diff --git a/libcxx/docs/ReleaseNotes.rst b/libcxx/docs/ReleaseNotes.rst
--- a/libcxx/docs/ReleaseNotes.rst
+++ b/libcxx/docs/ReleaseNotes.rst
@@ -43,4 +43,15 @@
 API Changes
 -----------
 
-- ...
+- There has been several changes in the tuple constructors provided by libc++.
+  Those changes were made as part of an effort to regularize libc++'s tuple
+  implementation, which contained several subtle bugs due to these extensions.
+  If you notice a build breakage when initializing a tuple, make sure you
+  properly initialize all the tuple elements - this is probably the culprit.
+
+  In particular, the extension allowing tuples to be constructed from fewer
+  elements than the number of elements in the tuple (in which case the remaining
+  elements would be default-constructed) has been removed. See https://godbolt.org/z/sqozjd.
+
+  Also, the extension allowing a tuple to be constructed from an array has been
+  removed. See https://godbolt.org/z/5esqbW.
diff --git a/libcxx/docs/UsingLibcxx.rst b/libcxx/docs/UsingLibcxx.rst
--- a/libcxx/docs/UsingLibcxx.rst
+++ b/libcxx/docs/UsingLibcxx.rst
@@ -165,33 +165,6 @@
   headers. The intended use case is for clients who wish to use the libc++
   headers without taking a dependency on the libc++ library itself.
 
-**_LIBCPP_ENABLE_TUPLE_IMPLICIT_REDUCED_ARITY_EXTENSION**:
-  This macro is used to re-enable an extension in `std::tuple` which allowed
-  it to be implicitly constructed from fewer initializers than contained
-  elements. Elements without an initializer are default constructed. For example:
-
-  .. code-block:: cpp
-
-    std::tuple<std::string, int, std::error_code> foo() {
-      return {"hello world", 42}; // default constructs error_code
-    }
-
-
-  Since libc++ 4.0 this extension has been disabled by default. This macro
-  may be defined to re-enable it in order to support existing code that depends
-  on the extension. New use of this extension should be discouraged.
-  See `PR 27374 <https://llvm.org/PR27374>`_ for more information.
-
-  Note: The "reduced-arity-initialization" extension is still offered but only
-  for explicit conversions. Example:
-
-  .. code-block:: cpp
-
-    auto foo() {
-      using Tup = std::tuple<std::string, int, std::error_code>;
-      return Tup{"hello world", 42}; // explicit constructor called. OK.
-    }
-
 **_LIBCPP_DISABLE_ADDITIONAL_DIAGNOSTICS**:
   This macro disables the additional diagnostics generated by libc++ using the
   `diagnose_if` attribute. These additional diagnostics include checks for:
diff --git a/libcxx/include/tuple b/libcxx/include/tuple
--- a/libcxx/include/tuple
+++ b/libcxx/include/tuple
@@ -449,165 +449,6 @@
 
     _BaseT __base_;
 
-#if defined(_LIBCPP_ENABLE_TUPLE_IMPLICIT_REDUCED_ARITY_EXTENSION)
-    static constexpr bool _EnableImplicitReducedArityExtension = true;
-#else
-    static constexpr bool _EnableImplicitReducedArityExtension = false;
-#endif
-
-    template <class ..._Args>
-    struct _PackExpandsToThisTuple : false_type {};
-
-    template <class _Arg>
-    struct _PackExpandsToThisTuple<_Arg>
-        : is_same<typename __uncvref<_Arg>::type, tuple> {};
-
-    template <bool _MaybeEnable, class _Dummy = void>
-    struct _CheckArgsConstructor : __check_tuple_constructor_fail {};
-
-    template <class _Dummy>
-    struct _CheckArgsConstructor<true, _Dummy>
-    {
-        template <int&...>
-        static constexpr bool __enable_implicit_default() {
-           return __all<__is_implicitly_default_constructible<_Tp>::value... >::value;
-        }
-
-        template <int&...>
-        static constexpr bool __enable_explicit_default() {
-            return
-                __all<is_default_constructible<_Tp>::value...>::value &&
-                !__enable_implicit_default< >();
-        }
-
-
-        template <class ..._Args>
-        static constexpr bool __enable_explicit() {
-            return
-                __tuple_constructible<
-                    tuple<_Args...>,
-                    typename __make_tuple_types<tuple,
-                             sizeof...(_Args) < sizeof...(_Tp) ?
-                                 sizeof...(_Args) :
-                                 sizeof...(_Tp)>::type
-                >::value &&
-                !__tuple_convertible<
-                    tuple<_Args...>,
-                    typename __make_tuple_types<tuple,
-                             sizeof...(_Args) < sizeof...(_Tp) ?
-                                 sizeof...(_Args) :
-                                 sizeof...(_Tp)>::type
-                >::value &&
-                __all_default_constructible<
-                    typename __make_tuple_types<tuple, sizeof...(_Tp),
-                             sizeof...(_Args) < sizeof...(_Tp) ?
-                                 sizeof...(_Args) :
-                                 sizeof...(_Tp)>::type
-                >::value;
-        }
-
-        template <class ..._Args>
-        static constexpr bool __enable_implicit() {
-            return
-               __tuple_constructible<
-                    tuple<_Args...>,
-                    typename __make_tuple_types<tuple,
-                             sizeof...(_Args) < sizeof...(_Tp) ?
-                                 sizeof...(_Args) :
-                                 sizeof...(_Tp)>::type
-                >::value &&
-                __tuple_convertible<
-                    tuple<_Args...>,
-                    typename __make_tuple_types<tuple,
-                             sizeof...(_Args) < sizeof...(_Tp) ?
-                                 sizeof...(_Args) :
-                                 sizeof...(_Tp)>::type
-                >::value &&
-                __all_default_constructible<
-                    typename __make_tuple_types<tuple, sizeof...(_Tp),
-                             sizeof...(_Args) < sizeof...(_Tp) ?
-                                 sizeof...(_Args) :
-                                 sizeof...(_Tp)>::type
-                >::value;
-        }
-    };
-
-    template <bool _MaybeEnable,
-              bool = sizeof...(_Tp) == 1,
-              class _Dummy = void>
-    struct _CheckTupleLikeConstructor : __check_tuple_constructor_fail {};
-
-    template <class _Dummy>
-    struct _CheckTupleLikeConstructor<true, false, _Dummy>
-    {
-        template <class _Tuple>
-        static constexpr bool __enable_implicit() {
-            return __tuple_constructible<_Tuple, tuple>::value
-                && __tuple_convertible<_Tuple, tuple>::value;
-        }
-
-        template <class _Tuple>
-        static constexpr bool __enable_explicit() {
-            return __tuple_constructible<_Tuple, tuple>::value
-               && !__tuple_convertible<_Tuple, tuple>::value;
-        }
-    };
-
-    template <class _Dummy>
-    struct _CheckTupleLikeConstructor<true, true, _Dummy>
-    {
-        // This trait is used to disable the tuple-like constructor when
-        // the UTypes... constructor should be selected instead.
-        // See LWG issue #2549.
-        template <class _Tuple>
-        using _PreferTupleLikeConstructor = _Or<
-            // Don't attempt the two checks below if the tuple we are given
-            // has the same type as this tuple.
-            _IsSame<__uncvref_t<_Tuple>, tuple>,
-            _Lazy<_And,
-                _Not<is_constructible<_Tp..., _Tuple>>,
-                _Not<is_convertible<_Tuple, _Tp...>>
-            >
-        >;
-
-        template <class _Tuple>
-        static constexpr bool __enable_implicit() {
-            return _And<
-                __tuple_constructible<_Tuple, tuple>,
-                __tuple_convertible<_Tuple, tuple>,
-                _PreferTupleLikeConstructor<_Tuple>
-            >::value;
-        }
-
-        template <class _Tuple>
-        static constexpr bool __enable_explicit() {
-            return _And<
-                __tuple_constructible<_Tuple, tuple>,
-                _PreferTupleLikeConstructor<_Tuple>,
-                _Not<__tuple_convertible<_Tuple, tuple>>
-            >::value;
-        }
-    };
-
-    template <class _Tuple, bool _DisableIfLValue>
-    using _EnableImplicitTupleLikeConstructor = _EnableIf<
-                         _CheckTupleLikeConstructor<
-                             __tuple_like_with_size<_Tuple, sizeof...(_Tp)>::value
-                             && !_PackExpandsToThisTuple<_Tuple>::value
-                             && (!is_lvalue_reference<_Tuple>::value || !_DisableIfLValue)
-                         >::template __enable_implicit<_Tuple>(),
-                         bool
-                      >;
-
-    template <class _Tuple, bool _DisableIfLValue>
-    using _EnableExplicitTupleLikeConstructor = _EnableIf<
-                         _CheckTupleLikeConstructor<
-                             __tuple_like_with_size<_Tuple, sizeof...(_Tp)>::value
-                             && !_PackExpandsToThisTuple<_Tuple>::value
-                             && (!is_lvalue_reference<_Tuple>::value || !_DisableIfLValue)
-                         >::template __enable_explicit<_Tuple>(),
-                         bool
-                      >;
     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
@@ -617,57 +458,69 @@
     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:
+    // [tuple.cnstr]
 
-    template <bool _Dummy = true, _EnableIf<
-        _CheckArgsConstructor<_Dummy>::__enable_implicit_default()
-    , void*> = nullptr>
+    // tuple() constructors (including allocator_arg_t variants)
+    template <template<class...> class _IsImpDefault = __is_implicitly_default_constructible, _EnableIf<
+        _And<
+            _IsImpDefault<_Tp>... // explicit check
+        >::value
+    , int> = 0>
     _LIBCPP_INLINE_VISIBILITY constexpr
     tuple()
-        _NOEXCEPT_(__all<is_nothrow_default_constructible<_Tp>::value...>::value) {}
-
-    template <bool _Dummy = true, _EnableIf<
-        _CheckArgsConstructor<_Dummy>::__enable_explicit_default()
-    , void*> = nullptr>
-    explicit _LIBCPP_INLINE_VISIBILITY constexpr
-    tuple()
-        _NOEXCEPT_(__all<is_nothrow_default_constructible<_Tp>::value...>::value) {}
+        _NOEXCEPT_(_And<is_nothrow_default_constructible<_Tp>...>::value)
+    { }
 
-    tuple(tuple const&) = default;
-    tuple(tuple&&) = default;
+    template <template<class...> class _IsImpDefault = __is_implicitly_default_constructible,
+              template<class...> class _IsDefault = is_default_constructible, _EnableIf<
+        _And<
+            _IsDefault<_Tp>...,
+            _Not<_Lazy<_And, _IsImpDefault<_Tp>...> > // explicit check
+        >::value
+    , int> = 0>
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR
+    explicit tuple()
+        _NOEXCEPT_(_And<is_nothrow_default_constructible<_Tp>...>::value)
+    { }
 
-    template <class _AllocArgT, class _Alloc, _EnableIf<
-             _CheckArgsConstructor<_IsSame<allocator_arg_t, _AllocArgT>::value >::__enable_implicit_default()
-      , void*> = nullptr
-    >
+    template <class _Alloc, template<class...> class _IsImpDefault = __is_implicitly_default_constructible, _EnableIf<
+        _And<
+            _IsImpDefault<_Tp>... // explicit check
+        >::value
+    , int> = 0>
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
-    tuple(_AllocArgT, _Alloc const& __a)
+    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...>()) {}
 
-    template <class _AllocArgT, class _Alloc, _EnableIf<
-             _CheckArgsConstructor<_IsSame<allocator_arg_t, _AllocArgT>::value>::__enable_explicit_default()
-      , void*> = nullptr
-    >
-    explicit _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
-    tuple(_AllocArgT, _Alloc const& __a)
+    template <class _Alloc,
+              template<class...> class _IsImpDefault = __is_implicitly_default_constructible,
+              template<class...> class _IsDefault = is_default_constructible, _EnableIf<
+        _And<
+            _IsDefault<_Tp>...,
+            _Not<_Lazy<_And, _IsImpDefault<_Tp>...> > // explicit check
+        >::value
+    , int> = 0>
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
+    explicit 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...>()) {}
 
-    template <bool _Dummy = true,
-              typename enable_if
-                      <
-                         _CheckArgsConstructor<
-                            _Dummy
-                         >::template __enable_implicit<_Tp const&...>(),
-                         bool
-                      >::type = false
-        >
+    // tuple(const T&...) constructors (including allocator_arg_t variants)
+    template <template<class...> class _And = _And, _EnableIf<
+        _And<
+            _BoolConstant<sizeof...(_Tp) >= 1>,
+            is_copy_constructible<_Tp>...,
+            is_convertible<const _Tp&, _Tp>... // explicit check
+        >::value
+    , int> = 0>
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
-    tuple(const _Tp& ... __t) _NOEXCEPT_((__all<is_nothrow_copy_constructible<_Tp>::value...>::value))
+    tuple(const _Tp& ... __t)
+        _NOEXCEPT_(_And<is_nothrow_copy_constructible<_Tp>...>::value)
         : __base_(typename __make_tuple_indices<sizeof...(_Tp)>::type(),
                 typename __make_tuple_types<tuple, sizeof...(_Tp)>::type(),
                 typename __make_tuple_indices<0>::type(),
@@ -675,17 +528,16 @@
                 __t...
                ) {}
 
-    template <bool _Dummy = true,
-              typename enable_if
-                      <
-                         _CheckArgsConstructor<
-                            _Dummy
-                         >::template __enable_explicit<_Tp const&...>(),
-                         bool
-                      >::type = false
-        >
+    template <template<class...> class _And = _And, _EnableIf<
+        _And<
+            _BoolConstant<sizeof...(_Tp) >= 1>,
+            is_copy_constructible<_Tp>...,
+            _Not<_Lazy<_And, is_convertible<const _Tp&, _Tp>...> > // explicit check
+        >::value
+    , int> = 0>
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
-    explicit tuple(const _Tp& ... __t) _NOEXCEPT_((__all<is_nothrow_copy_constructible<_Tp>::value...>::value))
+    explicit tuple(const _Tp& ... __t)
+        _NOEXCEPT_(_And<is_nothrow_copy_constructible<_Tp>...>::value)
         : __base_(typename __make_tuple_indices<sizeof...(_Tp)>::type(),
                 typename __make_tuple_types<tuple, sizeof...(_Tp)>::type(),
                 typename __make_tuple_indices<0>::type(),
@@ -693,17 +545,15 @@
                 __t...
                ) {}
 
-    template <class _Alloc, bool _Dummy = true,
-              typename enable_if
-                      <
-                         _CheckArgsConstructor<
-                            _Dummy
-                         >::template __enable_implicit<_Tp const&...>(),
-                         bool
-                      >::type = false
-        >
-      _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
-      tuple(allocator_arg_t, const _Alloc& __a, const _Tp& ... __t)
+    template <class _Alloc, template<class...> class _And = _And, _EnableIf<
+        _And<
+            _BoolConstant<sizeof...(_Tp) >= 1>,
+            is_copy_constructible<_Tp>...,
+            is_convertible<const _Tp&, _Tp>... // explicit check
+        >::value
+    , int> = 0>
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
+    tuple(allocator_arg_t, const _Alloc& __a, const _Tp& ... __t)
         : __base_(allocator_arg_t(), __a,
                 typename __make_tuple_indices<sizeof...(_Tp)>::type(),
                 typename __make_tuple_types<tuple, sizeof...(_Tp)>::type(),
@@ -712,18 +562,15 @@
                 __t...
                ) {}
 
-    template <class _Alloc, bool _Dummy = true,
-              typename enable_if
-                      <
-                         _CheckArgsConstructor<
-                            _Dummy
-                         >::template __enable_explicit<_Tp const&...>(),
-                         bool
-                      >::type = false
-        >
-      _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
-      explicit
-      tuple(allocator_arg_t, const _Alloc& __a, const _Tp& ... __t)
+    template <class _Alloc, template<class...> class _And = _And, _EnableIf<
+        _And<
+            _BoolConstant<sizeof...(_Tp) >= 1>,
+            is_copy_constructible<_Tp>...,
+            _Not<_Lazy<_And, is_convertible<const _Tp&, _Tp>...> > // explicit check
+        >::value
+    , int> = 0>
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
+    explicit tuple(allocator_arg_t, const _Alloc& __a, const _Tp& ... __t)
         : __base_(allocator_arg_t(), __a,
                 typename __make_tuple_indices<sizeof...(_Tp)>::type(),
                 typename __make_tuple_types<tuple, sizeof...(_Tp)>::type(),
@@ -732,158 +579,368 @@
                 __t...
                ) {}
 
-    template <class ..._Up,
-              bool _PackIsTuple = _PackExpandsToThisTuple<_Up...>::value,
-              typename enable_if
-                      <
-                         _CheckArgsConstructor<
-                             sizeof...(_Up) == sizeof...(_Tp)
-                             && !_PackIsTuple
-                         >::template __enable_implicit<_Up...>() ||
-                        _CheckArgsConstructor<
-                            _EnableImplicitReducedArityExtension
-                            && sizeof...(_Up) < sizeof...(_Tp)
-                            && !_PackIsTuple
-                         >::template __enable_implicit<_Up...>(),
-                         bool
-                      >::type = false
-             >
-        _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
-        tuple(_Up&&... __u)
-            _NOEXCEPT_((
-                is_nothrow_constructible<_BaseT,
-                    typename __make_tuple_indices<sizeof...(_Up)>::type,
-                    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,
-                    _Up...
-                >::value
-            ))
-            : __base_(typename __make_tuple_indices<sizeof...(_Up)>::type(),
+    // tuple(U&& ...) constructors (including allocator_arg_t variants)
+    template <class ..._Up> struct _IsThisTuple : false_type { };
+    template <class _Up> struct _IsThisTuple<_Up> : is_same<__uncvref_t<_Up>, tuple> { };
+
+    template <class ..._Up>
+    struct _EnableUTypesCtor : _And<
+        _BoolConstant<sizeof...(_Tp) >= 1>,
+        _Not<_IsThisTuple<_Up...> >, // extension to allow mis-behaved user constructors
+        is_constructible<_Tp, _Up>...
+    > { };
+
+    template <class ..._Up, _EnableIf<
+        _And<
+            _BoolConstant<sizeof...(_Up) == sizeof...(_Tp)>,
+            _EnableUTypesCtor<_Up...>,
+            is_convertible<_Up, _Tp>... // explicit check
+        >::value
+    , int> = 0>
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
+    tuple(_Up&&... __u)
+        _NOEXCEPT_((_And<is_nothrow_constructible<_Tp, _Up>...>::value))
+        : __base_(typename __make_tuple_indices<sizeof...(_Up)>::type(),
                     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 ..._Up,
-              typename enable_if
-                      <
-                         _CheckArgsConstructor<
-                             sizeof...(_Up) <= sizeof...(_Tp)
-                             && !_PackExpandsToThisTuple<_Up...>::value
-                         >::template __enable_explicit<_Up...>() ||
-                         _CheckArgsConstructor<
-                            !_EnableImplicitReducedArityExtension
-                            && sizeof...(_Up) < sizeof...(_Tp)
-                            && !_PackExpandsToThisTuple<_Up...>::value
-                         >::template __enable_implicit<_Up...>(),
-                         bool
-                      >::type = false
-             >
-        _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
-        explicit
-        tuple(_Up&&... __u)
-            _NOEXCEPT_((
-                is_nothrow_constructible<_BaseT,
-                    typename __make_tuple_indices<sizeof...(_Up)>::type,
-                    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,
-                    _Up...
-                >::value
-            ))
-            : __base_(typename __make_tuple_indices<sizeof...(_Up)>::type(),
+    template <class ..._Up, _EnableIf<
+        _And<
+            _BoolConstant<sizeof...(_Up) == sizeof...(_Tp)>,
+            _EnableUTypesCtor<_Up...>,
+            _Not<_Lazy<_And, is_convertible<_Up, _Tp>...> > // explicit check
+        >::value
+    , int> = 0>
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
+    explicit tuple(_Up&&... __u)
+        _NOEXCEPT_((_And<is_nothrow_constructible<_Tp, _Up>...>::value))
+        : __base_(typename __make_tuple_indices<sizeof...(_Up)>::type(),
                     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,
-              typename enable_if
-                      <
-                         _CheckArgsConstructor<
-                             sizeof...(_Up) == sizeof...(_Tp) &&
-                             !_PackExpandsToThisTuple<_Up...>::value
-                         >::template __enable_implicit<_Up...>(),
-                         bool
-                      >::type = false
-             >
-        _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
-        tuple(allocator_arg_t, const _Alloc& __a, _Up&&... __u)
-            : __base_(allocator_arg_t(), __a,
+    template <class _Alloc, class ..._Up, _EnableIf<
+        _And<
+            _BoolConstant<sizeof...(_Up) == sizeof...(_Tp)>,
+            _EnableUTypesCtor<_Up...>,
+            is_convertible<_Up, _Tp>... // explicit check
+        >::value
+    , int> = 0>
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
+    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(),
                     typename __make_tuple_types<tuple, sizeof...(_Tp), sizeof...(_Up)>::type(),
                     _VSTD::forward<_Up>(__u)...) {}
 
-    template <class _Alloc, class ..._Up,
-              typename enable_if
-                      <
-                         _CheckArgsConstructor<
-                             sizeof...(_Up) == sizeof...(_Tp) &&
-                             !_PackExpandsToThisTuple<_Up...>::value
-                         >::template __enable_explicit<_Up...>(),
-                         bool
-                      >::type = false
-             >
-        _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
-        explicit
-        tuple(allocator_arg_t, const _Alloc& __a, _Up&&... __u)
-            : __base_(allocator_arg_t(), __a,
+    template <class _Alloc, class ..._Up, _EnableIf<
+        _And<
+            _BoolConstant<sizeof...(_Up) == sizeof...(_Tp)>,
+            _EnableUTypesCtor<_Up...>,
+            _Not<_Lazy<_And, is_convertible<_Up, _Tp>...> > // explicit check
+        >::value
+    , int> = 0>
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
+    explicit 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(),
                     typename __make_tuple_types<tuple, sizeof...(_Tp), sizeof...(_Up)>::type(),
                     _VSTD::forward<_Up>(__u)...) {}
 
-    template <class _Tuple, _EnableImplicitTupleLikeConstructor<_Tuple, true> = false>
-        _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
-        tuple(_Tuple&& __t) _NOEXCEPT_((is_nothrow_constructible<_BaseT, _Tuple>::value))
-            : __base_(_VSTD::forward<_Tuple>(__t)) {}
+    // Copy and move constructors (including the allocator_arg_t variants)
+    tuple(const tuple&) = default;
+    tuple(tuple&&) = default;
 
-    template <class _Tuple, _EnableImplicitTupleLikeConstructor<const _Tuple&, false> = false>
-        _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
-        tuple(const _Tuple& __t) _NOEXCEPT_((is_nothrow_constructible<_BaseT, const _Tuple&>::value))
-            : __base_(__t) {}
-    template <class _Tuple, _EnableExplicitTupleLikeConstructor<_Tuple, true> = false>
-        _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
-        explicit
-        tuple(_Tuple&& __t) _NOEXCEPT_((is_nothrow_constructible<_BaseT, _Tuple>::value))
-            : __base_(_VSTD::forward<_Tuple>(__t)) {}
+    template <class _Alloc, template<class...> class _And = _And, _EnableIf<
+        _And<is_copy_constructible<_Tp>...>::value
+    , int> = 0>
+    tuple(allocator_arg_t, const _Alloc& __alloc, const tuple& __t)
+        : __base_(allocator_arg_t(), __alloc, __t)
+    { }
+
+    template <class _Alloc, template<class...> class _And = _And, _EnableIf<
+        _And<is_move_constructible<_Tp>...>::value
+    , int> = 0>
+    tuple(allocator_arg_t, const _Alloc& __alloc, tuple&& __t)
+        : __base_(allocator_arg_t(), __alloc, _VSTD::move(__t))
+    { }
+
+    // tuple(const tuple<U...>&) constructors (including allocator_arg_t variants)
+    template <class ..._Up>
+    struct _EnableCopyFromOtherTuple : _And<
+        _Or<
+            _BoolConstant<sizeof...(_Tp) != 1>,
+            // _Tp and _Up are 1-element packs - the pack expansions look
+            // weird to avoid tripping up the type traits in degenerate cases
+            _Lazy<_And,
+                _Not<is_same<_Tp, _Up> >...,
+                _Not<is_convertible<const tuple<_Up>&, _Tp> >...,
+                _Not<is_constructible<_Tp, const tuple<_Up>&> >...
+            >
+        >,
+        is_constructible<_Tp, const _Up&>...
+    > { };
 
-    template <class _Tuple, _EnableExplicitTupleLikeConstructor<const _Tuple&, false> = false>
-        _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
-        explicit
-        tuple(const _Tuple& __t) _NOEXCEPT_((is_nothrow_constructible<_BaseT, const _Tuple&>::value))
-            : __base_(__t) {}
+    template <class ..._Up, _EnableIf<
+        _And<
+            _BoolConstant<sizeof...(_Up) == sizeof...(_Tp)>,
+            _EnableCopyFromOtherTuple<_Up...>,
+            is_convertible<const _Up&, _Tp>... // explicit check
+        >::value
+    , int> = 0>
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
+    tuple(const tuple<_Up...>& __t)
+        _NOEXCEPT_((_And<is_nothrow_constructible<_Tp, const _Up&>...>::value))
+        : __base_(__t)
+    { }
 
-    template <class _Alloc, class _Tuple,
-              typename enable_if
-                      <
-                         _CheckTupleLikeConstructor<
-                             __tuple_like_with_size<_Tuple, sizeof...(_Tp)>::value
-                         >::template __enable_implicit<_Tuple>(),
-                         bool
-                      >::type = false
-             >
-        _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
-        tuple(allocator_arg_t, const _Alloc& __a, _Tuple&& __t)
-            : __base_(allocator_arg_t(), __a, _VSTD::forward<_Tuple>(__t)) {}
+    template <class ..._Up, _EnableIf<
+        _And<
+            _BoolConstant<sizeof...(_Up) == sizeof...(_Tp)>,
+            _EnableCopyFromOtherTuple<_Up...>,
+            _Not<_Lazy<_And, is_convertible<const _Up&, _Tp>...> > // explicit check
+        >::value
+    , int> = 0>
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
+    explicit tuple(const tuple<_Up...>& __t)
+        _NOEXCEPT_((_And<is_nothrow_constructible<_Tp, const _Up&>...>::value))
+        : __base_(__t)
+    { }
 
-    template <class _Alloc, class _Tuple,
-              typename enable_if
-                      <
-                         _CheckTupleLikeConstructor<
-                             __tuple_like_with_size<_Tuple, sizeof...(_Tp)>::value
-                         >::template __enable_explicit<_Tuple>(),
-                         bool
-                      >::type = false
-             >
-        _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
-        explicit
-        tuple(allocator_arg_t, const _Alloc& __a, _Tuple&& __t)
-            : __base_(allocator_arg_t(), __a, _VSTD::forward<_Tuple>(__t)) {}
+    template <class ..._Up, class _Alloc, _EnableIf<
+        _And<
+            _BoolConstant<sizeof...(_Up) == sizeof...(_Tp)>,
+            _EnableCopyFromOtherTuple<_Up...>,
+            is_convertible<const _Up&, _Tp>... // explicit check
+        >::value
+    , int> = 0>
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
+    tuple(allocator_arg_t, const _Alloc& __a, const tuple<_Up...>& __t)
+        : __base_(allocator_arg_t(), __a, __t)
+    { }
+
+    template <class ..._Up, class _Alloc, _EnableIf<
+        _And<
+            _BoolConstant<sizeof...(_Up) == sizeof...(_Tp)>,
+            _EnableCopyFromOtherTuple<_Up...>,
+            _Not<_Lazy<_And, is_convertible<const _Up&, _Tp>...> > // explicit check
+        >::value
+    , int> = 0>
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
+    explicit tuple(allocator_arg_t, const _Alloc& __a, const tuple<_Up...>& __t)
+        : __base_(allocator_arg_t(), __a, __t)
+    { }
+
+    // tuple(tuple<U...>&&) constructors (including allocator_arg_t variants)
+    template <class ..._Up>
+    struct _EnableMoveFromOtherTuple : _And<
+        _Or<
+            _BoolConstant<sizeof...(_Tp) != 1>,
+            // _Tp and _Up are 1-element packs - the pack expansions look
+            // weird to avoid tripping up the type traits in degenerate cases
+            _Lazy<_And,
+                _Not<is_same<_Tp, _Up> >...,
+                _Not<is_convertible<tuple<_Up>, _Tp> >...,
+                _Not<is_constructible<_Tp, tuple<_Up> > >...
+            >
+        >,
+        is_constructible<_Tp, _Up>...
+    > { };
+
+    template <class ..._Up, _EnableIf<
+        _And<
+            _BoolConstant<sizeof...(_Up) == sizeof...(_Tp)>,
+            _EnableMoveFromOtherTuple<_Up...>,
+            is_convertible<_Up, _Tp>... // explicit check
+        >::value
+    , int> = 0>
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
+    tuple(tuple<_Up...>&& __t)
+        _NOEXCEPT_((_And<is_nothrow_constructible<_Tp, _Up>...>::value))
+        : __base_(_VSTD::move(__t))
+    { }
+
+    template <class ..._Up, _EnableIf<
+        _And<
+            _BoolConstant<sizeof...(_Up) == sizeof...(_Tp)>,
+            _EnableMoveFromOtherTuple<_Up...>,
+            _Not<_Lazy<_And, is_convertible<_Up, _Tp>...> > // explicit check
+        >::value
+    , int> = 0>
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
+    explicit tuple(tuple<_Up...>&& __t)
+        _NOEXCEPT_((_And<is_nothrow_constructible<_Tp, _Up>...>::value))
+        : __base_(_VSTD::move(__t))
+    { }
+
+    template <class _Alloc, class ..._Up, _EnableIf<
+        _And<
+            _BoolConstant<sizeof...(_Up) == sizeof...(_Tp)>,
+            _EnableMoveFromOtherTuple<_Up...>,
+            is_convertible<_Up, _Tp>... // explicit check
+        >::value
+    , int> = 0>
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
+    tuple(allocator_arg_t, const _Alloc& __a, tuple<_Up...>&& __t)
+        : __base_(allocator_arg_t(), __a, _VSTD::move(__t))
+    { }
+
+    template <class _Alloc, class ..._Up, _EnableIf<
+        _And<
+            _BoolConstant<sizeof...(_Up) == sizeof...(_Tp)>,
+            _EnableMoveFromOtherTuple<_Up...>,
+            _Not<_Lazy<_And, is_convertible<_Up, _Tp>...> > // explicit check
+        >::value
+    , int> = 0>
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
+    explicit tuple(allocator_arg_t, const _Alloc& __a, tuple<_Up...>&& __t)
+        : __base_(allocator_arg_t(), __a, _VSTD::move(__t))
+    { }
+
+    // tuple(const pair<U1, U2>&) constructors (including allocator_arg_t variants)
+    template <class _Up1, class _Up2, class ..._DependentTp>
+    struct _EnableImplicitCopyFromPair : _And<
+        is_constructible<_FirstType<_DependentTp...>, const _Up1&>,
+        is_constructible<_SecondType<_DependentTp...>, const _Up2&>,
+        is_convertible<const _Up1&, _FirstType<_DependentTp...> >, // explicit check
+        is_convertible<const _Up2&, _SecondType<_DependentTp...> >
+    > { };
+
+    template <class _Up1, class _Up2, class ..._DependentTp>
+    struct _EnableExplicitCopyFromPair : _And<
+        is_constructible<_FirstType<_DependentTp...>, const _Up1&>,
+        is_constructible<_SecondType<_DependentTp...>, const _Up2&>,
+        _Not<is_convertible<const _Up1&, _FirstType<_DependentTp...> > >, // explicit check
+        _Not<is_convertible<const _Up2&, _SecondType<_DependentTp...> > >
+    > { };
+
+    template <class _Up1, class _Up2, template<class...> class _And = _And, _EnableIf<
+        _And<
+            _BoolConstant<sizeof...(_Tp) == 2>,
+            _EnableImplicitCopyFromPair<_Up1, _Up2, _Tp...>
+        >::value
+    , int> = 0>
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
+    tuple(const pair<_Up1, _Up2>& __p)
+        _NOEXCEPT_((_And<
+            is_nothrow_constructible<_FirstType<_Tp...>, const _Up1&>,
+            is_nothrow_constructible<_SecondType<_Tp...>, const _Up2&>
+        >::value))
+        : __base_(__p)
+    { }
+
+    template <class _Up1, class _Up2, template<class...> class _And = _And, _EnableIf<
+        _And<
+            _BoolConstant<sizeof...(_Tp) == 2>,
+            _EnableExplicitCopyFromPair<_Up1, _Up2, _Tp...>
+        >::value
+    , int> = 0>
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
+    explicit tuple(const pair<_Up1, _Up2>& __p)
+        _NOEXCEPT_((_And<
+            is_nothrow_constructible<_FirstType<_Tp...>, const _Up1&>,
+            is_nothrow_constructible<_SecondType<_Tp...>, const _Up2&>
+        >::value))
+        : __base_(__p)
+    { }
+
+    template <class _Alloc, class _Up1, class _Up2, template<class...> class _And = _And, _EnableIf<
+        _And<
+            _BoolConstant<sizeof...(_Tp) == 2>,
+            _EnableImplicitCopyFromPair<_Up1, _Up2, _Tp...>
+        >::value
+    , int> = 0>
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
+    tuple(allocator_arg_t, const _Alloc& __a, const pair<_Up1, _Up2>& __p)
+        : __base_(allocator_arg_t(), __a, __p)
+    { }
+
+    template <class _Alloc, class _Up1, class _Up2, template<class...> class _And = _And, _EnableIf<
+        _And<
+            _BoolConstant<sizeof...(_Tp) == 2>,
+            _EnableExplicitCopyFromPair<_Up1, _Up2, _Tp...>
+        >::value
+    , int> = 0>
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
+    explicit tuple(allocator_arg_t, const _Alloc& __a, const pair<_Up1, _Up2>& __p)
+        : __base_(allocator_arg_t(), __a, __p)
+    { }
+
+    // tuple(pair<U1, U2>&&) constructors (including allocator_arg_t variants)
+    template <class _Up1, class _Up2, class ..._DependentTp>
+    struct _EnableImplicitMoveFromPair : _And<
+        is_constructible<_FirstType<_DependentTp...>, _Up1>,
+        is_constructible<_SecondType<_DependentTp...>, _Up2>,
+        is_convertible<_Up1, _FirstType<_DependentTp...> >, // explicit check
+        is_convertible<_Up2, _SecondType<_DependentTp...> >
+    > { };
+
+    template <class _Up1, class _Up2, class ..._DependentTp>
+    struct _EnableExplicitMoveFromPair : _And<
+        is_constructible<_FirstType<_DependentTp...>, _Up1>,
+        is_constructible<_SecondType<_DependentTp...>, _Up2>,
+        _Not<is_convertible<_Up1, _FirstType<_DependentTp...> > >, // explicit check
+        _Not<is_convertible<_Up2, _SecondType<_DependentTp...> > >
+    > { };
+
+    template <class _Up1, class _Up2, template<class...> class _And = _And, _EnableIf<
+        _And<
+            _BoolConstant<sizeof...(_Tp) == 2>,
+            _EnableImplicitMoveFromPair<_Up1, _Up2, _Tp...>
+        >::value
+    , int> = 0>
+    _LIBCPP_INLINE_VISIBILITY
+    tuple(pair<_Up1, _Up2>&& __p)
+        _NOEXCEPT_((_And<
+            is_nothrow_constructible<_FirstType<_Tp...>, _Up1>,
+            is_nothrow_constructible<_SecondType<_Tp...>, _Up2>
+        >::value))
+        : __base_(_VSTD::move(__p))
+    { }
+
+    template <class _Up1, class _Up2, template<class...> class _And = _And, _EnableIf<
+        _And<
+            _BoolConstant<sizeof...(_Tp) == 2>,
+            _EnableExplicitMoveFromPair<_Up1, _Up2, _Tp...>
+        >::value
+    , int> = 0>
+    _LIBCPP_INLINE_VISIBILITY
+    explicit tuple(pair<_Up1, _Up2>&& __p)
+        _NOEXCEPT_((_And<
+            is_nothrow_constructible<_FirstType<_Tp...>, _Up1>,
+            is_nothrow_constructible<_SecondType<_Tp...>, _Up2>
+        >::value))
+        : __base_(_VSTD::move(__p))
+    { }
+
+    template <class _Alloc, class _Up1, class _Up2, template<class...> class _And = _And, _EnableIf<
+        _And<
+            _BoolConstant<sizeof...(_Tp) == 2>,
+            _EnableImplicitMoveFromPair<_Up1, _Up2, _Tp...>
+        >::value
+    , int> = 0>
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
+    tuple(allocator_arg_t, const _Alloc& __a, pair<_Up1, _Up2>&& __p)
+        : __base_(allocator_arg_t(), __a, _VSTD::move(__p))
+    { }
+
+    template <class _Alloc, class _Up1, class _Up2, template<class...> class _And = _And, _EnableIf<
+        _And<
+            _BoolConstant<sizeof...(_Tp) == 2>,
+            _EnableExplicitMoveFromPair<_Up1, _Up2, _Tp...>
+        >::value
+    , int> = 0>
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
+    explicit tuple(allocator_arg_t, const _Alloc& __a, pair<_Up1, _Up2>&& __p)
+        : __base_(allocator_arg_t(), __a, _VSTD::move(__p))
+    { }
 
     // [tuple.assign]
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
diff --git a/libcxx/include/type_traits b/libcxx/include/type_traits
--- a/libcxx/include/type_traits
+++ b/libcxx/include/type_traits
@@ -3206,18 +3206,18 @@
 template <class _Tp>
 void __test_implicit_default_constructible(_Tp);
 
-template <class _Tp, class = void, bool = is_default_constructible<_Tp>::value>
+template <class _Tp, class = void, class = typename is_default_constructible<_Tp>::type>
 struct __is_implicitly_default_constructible
     : false_type
 { };
 
 template <class _Tp>
-struct __is_implicitly_default_constructible<_Tp, decltype(__test_implicit_default_constructible<_Tp const&>({})), true>
+struct __is_implicitly_default_constructible<_Tp, decltype(__test_implicit_default_constructible<_Tp const&>({})), true_type>
     : true_type
 { };
 
 template <class _Tp>
-struct __is_implicitly_default_constructible<_Tp, decltype(__test_implicit_default_constructible<_Tp const&>({})), false>
+struct __is_implicitly_default_constructible<_Tp, decltype(__test_implicit_default_constructible<_Tp const&>({})), false_type>
     : false_type
 { };
 #endif // !C++03
diff --git a/libcxx/test/libcxx/utilities/tuple/tuple.tuple/tuple.cnstr/disable_reduced_arity_initialization_extension.pass.cpp b/libcxx/test/libcxx/utilities/tuple/tuple.tuple/tuple.cnstr/disable_reduced_arity_initialization_extension.pass.cpp
deleted file mode 100644
--- a/libcxx/test/libcxx/utilities/tuple/tuple.tuple/tuple.cnstr/disable_reduced_arity_initialization_extension.pass.cpp
+++ /dev/null
@@ -1,109 +0,0 @@
-//===----------------------------------------------------------------------===//
-//
-// 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... UTypes>
-//   explicit tuple(UTypes&&... u);
-
-// UNSUPPORTED: c++03
-
-#include <tuple>
-#include <cassert>
-#include <type_traits>
-#include <string>
-#include <system_error>
-
-#include "test_macros.h"
-#include "test_convertible.h"
-#include "MoveOnly.h"
-
-#if defined(_LIBCPP_ENABLE_TUPLE_IMPLICIT_REDUCED_ARITY_EXTENSION)
-#error This macro should not be defined by default
-#endif
-
-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()
-{
-    typedef MoveOnly MO;
-    typedef NoDefault ND;
-    {
-        typedef std::tuple<MO, ND> Tuple;
-        static_assert(!std::is_constructible<Tuple, MO>::value, "");
-        static_assert(std::is_constructible<Tuple, MO, ND>::value, "");
-        static_assert(test_convertible<Tuple, MO, ND>(), "");
-    }
-    {
-        typedef std::tuple<MO, MO, ND> Tuple;
-        static_assert(!std::is_constructible<Tuple, MO, MO>::value, "");
-        static_assert(std::is_constructible<Tuple, MO, MO, ND>::value, "");
-        static_assert(test_convertible<Tuple, MO, MO, ND>(), "");
-    }
-    {
-        // Same idea as above but with a nested tuple type.
-        typedef std::tuple<MO, ND> Tuple;
-        typedef std::tuple<MO, Tuple, MO, MO> NestedTuple;
-
-        static_assert(!std::is_constructible<
-            NestedTuple, MO, MO, MO, MO>::value, "");
-        static_assert(std::is_constructible<
-            NestedTuple, MO, Tuple, MO, MO>::value, "");
-    }
-}
-
-using ExplicitTup = std::tuple<std::string, int, std::error_code>;
-ExplicitTup doc_example() {
-      return ExplicitTup{"hello world", 42}; // explicit constructor called. OK.
-}
-
-// Test that the example given in UsingLibcxx.rst actually works.
-void test_example_from_docs() {
-  auto tup = doc_example();
-  assert(std::get<0>(tup) == "hello world");
-  assert(std::get<1>(tup) == 42);
-  assert(std::get<2>(tup) == std::error_code{});
-}
-
-int main(int, char**)
-{
-    {
-        using E = MoveOnly;
-        using Tup = std::tuple<E, E, E>;
-        // Test that the reduced arity initialization extension is only
-        // allowed on the explicit constructor.
-        static_assert(test_convertible<Tup, E, E, E>(), "");
-
-        Tup t(E(0), E(1));
-        static_assert(std::is_constructible<Tup, E, E>::value, "");
-        static_assert(!test_convertible<Tup, E, E>(), "");
-        assert(std::get<0>(t) == E(0));
-        assert(std::get<1>(t) == E(1));
-        assert(std::get<2>(t) == E());
-
-        Tup t2(E(0));
-        static_assert(std::is_constructible<Tup, E>::value, "");
-        static_assert(!test_convertible<Tup, E>(), "");
-        assert(std::get<0>(t2) == E(0));
-        assert(std::get<1>(t2) == E());
-        assert(std::get<2>(t2) == E());
-    }
-    // Check that SFINAE is properly applied with the default reduced arity
-    // constructor extensions.
-    test_default_constructible_extension_sfinae();
-    test_example_from_docs();
-
-  return 0;
-}
diff --git a/libcxx/test/libcxx/utilities/tuple/tuple.tuple/tuple.cnstr/enable_reduced_arity_initialization_extension.pass.cpp b/libcxx/test/libcxx/utilities/tuple/tuple.tuple/tuple.cnstr/enable_reduced_arity_initialization_extension.pass.cpp
deleted file mode 100644
--- a/libcxx/test/libcxx/utilities/tuple/tuple.tuple/tuple.cnstr/enable_reduced_arity_initialization_extension.pass.cpp
+++ /dev/null
@@ -1,118 +0,0 @@
-//===----------------------------------------------------------------------===//
-//
-// 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... UTypes>
-//   explicit tuple(UTypes&&... u);
-
-// UNSUPPORTED: c++03
-
-// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_TUPLE_IMPLICIT_REDUCED_ARITY_EXTENSION
-
-#include <tuple>
-#include <cassert>
-#include <type_traits>
-#include <string>
-#include <system_error>
-
-#include "test_macros.h"
-#include "test_convertible.h"
-#include "MoveOnly.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()
-{
-    typedef MoveOnly MO;
-    typedef NoDefault ND;
-    {
-        typedef std::tuple<MO, ND> Tuple;
-        static_assert(!std::is_constructible<Tuple, MO>::value, "");
-        static_assert(std::is_constructible<Tuple, MO, ND>::value, "");
-        static_assert(test_convertible<Tuple, MO, ND>(), "");
-    }
-    {
-        typedef std::tuple<MO, MO, ND> Tuple;
-        static_assert(!std::is_constructible<Tuple, MO, MO>::value, "");
-        static_assert(std::is_constructible<Tuple, MO, MO, ND>::value, "");
-        static_assert(test_convertible<Tuple, MO, MO, ND>(), "");
-    }
-    {
-        // Same idea as above but with a nested tuple type.
-        typedef std::tuple<MO, ND> Tuple;
-        typedef std::tuple<MO, Tuple, MO, MO> NestedTuple;
-
-        static_assert(!std::is_constructible<
-            NestedTuple, MO, MO, MO, MO>::value, "");
-        static_assert(std::is_constructible<
-            NestedTuple, MO, Tuple, MO, MO>::value, "");
-    }
-    {
-        typedef std::tuple<MO, int> Tuple;
-        typedef std::tuple<MO, Tuple, MO, MO> NestedTuple;
-
-        static_assert(std::is_constructible<
-            NestedTuple, MO, MO, MO, MO>::value, "");
-        static_assert(test_convertible<
-            NestedTuple, MO, MO, MO, MO>(), "");
-
-        static_assert(std::is_constructible<
-            NestedTuple, MO, Tuple, MO, MO>::value, "");
-        static_assert(test_convertible<
-            NestedTuple, MO, Tuple, MO, MO>(), "");
-    }
-}
-
-std::tuple<std::string, int, std::error_code> doc_example() {
-      return {"hello world", 42};
-}
-
-// Test that the example given in UsingLibcxx.rst actually works.
-void test_example_from_docs() {
-  auto tup = doc_example();
-  assert(std::get<0>(tup) == "hello world");
-  assert(std::get<1>(tup) == 42);
-  assert(std::get<2>(tup) == std::error_code{});
-}
-
-int main(int, char**)
-{
-
-    {
-        using E = MoveOnly;
-        using Tup = std::tuple<E, E, E>;
-        static_assert(test_convertible<Tup, E, E, E>(), "");
-
-        Tup t = {E(0), E(1)};
-        static_assert(test_convertible<Tup, E, E>(), "");
-        assert(std::get<0>(t) == E(0));
-        assert(std::get<1>(t) == E(1));
-        assert(std::get<2>(t) == E());
-
-        Tup t2 = {E(0)};
-        static_assert(test_convertible<Tup, E>(), "");
-        assert(std::get<0>(t2) == E(0));
-        assert(std::get<1>(t2) == E());
-        assert(std::get<2>(t2) == E());
-    }
-    // Check that SFINAE is properly applied with the default reduced arity
-    // constructor extensions.
-    test_default_constructible_extension_sfinae();
-    test_example_from_docs();
-
-  return 0;
-}
diff --git a/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/PR20855_tuple_ref_binding_diagnostics.pass.cpp b/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/PR20855_tuple_ref_binding_diagnostics.pass.cpp
--- a/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/PR20855_tuple_ref_binding_diagnostics.pass.cpp
+++ b/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/PR20855_tuple_ref_binding_diagnostics.pass.cpp
@@ -11,7 +11,7 @@
 
 // <tuple>
 
-// See llvm.org/PR20855
+// See https://llvm.org/PR20855.
 
 #include <functional>
 #include <tuple>
diff --git a/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/PR22806_constrain_tuple_like_ctor.pass.cpp b/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/PR22806_constrain_tuple_like_ctor.pass.cpp
--- a/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/PR22806_constrain_tuple_like_ctor.pass.cpp
+++ b/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/PR22806_constrain_tuple_like_ctor.pass.cpp
@@ -12,13 +12,10 @@
 
 // template <class... Types> class tuple;
 
-// template <class TupleLike>
-//   tuple(TupleLike&&);
-// template <class Alloc, class TupleLike>
-//   tuple(std::allocator_arg_t, Alloc const&, TupleLike&&);
-
 // Check that the tuple-like ctors are properly disabled when the UTypes...
-// constructor should be selected. See PR22806.
+// constructor should be selected.
+//
+// See https://llvm.org/PR22806.
 
 #include <tuple>
 #include <memory>
diff --git a/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/PR23256_constrain_UTypes_ctor.pass.cpp b/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/PR23256_constrain_UTypes_ctor.pass.cpp
--- a/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/PR23256_constrain_UTypes_ctor.pass.cpp
+++ b/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/PR23256_constrain_UTypes_ctor.pass.cpp
@@ -16,9 +16,11 @@
 //    EXPLICIT(...) tuple(UTypes&&...)
 
 // Check that the UTypes... ctor is properly disabled before evaluating any
-// SFINAE when the tuple-like copy/move ctor should *clearly* be selected
+// SFINAE when the copy/move ctor from another tuple should clearly be selected
 // instead. This happens 'sizeof...(UTypes) == 1' and the first element of
-// 'UTypes...' is an instance of the tuple itself. See PR23256.
+// 'UTypes...' is an instance of the tuple itself.
+//
+// See https://llvm.org/PR23256.
 
 #include <tuple>
 #include <memory>
diff --git a/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/PR27684_contains_ref_to_incomplete_type.pass.cpp b/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/PR27684_contains_ref_to_incomplete_type.pass.cpp
--- a/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/PR27684_contains_ref_to_incomplete_type.pass.cpp
+++ b/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/PR27684_contains_ref_to_incomplete_type.pass.cpp
@@ -14,11 +14,7 @@
 
 // template <class Alloc> tuple(allocator_arg_t, Alloc const&)
 
-// Libc++ has to deduce the 'allocator_arg_t' parameter for this constructor
-// as 'AllocArgT'. Previously libc++ has tried to support tags derived from
-// 'allocator_arg_t' by using 'is_base_of<AllocArgT, allocator_arg_t>'.
-// However this breaks whenever a 2-tuple contains a reference to an incomplete
-// type as its first parameter. See PR27684.
+// See https://llvm.org/PR27684.
 
 #include <tuple>
 #include <cassert>
diff --git a/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/PR31384.pass.cpp b/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/PR31384.pass.cpp
--- a/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/PR31384.pass.cpp
+++ b/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/PR31384.pass.cpp
@@ -9,11 +9,8 @@
 
 // UNSUPPORTED: c++03
 
-// <tuple>
+// See https://llvm.org/PR31384.
 
-// template <class TupleLike> tuple(TupleLike&&); // libc++ extension
-
-// See llvm.org/PR31384
 #include <tuple>
 #include <cassert>
 
@@ -50,42 +47,51 @@
   {
     std::tuple<Explicit> foo = Derived<int>{42}; ((void)foo);
     assert(count == 1);
-    std::tuple<Explicit> bar(Derived<int>{42}); ((void)bar);
+    Derived<int> d{42};
+    std::tuple<Explicit> bar(std::move(d)); ((void)bar);
+#if TEST_STD_VER < 17
+    assert(count == 1);
+#else
     assert(count == 2);
+#endif
   }
   count = 0;
   {
     std::tuple<Implicit> foo = Derived<int>{42}; ((void)foo);
     assert(count == 1);
-    std::tuple<Implicit> bar(Derived<int>{42}); ((void)bar);
+    Derived<int> d{42};
+    std::tuple<Implicit> bar(std::move(d)); ((void)bar);
+#if TEST_STD_VER < 17
+    assert(count == 1);
+#else
     assert(count == 2);
+#endif
   }
   count = 0;
   {
-    static_assert(!std::is_convertible<
-        ExplicitDerived<int>, std::tuple<Explicit>>::value, "");
-    std::tuple<Explicit> bar(ExplicitDerived<int>{42}); ((void)bar);
+    static_assert(!std::is_convertible<ExplicitDerived<int>, std::tuple<Explicit>>::value, "");
+    ExplicitDerived<int> d{42};
+    std::tuple<Explicit> bar(std::move(d)); ((void)bar);
+#if TEST_STD_VER < 17
+    assert(count == 0);
+#else
     assert(count == 1);
+#endif
   }
   count = 0;
   {
-    // FIXME: Libc++ incorrectly rejects this code.
-#ifndef _LIBCPP_VERSION
     std::tuple<Implicit> foo = ExplicitDerived<int>{42}; ((void)foo);
-    static_assert(std::is_convertible<
-        ExplicitDerived<int>, std::tuple<Implicit>>::value,
-        "correct STLs accept this");
-#else
-    static_assert(!std::is_convertible<
-        ExplicitDerived<int>, std::tuple<Implicit>>::value,
-        "libc++ incorrectly rejects this");
-#endif
+    static_assert(std::is_convertible<ExplicitDerived<int>, std::tuple<Implicit>>::value, "");
     assert(count == 0);
-    std::tuple<Implicit> bar(ExplicitDerived<int>{42}); ((void)bar);
+    ExplicitDerived<int> d{42};
+    std::tuple<Implicit> bar(std::move(d)); ((void)bar);
+#if TEST_STD_VER < 17
+    assert(count == 0);
+#else
     assert(count == 1);
+#endif
   }
   count = 0;
 
-
   return 0;
 }
diff --git a/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/UTypes.pass.cpp b/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/UTypes.pass.cpp
--- a/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/UTypes.pass.cpp
+++ b/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/UTypes.pass.cpp
@@ -36,11 +36,9 @@
 
 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()
+// Make sure the _Up... constructor SFINAEs out when there are fewer
+// constructor arguments than tuple elements.
+void test_sfinae_missing_elements()
 {
     {
         typedef std::tuple<MoveOnly, NoDefault> Tuple;
@@ -83,23 +81,6 @@
             MoveOnly, Tuple, MoveOnly, MoveOnly
         >::value, "");
     }
-    // testing extensions
-#ifdef _LIBCPP_VERSION
-    {
-        typedef std::tuple<MoveOnly, int> Tuple;
-        typedef std::tuple<MoveOnly, Tuple, MoveOnly, MoveOnly> NestedTuple;
-
-        static_assert(std::is_constructible<
-            NestedTuple,
-            MoveOnly, MoveOnly, MoveOnly, MoveOnly
-        >::value, "");
-
-        static_assert(std::is_constructible<
-            NestedTuple,
-            MoveOnly, Tuple, MoveOnly, MoveOnly
-        >::value, "");
-    }
-#endif
 }
 
 int main(int, char**)
@@ -121,28 +102,6 @@
         assert(std::get<1>(t) == 1);
         assert(std::get<2>(t) == 2);
     }
-    // extensions
-#ifdef _LIBCPP_VERSION
-    {
-        using E = MoveOnly;
-        using Tup = std::tuple<E, E, E>;
-        // Test that the reduced arity initialization extension is only
-        // allowed on the explicit constructor.
-        static_assert(test_convertible<Tup, E, E, E>(), "");
-
-        Tup t(E(0), E(1));
-        static_assert(!test_convertible<Tup, E, E>(), "");
-        assert(std::get<0>(t) == E(0));
-        assert(std::get<1>(t) == E(1));
-        assert(std::get<2>(t) == E());
-
-        Tup t2(E(0));
-        static_assert(!test_convertible<Tup, E>(), "");
-        assert(std::get<0>(t2) == E(0));
-        assert(std::get<1>(t2) == E());
-        assert(std::get<2>(t2) == E());
-    }
-#endif
 #if TEST_STD_VER > 11
     {
         constexpr std::tuple<Empty> t0{Empty()};
@@ -153,9 +112,8 @@
         static_assert(std::get<0>(t).id_ == 3, "");
     }
 #endif
-    // Check that SFINAE is properly applied with the default reduced arity
-    // constructor extensions.
-    test_default_constructible_extension_sfinae();
+
+    test_sfinae_missing_elements();
 
   return 0;
 }
diff --git a/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/alloc.pass.cpp b/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/alloc.pass.cpp
--- a/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/alloc.pass.cpp
+++ b/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/alloc.pass.cpp
@@ -15,10 +15,6 @@
 // template <class Alloc>
 //   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://llvm.org/PR27684)
-
 #include <tuple>
 #include <cassert>
 
@@ -94,6 +90,14 @@
         assert(!alloc_last::allocator_constructed);
         assert(std::get<2>(t) == alloc_last());
     }
+    {
+        // Test that we can use a tag derived from allocator_arg_t
+        struct DerivedFromAllocatorArgT : std::allocator_arg_t { };
+        DerivedFromAllocatorArgT derived;
+        std::tuple<> t1(derived, A1<int>());
+        std::tuple<int> t2(derived, A1<int>());
+        std::tuple<int, int> t3(derived, A1<int>());
+    }
     {
         // Test that the uses-allocator default constructor does not evaluate
         // its SFINAE when it otherwise shouldn't be selected. Do this by
diff --git a/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/alloc_const_Types.pass.cpp b/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/alloc_const_Types.pass.cpp
--- a/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/alloc_const_Types.pass.cpp
+++ b/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/alloc_const_Types.pass.cpp
@@ -95,6 +95,14 @@
         assert(!alloc_last::allocator_constructed);
         assert(std::get<2>(t) == alloc_last(3));
     }
+    {
+        // Test that we can use a tag derived from allocator_arg_t
+        struct DerivedFromAllocatorArgT : std::allocator_arg_t { };
+        DerivedFromAllocatorArgT derived;
+        std::tuple<> t1(derived, A1<int>());
+        std::tuple<int> t2(derived, A1<int>(), 1);
+        std::tuple<int, int> t3(derived, A1<int>(), 1, 2);
+    }
 
-  return 0;
+    return 0;
 }
diff --git a/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/alloc_const_pair.pass.cpp b/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/alloc_const_pair.pass.cpp
--- a/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/alloc_const_pair.pass.cpp
+++ b/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/alloc_const_pair.pass.cpp
@@ -56,6 +56,13 @@
         assert(std::get<0>(t1) == 2);
         assert(std::get<1>(t1) == 3);
     }
+    {
+        // Test that we can use a tag derived from allocator_arg_t
+        struct DerivedFromAllocatorArgT : std::allocator_arg_t { };
+        DerivedFromAllocatorArgT derived;
+        std::pair<int, int> p(1, 2);
+        std::tuple<int, int> t(derived, A1<int>(), p);
+    }
 
-  return 0;
+    return 0;
 }
diff --git a/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/alloc_convert_copy.pass.cpp b/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/alloc_convert_copy.pass.cpp
--- a/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/alloc_convert_copy.pass.cpp
+++ b/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/alloc_convert_copy.pass.cpp
@@ -87,6 +87,13 @@
         std::tuple<Implicit> t2 = {std::allocator_arg, std::allocator<int>{}, t1};
         assert(std::get<0>(t2).value == 42);
     }
+    {
+        // Test that we can use a tag derived from allocator_arg_t
+        struct DerivedFromAllocatorArgT : std::allocator_arg_t { };
+        DerivedFromAllocatorArgT derived;
+        std::tuple<long> from(3l);
+        std::tuple<long long> t0(derived, A1<int>(), from);
+    }
 
-  return 0;
+    return 0;
 }
diff --git a/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/alloc_convert_move.pass.cpp b/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/alloc_convert_move.pass.cpp
--- a/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/alloc_convert_move.pass.cpp
+++ b/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/alloc_convert_move.pass.cpp
@@ -101,6 +101,13 @@
         std::tuple<Implicit> t2 = {std::allocator_arg, std::allocator<int>{}, std::move(t1)};
         assert(std::get<0>(t2).value == 42);
     }
+    {
+        // Test that we can use a tag derived from allocator_arg_t
+        struct DerivedFromAllocatorArgT : std::allocator_arg_t { };
+        DerivedFromAllocatorArgT derived;
+        std::tuple<long> from(3l);
+        std::tuple<long long> t0(derived, A1<int>(), std::move(from));
+    }
 
-  return 0;
+    return 0;
 }
diff --git a/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/alloc_copy.pass.cpp b/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/alloc_copy.pass.cpp
--- a/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/alloc_copy.pass.cpp
+++ b/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/alloc_copy.pass.cpp
@@ -52,8 +52,6 @@
         assert(alloc_last::allocator_constructed);
         assert(std::get<0>(t) == 2);
     }
-// testing extensions
-#ifdef _LIBCPP_VERSION
     {
         typedef std::tuple<alloc_first, alloc_last> T;
         T t0(2, 3);
@@ -77,7 +75,13 @@
         assert(std::get<1>(t) == 2);
         assert(std::get<2>(t) == 3);
     }
-#endif
+    {
+        // Test that we can use a tag derived from allocator_arg_t
+        struct DerivedFromAllocatorArgT : std::allocator_arg_t { };
+        DerivedFromAllocatorArgT derived;
+        std::tuple<int> from(3);
+        std::tuple<int> t0(derived, A1<int>(), from);
+    }
 
-  return 0;
+    return 0;
 }
diff --git a/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/alloc_move.pass.cpp b/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/alloc_move.pass.cpp
--- a/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/alloc_move.pass.cpp
+++ b/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/alloc_move.pass.cpp
@@ -53,8 +53,6 @@
         assert(alloc_last::allocator_constructed);
         assert(std::get<0>(t) == 1);
     }
-// testing extensions
-#ifdef _LIBCPP_VERSION
     {
         typedef std::tuple<MoveOnly, alloc_first> T;
         T t0(0 ,1);
@@ -76,7 +74,13 @@
         assert(std::get<1>(t) == 2);
         assert(std::get<2>(t) == 3);
     }
-#endif
+    {
+        // Test that we can use a tag derived from allocator_arg_t
+        struct DerivedFromAllocatorArgT : std::allocator_arg_t { };
+        DerivedFromAllocatorArgT derived;
+        std::tuple<int> from(3);
+        std::tuple<int> t0(derived, A1<int>(), std::move(from));
+    }
 
-  return 0;
+    return 0;
 }
diff --git a/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/alloc_move_pair.pass.cpp b/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/alloc_move_pair.pass.cpp
--- a/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/alloc_move_pair.pass.cpp
+++ b/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/alloc_move_pair.pass.cpp
@@ -52,6 +52,13 @@
         assert(std::get<0>(t1) == 2);
         assert(std::get<1>(t1)->id_ == 3);
     }
+    {
+        // Test that we can use a tag derived from allocator_arg_t
+        struct DerivedFromAllocatorArgT : std::allocator_arg_t { };
+        DerivedFromAllocatorArgT derived;
+        std::pair<int, int> from(1, 2);
+        std::tuple<int, int> t0(derived, A1<int>(), std::move(from));
+    }
 
-  return 0;
+    return 0;
 }
diff --git a/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/const_Types.pass.cpp b/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/const_Types.pass.cpp
--- a/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/const_Types.pass.cpp
+++ b/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/const_Types.pass.cpp
@@ -137,28 +137,6 @@
         assert(std::get<2>(t) == 2);
         assert(std::get<3>(t) == 3);
     }
-// extensions
-#ifdef _LIBCPP_VERSION
-    {
-        std::tuple<int, char*, std::string> t(2);
-        assert(std::get<0>(t) == 2);
-        assert(std::get<1>(t) == nullptr);
-        assert(std::get<2>(t) == "");
-    }
-    {
-        std::tuple<int, char*, std::string> t(2, nullptr);
-        assert(std::get<0>(t) == 2);
-        assert(std::get<1>(t) == nullptr);
-        assert(std::get<2>(t) == "");
-    }
-    {
-        std::tuple<int, char*, std::string, double> t(2, nullptr, "text");
-        assert(std::get<0>(t) == 2);
-        assert(std::get<1>(t) == nullptr);
-        assert(std::get<2>(t) == "text");
-        assert(std::get<3>(t) == 0.0);
-    }
-#endif
 
   return 0;
 }
diff --git a/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/deduct.pass.cpp b/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/deduct.pass.cpp
--- a/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/deduct.pass.cpp
+++ b/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/deduct.pass.cpp
@@ -13,7 +13,7 @@
 // GCC's implementation of class template deduction is still immature and runs
 // into issues with libc++. However GCC accepts this code when compiling
 // against libstdc++.
-// XFAIL: gcc-5, gcc-6, gcc-7
+// XFAIL: gcc-5, gcc-6, gcc-7, gcc-8, gcc-9, gcc-10
 
 // <tuple>
 
diff --git a/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/default.lazy.verify.cpp b/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/default.lazy.verify.cpp
new file mode 100644
--- /dev/null
+++ b/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/default.lazy.verify.cpp
@@ -0,0 +1,26 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// This test makes sure that we don't evaluate `is_default_constructible<T>`
+// too early in std::tuple's default constructor.
+
+// UNSUPPORTED: c++03
+
+#include <tuple>
+
+#include "test_macros.h"
+
+struct Outer {
+    template <class T>
+    struct Inner {
+        bool foo = false;
+    };
+    std::tuple<Inner<int>> tup;
+};
+
+Outer x; // expected-no-diagnostics
diff --git a/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/test_lazy_sfinae.pass.cpp b/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/test_lazy_sfinae.pass.cpp
--- a/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/test_lazy_sfinae.pass.cpp
+++ b/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/test_lazy_sfinae.pass.cpp
@@ -12,61 +12,18 @@
 
 // UNSUPPORTED: c++03
 
+// Test the following constructors:
+// (1) tuple(Types const&...)
+// (2) tuple(UTypes&&...)
+// Test that (1) short circuits before evaluating the copy constructor of the
+// second argument. Constructor (2) should be selected.
+
 #include <tuple>
 #include <utility>
 #include <cassert>
 
 #include "test_macros.h"
 
-template <class ConstructFrom>
-struct ConstructibleFromT {
-  ConstructibleFromT() = default;
-  ConstructibleFromT(ConstructFrom v) : value(v) {}
-  ConstructFrom value;
-};
-
-template <class AssertOn>
-struct CtorAssertsT {
-  bool defaulted;
-  CtorAssertsT() : defaulted(true) {}
-  template <class T>
-  constexpr CtorAssertsT(T) : defaulted(false) {
-      static_assert(!std::is_same<T, AssertOn>::value, "");
-  }
-};
-
-template <class AllowT, class AssertT>
-struct AllowAssertT {
-  AllowAssertT() = default;
-  AllowAssertT(AllowT) {}
-  template <class U>
-  constexpr AllowAssertT(U) {
-      static_assert(!std::is_same<U, AssertT>::value, "");
-  }
-};
-
-// Construct a tuple<T1, T2> from pair<int, int> where T1 and T2
-// are not constructible from ints but T1 is constructible from std::pair.
-// This considers the following constructors:
-// (1) tuple(TupleLike) -> checks is_constructible<Tn, int>
-// (2) tuple(UTypes...) -> checks is_constructible<T1, pair<int, int>>
-//                            and is_default_constructible<T2>
-// The point of this test is to ensure that the consideration of (1)
-// short circuits before evaluating is_constructible<T2, int>, which
-// will cause a static assertion.
-void test_tuple_like_lazy_sfinae() {
-#if defined(_LIBCPP_VERSION)
-    // This test requires libc++'s reduced arity initialization.
-    using T1 = ConstructibleFromT<std::pair<int, int>>;
-    using T2 = CtorAssertsT<int>;
-    std::pair<int, int> p(42, 100);
-    std::tuple<T1, T2> t(p);
-    assert(std::get<0>(t).value == p);
-    assert(std::get<1>(t).defaulted);
-#endif
-}
-
-
 struct NonConstCopyable {
   NonConstCopyable() = default;
   explicit NonConstCopyable(int v) : value(v) {}
@@ -84,22 +41,11 @@
   BlowsUpOnConstCopy(BlowsUpOnConstCopy&) = default;
 };
 
-// Test the following constructors:
-// (1) tuple(Types const&...)
-// (2) tuple(UTypes&&...)
-// Test that (1) short circuits before evaluating the copy constructor of the
-// second argument. Constructor (2) should be selected.
-void test_const_Types_lazy_sfinae()
-{
-    NonConstCopyable v(42);
-    BlowsUpOnConstCopy<int> b;
-    std::tuple<NonConstCopyable, BlowsUpOnConstCopy<int>> t(v, b);
-    assert(std::get<0>(t).value == 42);
-}
-
 int main(int, char**) {
-    test_tuple_like_lazy_sfinae();
-    test_const_Types_lazy_sfinae();
+  NonConstCopyable v(42);
+  BlowsUpOnConstCopy<int> b;
+  std::tuple<NonConstCopyable, BlowsUpOnConstCopy<int>> t(v, b);
+  assert(std::get<0>(t).value == 42);
 
   return 0;
 }
diff --git a/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/tuple_array_template_depth.pass.cpp b/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/tuple_array_template_depth.pass.cpp
deleted file mode 100644
--- a/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/tuple_array_template_depth.pass.cpp
+++ /dev/null
@@ -1,39 +0,0 @@
-//===----------------------------------------------------------------------===//
-//
-// 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++03
-
-// <tuple>
-
-// template <class... Types> class tuple;
-
-// template <class Tuple, __tuple_convertible<Tuple, tuple> >
-//   tuple(Tuple &&);
-//
-// template <class Tuple, __tuple_constructible<Tuple, tuple> >
-//   tuple(Tuple &&);
-
-// This test checks that we do not evaluate __make_tuple_types
-// on the array.
-
-#include <array>
-#include <tuple>
-
-#include "test_macros.h"
-
-// Use 1256 to try and blow the template instantiation depth for all compilers.
-typedef std::array<char, 1256> array_t;
-typedef std::tuple<array_t> tuple_t;
-
-int main(int, char**)
-{
-    array_t arr;
-    tuple_t tup(arr);
-
-  return 0;
-}