type>::value &&
                       is_nothrow_default_constructible<second_type>::value)
         : first(), second() {}
 
     template <bool _Dummy = true, _EnableB<
              _CheckArgsDep<_Dummy>::template __enable_explicit<_T1 const&, _T2 const&>()

libcxx/src/mutex.cpp

 #if defined(__unix__) && !defined(__ANDROID__) && defined(__ELF__) && defined(_LIBCPP_HAS_COMMENT_LIB_PRAGMA)
 #pragma comment(lib, "pthread")
 #endif
 #endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 #ifndef _LIBCPP_HAS_NO_THREADS
 
-const defer_lock_t  defer_lock = {};
-const try_to_lock_t try_to_lock = {};
-const adopt_lock_t  adopt_lock = {};
+const defer_lock_t  defer_lock{};
+const try_to_lock_t try_to_lock{};
+const adopt_lock_t  adopt_lock{};
 
 // ~mutex is defined elsewhere
 
 void
 mutex::lock()
 {
     int ec = __libcpp_mutex_lock(&__m_);
     if (ec)

libcxx/src/utility.cpp

 //===------------------------ utility.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
 //
 //===----------------------------------------------------------------------===//
 
 #include "utility"
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-const piecewise_construct_t piecewise_construct = {};
+const piecewise_construct_t piecewise_construct{};
 
 _LIBCPP_END_NAMESPACE_STD

libcxx/test/libcxx/type_traits/is_implicitly_default_constructible.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: c++98, c++03
+
+// <type_traits>
+
+// __is_implicitly_default_constructible<Tp>
+
+#include <type_traits>
+
+
+struct ExplicitlyDefaultConstructible1 {
+    explicit ExplicitlyDefaultConstructible1() = default;
+};
+
+struct ExplicitlyDefaultConstructible2 {
+    explicit ExplicitlyDefaultConstructible2() { }
+};
+
+struct ImplicitlyDefaultConstructible1 {
+    ImplicitlyDefaultConstructible1() { }
+};
+
+struct ImplicitlyDefaultConstructible2 {
+    ImplicitlyDefaultConstructible2() = default;
+};
+
+struct NonDefaultConstructible1 {
+    NonDefaultConstructible1() = delete;
+};
+
+struct NonDefaultConstructible2 {
+    explicit NonDefaultConstructible2() = delete;
+};
+
+struct NonDefaultConstructible3 {
+    NonDefaultConstructible3(NonDefaultConstructible3&&) { }
+};
+
+static_assert(!std::__is_implicitly_default_constructible<ExplicitlyDefaultConstructible1>::value, "");
+static_assert(!std::__is_implicitly_default_constructible<ExplicitlyDefaultConstructible2>::value, "");
+static_assert(std::__is_implicitly_default_constructible<ImplicitlyDefaultConstructible1>::value, "");
+static_assert(std::__is_implicitly_default_constructible<ImplicitlyDefaultConstructible2>::value, "");
+static_assert(!std::__is_implicitly_default_constructible<NonDefaultConstructible1>::value, "");
+static_assert(!std::__is_implicitly_default_constructible<NonDefaultConstructible2>::value, "");
+static_assert(!std::__is_implicitly_default_constructible<NonDefaultConstructible3>::value, "");
+
+int main(int, char**) {
+    return 0;
+}

libcxx/test/std/language.support/support.dynamic/nothrow_t.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: c++98, c++03
+
+// struct nothrow_t {
+//   explicit nothrow_t() = default;
+// };
+// extern const nothrow_t nothrow;
+
+// This test checks for LWG 2510.
+
+#include <new>
+
+
+std::nothrow_t f() { return {}; } // expected-error 1 {{chosen constructor is explicit in copy-initialization}}
+
+int main(int, char**) {
+    return 0;
+}

libcxx/test/std/language.support/support.dynamic/nothrow_t.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.
+//
+//===----------------------------------------------------------------------===//
+
+// struct nothrow_t {
+//   explicit nothrow_t() = default;
+// };
+// extern const nothrow_t nothrow;
+
+#include <new>
+
+
+int main(int, char**) {
+  std::nothrow_t x = std::nothrow;
+  (void)x;
+
+  return 0;
+}

libcxx/test/std/thread/thread.mutex/thread.lock/types.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: c++98, c++03
+
+// UNSUPPORTED: libcpp-has-no-threads
+
+// <mutex>
+
+// struct defer_lock_t { explicit defer_lock_t() = default; };
+// struct try_to_lock_t { explicit try_to_lock_t() = default; };
+// struct adopt_lock_t { explicit adopt_lock_t() = default; };
+
+// This test checks for LWG 2510.
+
+#include <mutex>
+
+
+std::defer_lock_t f1() { return {}; } // expected-error 1 {{chosen constructor is explicit in copy-initialization}}
+std::try_to_lock_t f2() { return {}; } // expected-error 1 {{chosen constructor is explicit in copy-initialization}}
+std::adopt_lock_t f3() { return {}; } // expected-error 1 {{chosen constructor is explicit in copy-initialization}}
+
+int main(int, char**) {
+    return 0;
+}

libcxx/test/std/thread/thread.mutex/thread.lock/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
 
 // <mutex>
 
-// struct defer_lock_t {};
-// struct try_to_lock_t {};
-// struct adopt_lock_t {};
+// struct defer_lock_t { explicit defer_lock_t() = default; };
+// struct try_to_lock_t { explicit try_to_lock_t() = default; };
+// struct adopt_lock_t { explicit adopt_lock_t() = default; };
 //
 // constexpr defer_lock_t  defer_lock{};
 // constexpr try_to_lock_t try_to_lock{};
 // constexpr adopt_lock_t  adopt_lock{};
 
 #include <mutex>
-#include <type_traits>
 
 #include "test_macros.h"
 
 int main(int, char**)
 {
     typedef std::defer_lock_t T1;
     typedef std::try_to_lock_t T2;
     typedef std::adopt_lock_t T3;
 
     T1 t1 = std::defer_lock; ((void)t1);
     T2 t2 = std::try_to_lock; ((void)t2);
     T3 t3 = std::adopt_lock; ((void)t3);
 
     return 0;
 }

libcxx/test/std/utilities/memory/allocator.tag/allocator_arg.fail.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: c++98, c++03
+
 // <memory>
 
-// struct allocator_arg_t { };
+// struct allocator_arg_t { explicit allocator_arg_t() = default; };
 // const allocator_arg_t allocator_arg = allocator_arg_t();
 
-#include <memory>
+// This test checks for LWG 2510.
 
-#include "test_macros.h"
+#include <memory>
 
-void test(std::allocator_arg_t) {}
 
-int main(int, char**)
-{
-    test(std::allocator_arg);
+std::allocator_arg_t f() { return {}; } // expected-error 1 {{chosen constructor is explicit in copy-initialization}}
 
+int main(int, char**) {
     return 0;
 }

libcxx/test/std/utilities/memory/allocator.tag/allocator_arg.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
 //
 //===----------------------------------------------------------------------===//
 
 // <memory>
 
-// struct allocator_arg_t { };
+// struct allocator_arg_t { explicit allocator_arg_t() = default; };
 // const allocator_arg_t allocator_arg = allocator_arg_t();
 
 #include <memory>
 
 #include "test_macros.h"
 
 void test(std::allocator_arg_t) {}
 
 int main(int, char**)
 {
     test(std::allocator_arg);
 
     return 0;
 }

libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/default.fail.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: c++98, c++03
+
+// <tuple>
+
+// template <class... Types> class tuple;
+
+// explicit(see-below) constexpr tuple();
+
+#include <tuple>
+
+
+struct Implicit {
+    Implicit() = default;
+};
+
+struct Explicit {
+    explicit Explicit() = default;
+};
+
+std::tuple<> test1() { return {}; }
+
+std::tuple<Implicit> test2() { return {}; }
+std::tuple<Explicit> test3() { return {}; } // expected-error 1 {{chosen constructor is explicit in copy-initialization}}
+
+std::tuple<Implicit, Implicit> test4() { return {}; }
+std::tuple<Explicit, Implicit> test5() { return {}; } // expected-error 1 {{chosen constructor is explicit in copy-initialization}}
+std::tuple<Implicit, Explicit> test6() { return {}; } // expected-error 1 {{chosen constructor is explicit in copy-initialization}}
+std::tuple<Explicit, Explicit> test7() { return {}; } // expected-error 1 {{chosen constructor is explicit in copy-initialization}}
+
+std::tuple<Implicit, Implicit, Implicit> test8() { return {}; }
+std::tuple<Implicit, Implicit, Explicit> test9() { return {}; } // expected-error 1 {{chosen constructor is explicit in copy-initialization}}
+std::tuple<Implicit, Explicit, Implicit> test10() { return {}; } // expected-error 1 {{chosen constructor is explicit in copy-initialization}}
+std::tuple<Implicit, Explicit, Explicit> test11() { return {}; } // expected-error 1 {{chosen constructor is explicit in copy-initialization}}
+std::tuple<Explicit, Implicit, Implicit> test12() { return {}; } // expected-error 1 {{chosen constructor is explicit in copy-initialization}}
+std::tuple<Explicit, Implicit, Explicit> test13() { return {}; } // expected-error 1 {{chosen constructor is explicit in copy-initialization}}
+std::tuple<Explicit, Explicit, Implicit> test14() { return {}; } // expected-error 1 {{chosen constructor is explicit in copy-initialization}}
+std::tuple<Explicit, Explicit, Explicit> test15() { return {}; } // expected-error 1 {{chosen constructor is explicit in copy-initialization}}
+
+int main(int, char**) {
+    return 0;
+}

libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/default.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
 //
 //===----------------------------------------------------------------------===//
 
 // <tuple>
 
 // template <class... Types> class tuple;
 
-// constexpr tuple();
+// explicit(see-below) constexpr tuple();
 
 // UNSUPPORTED: c++98, c++03
 
 #include <tuple>
 #include <string>
 #include <cassert>
 #include <type_traits>
 

libcxx/test/std/utilities/utility/pairs/pair.piecewise/piecewise_construct.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: c++98, c++03
 
 // <utility>
 
 // template <class T1, class T2> struct pair
 
-// struct piecewise_construct_t { };
+// struct piecewise_construct_t { explicit piecewise_construct_t() = default; };
 // constexpr piecewise_construct_t piecewise_construct = piecewise_construct_t();
 
 #include <utility>
 #include <tuple>
 #include <cassert>
 
 #include "test_macros.h"
 

libcxx/test/std/utilities/utility/pairs/pair.piecewise/piecewise_construct_t.fail.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: c++98, c++03
+
+// <utility>
+
+// struct piecewise_construct_t { explicit piecewise_construct_t() = default; };
+// constexpr piecewise_construct_t piecewise_construct = piecewise_construct_t();
+
+// This test checks for LWG 2510.
+
+#include <utility>
+
+
+std::piecewise_construct_t f() { return {}; } // expected-error 1 {{chosen constructor is explicit in copy-initialization}}
+
+int main(int, char**) {
+    return 0;
+}

libcxx/test/std/utilities/utility/pairs/pair.piecewise/piecewise_construct_t.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
 //
 //===----------------------------------------------------------------------===//
 
-// <memory>
+// UNSUPPORTED: c++98, c++03
 
-// struct allocator_arg_t { };
-// const allocator_arg_t allocator_arg = allocator_arg_t();
+// <utility>
 
-#include <memory>
+// struct piecewise_construct_t { explicit piecewise_construct_t() = default; };
+// constexpr piecewise_construct_t piecewise_construct = piecewise_construct_t();
 
-#include "test_macros.h"
+#include <utility>
 
-void test(std::allocator_arg_t) {}
 
-int main(int, char**)
-{
-    test(std::allocator_arg);
+int main(int, char**) {
+    std::piecewise_construct_t x = std::piecewise_construct;
+    (void)x;
 
     return 0;
 }

libcxx/test/std/utilities/utility/pairs/pairs.pair/default.explicit.fail.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: c++98, c++03
+
+// <utility>
+
+// template <class T1, class T2> struct pair
+
+// explicit(see-below) constexpr pair();
+
+// This test checks the conditional explicitness of std::pair's default
+// constructor as introduced by the resolution of LWG 2510.
+
+#include <utility>
+
+
+struct ImplicitlyDefaultConstructible {
+    ImplicitlyDefaultConstructible() = default;
+};
+
+struct ExplicitlyDefaultConstructible {
+    explicit ExplicitlyDefaultConstructible() = default;
+};
+
+std::pair<ImplicitlyDefaultConstructible, ExplicitlyDefaultConstructible> test1() { return {}; } // expected-error 1 {{chosen constructor is explicit in copy-initialization}}
+std::pair<ExplicitlyDefaultConstructible, ImplicitlyDefaultConstructible> test2() { return {}; } // expected-error 1 {{chosen constructor is explicit in copy-initialization}}
+std::pair<ExplicitlyDefaultConstructible, ExplicitlyDefaultConstructible> test3() { return {}; } // expected-error 1 {{chosen constructor is explicit in copy-initialization}}
+std::pair<ImplicitlyDefaultConstructible, ImplicitlyDefaultConstructible> test4() { return {}; }
+
+int main(int, char**) {
+    return 0;
+}

libcxx/test/std/utilities/utility/pairs/pairs.pair/default.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
 //
 //===----------------------------------------------------------------------===//
 
 // <utility>
 
 // template <class T1, class T2> struct pair
 
-// constexpr pair();
+// explicit(see-below) constexpr pair();
 
 // This test doesn't pass due to a constexpr bug in GCC 4.9 that fails
 // to initialize any type without a user provided constructor in a constant
 // expression (e.g. float).
 // XFAIL: gcc-4.9
 
 // NOTE: The SFINAE on the default constructor is tested in
 //       default-sfinae.pass.cpp

libcxx/www/cxx1z_status.html

<!--   <I>Note: "NAD" means that the issue was deemed "Not a defect"</I> -->
   <tr><td><a href="https://wg21.link/LWG2221">2221</a></td><td>No formatted output operator for nullptr</td><td>Issaquah</td><td>Complete</td></tr>
   <tr><td><a href="https://wg21.link/LWG2223">2223</a></td><td>shrink_to_fit effect on iterator validity</td><td>Issaquah</td><td>Complete</td></tr>
   <tr><td><a href="https://wg21.link/LWG2261">2261</a></td><td>Are containers required to use their 'pointer' type internally?</td><td>Issaquah</td><td></td></tr>
   <tr><td><a href="https://wg21.link/LWG2394">2394</a></td><td>locale::name specification unclear - what is implementation-defined?</td><td>Issaquah</td><td>Complete</td></tr>
   <tr><td><a href="https://wg21.link/LWG2460">2460</a></td><td>LWG issue 2408 and value categories</td><td>Issaquah</td><td>Complete</td></tr>
   <tr><td><a href="https://wg21.link/LWG2468">2468</a></td><td>Self-move-assignment of library types</td><td>Issaquah</td><td></td></tr>
   <tr><td><a href="https://wg21.link/LWG2475">2475</a></td><td>Allow overwriting of std::basic_string terminator with charT() to allow cleaner interoperation with legacy APIs</td><td>Issaquah</td><td>Complete</td></tr>
   <tr><td><a href="https://wg21.link/LWG2503">2503</a></td><td>multiline option should be added to syntax_option_type</td><td>Issaquah</td><td></td></tr>
-  <tr><td><a href="https://wg21.link/LWG2510">2510</a></td><td>Tag types should not be DefaultConstructible</td><td>Issaquah</td><td></td></tr>
+  <tr><td><a href="https://wg21.link/LWG2510">2510</a></td><td>Tag types should not be DefaultConstructible</td><td>Issaquah</td><td>Complete</td></tr>
   <tr><td><a href="https://wg21.link/LWG2514">2514</a></td><td>Type traits must not be final</td><td>Issaquah</td><td>Complete</td></tr>
   <tr><td><a href="https://wg21.link/LWG2518">2518</a></td><td>[fund.ts.v2] Non-member swap for propagate_const should call member swap</td><td>Issaquah</td><td>Complete</td></tr>
   <tr><td><a href="https://wg21.link/LWG2519">2519</a></td><td>Iterator operator-= has gratuitous undefined behaviour</td><td>Issaquah</td><td>Complete</td></tr>
   <tr><td><a href="https://wg21.link/LWG2521">2521</a></td><td>[fund.ts.v2] weak_ptr's converting move constructor should be modified as well for array support</td><td>Issaquah</td><td></td></tr>
   <tr><td><a href="https://wg21.link/LWG2525">2525</a></td><td>[fund.ts.v2] get_memory_resource should be const and noexcept</td><td>Issaquah</td><td></td></tr>
   <tr><td><a href="https://wg21.link/LWG2527">2527</a></td><td>[fund.ts.v2] ALLOCATOR_OF for function::operator= has incorrect default</td><td>Issaquah</td><td></td></tr>
   <tr><td><a href="https://wg21.link/LWG2531">2531</a></td><td>future::get should explicitly state that the shared state is released</td><td>Issaquah</td><td></td></tr>
   <tr><td><a href="https://wg21.link/LWG2534">2534</a></td><td>Constrain rvalue stream operators</td><td>Issaquah</td><td></td></tr>