[libcxx] Remove the "reduced-arity-initialization" extension from the uses-allocator constructors

[EricWF]

Summary

A default uses-allocator constructor has been added since that overload was previously provided by the extended constructor.

Since Clang does implicit conversion checking after substitution this constructor has to deduce the allocator_arg_t parameter so that it can prevent the evaluation of "is_default_constructible" if the first argument doesn't match. See http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#1391 for more information.

This patch fixes PR24779 (https://llvm.org/bugs/show_bug.cgi?id=24779)

include/tuple

public:
 
     template <bool _Dummy = true, class = typename enable_if<
         __all<__dependent_type<is_default_constructible<_Tp>, _Dummy>::value...>::value
     >::type>
     _LIBCPP_INLINE_VISIBILITY
     _LIBCPP_CONSTEXPR tuple()
         _NOEXCEPT_(__all<is_nothrow_default_constructible<_Tp>::value...>::value) {}
 
+    template <class _Alloc, bool _Dummy = true, class = typename enable_if<
+        __all<__dependent_type<is_default_constructible<_Tp>, _Dummy>::value...>::value
+    >::type>
+    _LIBCPP_INLINE_VISIBILITY
+    tuple(allocator_arg_t, _Alloc const& __a)
+      : base_(allocator_arg_t(), __a,
+                    __tuple_indices<>(), __tuple_types<>(),
+                    typename __make_tuple_indices<sizeof...(_Tp), 0>::type(),
+                    __tuple_types<_Tp...>()) {}
+
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
     explicit tuple(const _Tp& ... __t) _NOEXCEPT_((__all<is_nothrow_copy_constructible<_Tp>::value...>::value)) 
         : base_(typename __make_tuple_indices<sizeof...(_Tp)>::type(),
                 typename __make_tuple_types<tuple, sizeof...(_Tp)>::type(),
                 typename __make_tuple_indices<0>::type(),
                 typename __make_tuple_types<tuple, 0>::type(),
                 __t...
                ) {}
template <class ..._Up,
                     typename __make_tuple_types<tuple, sizeof...(_Up)>::type(),
                     typename __make_tuple_indices<sizeof...(_Tp), sizeof...(_Up)>::type(),
                     typename __make_tuple_types<tuple, sizeof...(_Tp), sizeof...(_Up)>::type(),
                     _VSTD::forward<_Up>(__u)...) {}
 
     template <class _Alloc, class ..._Up,
               class = typename enable_if
                       <
-                         sizeof...(_Up) <= sizeof...(_Tp) &&
-                         __tuple_convertible
-                         <
-                            tuple<_Up...>,
-                            typename __make_tuple_types<tuple,
-                                     sizeof...(_Up) < sizeof...(_Tp) ?
-                                        sizeof...(_Up) :
-                                        sizeof...(_Tp)>::type
-                         >::value &&
-                         __all_default_constructible<
-                            typename __make_tuple_types<tuple, sizeof...(_Tp),
-                                sizeof...(_Up) < sizeof...(_Tp) ?
-                                    sizeof...(_Up) :
-                                    sizeof...(_Tp)>::type
-                         >::value
+                         sizeof...(_Up) == sizeof...(_Tp) &&
+                         __tuple_convertible<tuple<_Up...>, tuple>::value
                       >::type
              >
         _LIBCPP_INLINE_VISIBILITY
         tuple(allocator_arg_t, const _Alloc& __a, _Up&&... __u)
             : base_(allocator_arg_t(), __a,
                     typename __make_tuple_indices<sizeof...(_Up)>::type(),
                     typename __make_tuple_types<tuple, sizeof...(_Up)>::type(),
                     typename __make_tuple_indices<sizeof...(_Tp), sizeof...(_Up)>::type(),

test/std/utilities/tuple/tuple.tuple/tuple.cnstr/alloc_UTypes.pass.cpp

 #include "../alloc_last.h"
 
 struct NoDefault { NoDefault() = delete; };
 
 // Make sure the _Up... constructor SFINAEs out when the types that
 // are not explicitly initialized are not all default constructible.
 // Otherwise, std::is_constructible would return true but instantiating
 // the constructor would fail.
-void test_default_constructible_extension_sfinae()
+void test_default_constructible_sfinae()
 {
     {
         typedef std::tuple<MoveOnly, NoDefault> Tuple;
 
         static_assert(!std::is_constructible<
             Tuple,
             std::allocator_arg_t, A1<int>, MoveOnly
         >::value, "");
int main()
         std::tuple<int, alloc_first, alloc_last> t(std::allocator_arg,
                                                    A1<int>(5), 1, 2, 3);
         assert(std::get<0>(t) == 1);
         assert(alloc_first::allocator_constructed);
         assert(std::get<1>(t) == alloc_first(2));
         assert(alloc_last::allocator_constructed);
         assert(std::get<2>(t) == alloc_last(3));
     }
-    // extensions
-    {
-        std::tuple<MoveOnly, MoveOnly, MoveOnly> t(std::allocator_arg, A1<int>(),
-                                                   0, 1);
-        assert(std::get<0>(t) == 0);
-        assert(std::get<1>(t) == 1);
-        assert(std::get<2>(t) == MoveOnly());
-    }
-    {
-        std::tuple<MoveOnly, MoveOnly, MoveOnly> t(std::allocator_arg, A1<int>(),
-                                                   0);
-        assert(std::get<0>(t) == 0);
-        assert(std::get<1>(t) == MoveOnly());
-        assert(std::get<2>(t) == MoveOnly());
-    }
     // Check that SFINAE is properly applied with the default reduced arity
     // constructor extensions.
-    test_default_constructible_extension_sfinae();
+    test_default_constructible_sfinae();
 }