[libc++] Implement LWG 3158

[ldionne]

Summary

LWG 3158 marks the allocator_arg_t constructor of std::tuple as
conditionnally explicit based on whether the default constructors
of the tuple's members are explicitly default constructible.

[mclow.lists]

This looks fine to me; please update www/cxx2a_status.html when you commit.

[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.

[ldionne]

Rebase on top of latest D65161 update

libcxx/include/tuple

#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_implicit_default() {
-            return __all<__is_implicitly_default_constructible<_Args>::value...>::value;
-        }
+        struct __enable_implicit_default
+            : __all<__is_implicitly_default_constructible<_Args>::value...>
+        { };
 
         template <class ..._Args>
-        static constexpr bool __enable_explicit_default() {
-            return __all<is_default_constructible<_Args>::value...>::value
-                && !__enable_implicit_default<_Args...>();
-        }
+        struct __enable_explicit_default
+            : integral_constant<bool,
+                __all<is_default_constructible<_Args>::value...>::value &&
+                !__enable_implicit_default<_Args...>::value
+            >
+        { };
 
         template <class ..._Args>
         static constexpr bool __enable_explicit() {
             return
                 __tuple_constructible<
                     tuple<_Args...>,
                     typename __make_tuple_types<tuple,
                              sizeof...(_Args) < sizeof...(_Tp) ?
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, _EnableIf<
-        _CheckArgsConstructor<_Dummy>::template __enable_implicit_default<_Tp...>()
+        _CheckArgsConstructor<_Dummy>::template __enable_implicit_default<_Tp...>::value
     , 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...>()
+        _CheckArgsConstructor<_Dummy>::template __enable_explicit_default<_Tp...>::value
     , 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<
+    template <class _AllocArgT, class _Alloc, bool _Dummy = true, _EnableIf<
         _And<
             _IsSame<allocator_arg_t, _AllocArgT>,
-           __dependent_type<is_default_constructible<_Tp>, _Dummy>...
+            typename _CheckArgsConstructor<_Dummy>::template __enable_implicit_default<_Tp...>
        >::value
-      >
+      , void*> = nullptr
     >
     _LIBCPP_INLINE_VISIBILITY
     tuple(_AllocArgT, _Alloc const& __a)
       : __base_(allocator_arg_t(), __a,
                     __tuple_indices<>(), __tuple_types<>(),
                     typename __make_tuple_indices<sizeof...(_Tp), 0>::type(),
                     __tuple_types<_Tp...>()) {}
 
+    template <class _AllocArgT, class _Alloc, bool _Dummy = true, _EnableIf<
+        _And<
+            _IsSame<allocator_arg_t, _AllocArgT>,
+            typename _CheckArgsConstructor<_Dummy>::template __enable_explicit_default<_Tp...>
+       >::value
+      , void*> = nullptr
+    >
+    explicit _LIBCPP_INLINE_VISIBILITY
+    tuple(_AllocArgT, _Alloc const& __a)
+      : __base_(allocator_arg_t(), __a,
+                    __tuple_indices<>(), __tuple_types<>(),
+                    typename __make_tuple_indices<sizeof...(_Tp), 0>::type(),
+                    __tuple_types<_Tp...>()) {}
+
     template <bool _Dummy = true,
               typename enable_if
                       <
                          _CheckArgsConstructor<
                             _Dummy
                          >::template __enable_implicit<_Tp const&...>(),
                          bool
                       >::type = false

libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/alloc.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
+//
+//===----------------------------------------------------------------------===//
+
+// <tuple>
+
+// template <class... Types> class tuple;
+
+// template <class Alloc>
+//   explicit(see-below) tuple(allocator_arg_t, const Alloc& a);
+
+// UNSUPPORTED: c++98, c++03
+
+// Make sure we get the explicit-ness of the constructor right.
+// This is LWG 3158.
+
+#include <tuple>
+#include <memory>
+
+
+struct ExplicitDefault { explicit ExplicitDefault() { } };
+
+std::tuple<ExplicitDefault> explicit_default_test() {
+    return {std::allocator_arg, std::allocator<int>()}; // expected-error {{chosen constructor is explicit in copy-initialization}}
+}
+
+int main(int, char**) {
+    return 0;
+}

libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/alloc.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
 
 // <tuple>
 
 // template <class... Types> class tuple;
 
 // template <class Alloc>
-//   tuple(allocator_arg_t, const Alloc& a);
+//   explicit(see-below) tuple(allocator_arg_t, const Alloc& a);
 
 // NOTE: this constructor does not currently support tags derived from
 // allocator_arg_t because libc++ has to deduce the parameter as a template
 // argument. See PR27684 (https://bugs.llvm.org/show_bug.cgi?id=27684)
 
 #include <tuple>
 #include <cassert>
 

libcxx/www/upcoming_meeting.html

<div id="content">
   <h3>Library Working group Issues Status</h3>
   <table id="issues" border="1">
   <tr><th>Issue #</th><th>Issue Name</th><th>Meeting</th><th>Status</th></tr>
 
 <table border="1">
 <tr><th>Issue #</th><th>Has P/R</th><th>Issue Name</th><th>Meeting</th><th>Status</th></tr>
 <tr><td><a href="https://wg21.link/LWG2899">2899</a></td><td>Yes</td><td><tt>is_(nothrow_)move_constructible</tt> and <tt>tuple</tt>, <tt>optional</tt> and <tt>unique_ptr</tt></td><td>Cologne</td><td></td></tr>
 <tr><td><a href="https://wg21.link/LWG3055">3055</a></td><td>Yes</td><td><tt>path::operator+=(<i>single-character</i>)</tt> misspecified</td><td>Cologne</td><td></td></tr>
-<tr><td><a href="https://wg21.link/LWG3158">3158</a></td><td>Yes</td><td><tt>tuple(allocator_arg_t, const Alloc&amp;)</tt> should be conditionally explicit</td><td>Cologne</td><td></td></tr>
+<tr><td><a href="https://wg21.link/LWG3158">3158</a></td><td>Yes</td><td><tt>tuple(allocator_arg_t, const Alloc&amp;)</tt> should be conditionally explicit</td><td>Cologne</td><td>Complete</td></tr>
 <tr><td><a href="https://wg21.link/LWG3169">3169</a></td><td>Yes</td><td><tt>ranges</tt> permutation generators discard useful information</td><td>Cologne</td><td></td></tr>
 <tr><td><a href="https://wg21.link/LWG3183">3183</a></td><td>Yes</td><td>Normative permission to specialize Ranges variable templates</td><td>Cologne</td><td></td></tr>
 <tr><td><a href="https://wg21.link/LWG3184">3184</a></td><td>Yes</td><td>Inconsistencies in <tt>bind_front</tt> wording</td><td>Cologne</td><td></td></tr>
 <tr><td><a href="https://wg21.link/LWG3185">3185</a></td><td>Yes</td><td>Uses-allocator construction functions missing <tt>constexpr</tt> and <tt>noexcept</tt></td><td>Cologne</td><td></td></tr>
 <tr><td><a href="https://wg21.link/LWG3186">3186</a></td><td>Yes</td><td><tt>ranges</tt> removal, partition, and <tt>partial_sort_copy</tt> algorithms discard useful information</td><td>Cologne</td><td></td></tr>
 <tr><td><a href="https://wg21.link/LWG3187">3187</a></td><td>Yes</td><td><a href="https://wg21.link/p0591r4">P0591R4</a> reverted DR 2586 fixes to <tt>scoped_allocator_adaptor::construct()</tt></td><td>Cologne</td><td></td></tr>
 <tr><td><a href="https://wg21.link/LWG3191">3191</a></td><td>Yes</td><td><tt>std::ranges::shuffle</tt> synopsis does not match algorithm definition</td><td>Cologne</td><td></td></tr>
 <tr><td><a href="https://wg21.link/LWG3196">3196</a></td><td>Yes</td><td><tt>std::optional&lt;T&gt;</tt> is ill-formed is <tt>T</tt> is an array</td><td>Cologne</td><td>Complete</td></tr>

 <li>P1355R2 - Paper in the mailing; Marshall to handle as part of 553/556</li>
 </ul>
 
 
 <h3>Comments about the issues</h3>
 <ul>
 <li>2899 - Louis</li>
 <li>3055 - Eric?</li>
-<li>3158 - Louis</li>
 <li>3169 - We don't do ranges yet</li>
 <li>3183 - We don't do ranges yet</li>
 <li>3184 - We don't do bind_front yet</li>
 <li>3185 - Let's make sure that the patch up for review does this (D58879)</li>
 <li>3186 - We don't do ranges yet</li>
 <li>3187 - We need to make sure this is fixed when we land PMRs</li>
 <li>3191 - We don't do ranges yet</li>
 <li>3196 - We already do this</li>