[libcxx][NFC] Make sequence containers slightly more SFINAE-friendly during CTAD.

Disable the constructors taking `(size_type, const value_type&,
allocator_type)` if `allocator_type` is not a valid allocator.
Otherwise, these constructors are considered when resolving e.g.
`(int*, int*, NotAnAllocator())`, leading to a hard error during
instantiation. A hard error makes the Standard's requirement to not
consider deduction guides of the form `(Iterator, Iterator,
BadAllocator)` during overload resolution essentially non-functional.

The previous approach was to SFINAE away `allocator_traits`. This patch
SFINAEs away the specific constructors instead, for consistency with
`basic_string` -- see [LWG3076](wg21.link/lwg3076) which describes
a very similar problem for strings (note, however, that unlike LWG3076,
no valid constructor call is affected by the bad instantiation).

Differential Revision: https://reviews.llvm.org/D114311

libcxx/include/__memory/allocator_traits.h

@@ -349,14 +349,6 @@ struct _LIBCPP_TEMPLATE_VIS allocator_traits
     }
 };
 
-// A version of `allocator_traits` for internal usage that SFINAEs away if the
-// given allocator doesn't have a nested `value_type`. This helps avoid hard
-// errors when forming implicit deduction guides for a container that has an
-// invalid Allocator type. See https://wg21.link/LWGXXXXX.
-// TODO(varconst): use the actual link once available.
-template <class _Alloc, class _ValueType = typename _Alloc::value_type>
-struct _LIBCPP_TEMPLATE_VIS __allocator_traits : allocator_traits<_Alloc> {};
-
 template <class _Traits, class _Tp>
 struct __rebind_alloc_helper {
 #ifndef _LIBCPP_CXX03_LANG

libcxx/include/deque

@@ -915,16 +915,16 @@ class __deque_base
     __deque_base(const __deque_base& __c);
     __deque_base& operator=(const __deque_base& __c);
 public:
-    typedef _Allocator                               allocator_type;
-    typedef allocator_traits<allocator_type>         __alloc_traits;
-    typedef typename __alloc_traits::size_type       size_type;
+    typedef _Allocator                                allocator_type;
+    typedef allocator_traits<allocator_type>          __alloc_traits;
+    typedef typename __alloc_traits::size_type        size_type;
 
-    typedef _Tp                                      value_type;
-    typedef value_type&                              reference;
-    typedef const value_type&                        const_reference;
-    typedef typename __alloc_traits::difference_type difference_type;
-    typedef typename __alloc_traits::pointer         pointer;
-    typedef typename __alloc_traits::const_pointer   const_pointer;
+    typedef _Tp                                       value_type;
+    typedef value_type&                               reference;
+    typedef const value_type&                         const_reference;
+    typedef typename __alloc_traits::difference_type  difference_type;
+    typedef typename __alloc_traits::pointer          pointer;
+    typedef typename __alloc_traits::const_pointer    const_pointer;
 
     static const difference_type __block_size;
 
@@ -1259,20 +1259,20 @@ public:
     static_assert((is_same<typename allocator_type::value_type, value_type>::value),
                   "Allocator::value_type must be same type as value_type");
 
-    typedef __deque_base<value_type, allocator_type>                 __base;
+    typedef __deque_base<value_type, allocator_type>      __base;
 
-    typedef typename __base::__alloc_traits                          __alloc_traits;
-    typedef typename __base::reference                               reference;
-    typedef typename __base::const_reference                         const_reference;
-    typedef typename __base::iterator                                iterator;
-    typedef typename __base::const_iterator                          const_iterator;
-    typedef typename __allocator_traits<allocator_type>::size_type   size_type;
-    typedef typename __base::difference_type                         difference_type;
+    typedef typename __base::__alloc_traits               __alloc_traits;
+    typedef typename __base::reference                    reference;
+    typedef typename __base::const_reference              const_reference;
+    typedef typename __base::iterator                     iterator;
+    typedef typename __base::const_iterator               const_iterator;
+    typedef typename __base::size_type                    size_type;
+    typedef typename __base::difference_type              difference_type;
 
-    typedef typename __base::pointer                                 pointer;
-    typedef typename __base::const_pointer                           const_pointer;
-    typedef _VSTD::reverse_iterator<iterator>                        reverse_iterator;
-    typedef _VSTD::reverse_iterator<const_iterator>                  const_reverse_iterator;
+    typedef typename __base::pointer                      pointer;
+    typedef typename __base::const_pointer                const_pointer;
+    typedef _VSTD::reverse_iterator<iterator>             reverse_iterator;
+    typedef _VSTD::reverse_iterator<const_iterator>       const_reverse_iterator;
 
     using typename __base::__deque_range;
     using typename __base::__deque_block_range;
@@ -1289,7 +1289,14 @@ public:
     explicit deque(size_type __n, const _Allocator& __a);
 #endif
     deque(size_type __n, const value_type& __v);
-    deque(size_type __n, const value_type& __v, const allocator_type& __a);
+
+    template <class = __enable_if_t<__is_allocator<_Allocator>::value> >
+    deque(size_type __n, const value_type& __v, const allocator_type& __a) : __base(__a)
+    {
+        if (__n > 0)
+            __append(__n, __v);
+    }
+
     template <class _InputIter>
         deque(_InputIter __f, _InputIter __l,
               typename enable_if<__is_cpp17_input_iterator<_InputIter>::value>::type* = 0);
@@ -1608,14 +1615,6 @@ deque<_Tp, _Allocator>::deque(size_type __n, const value_type& __v)
         __append(__n, __v);
 }
 
-template <class _Tp, class _Allocator>
-deque<_Tp, _Allocator>::deque(size_type __n, const value_type& __v, const allocator_type& __a)
-    : __base(__a)
-{
-    if (__n > 0)
-        __append(__n, __v);
-}
-
 template <class _Tp, class _Allocator>
 template <class _InputIter>
 deque<_Tp, _Allocator>::deque(_InputIter __f, _InputIter __l,

libcxx/include/forward_list

@@ -186,6 +186,7 @@ template <class T, class Allocator, class Predicate>
 #include <iterator>
 #include <limits>
 #include <memory>
+#include <type_traits>
 #include <version>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
@@ -643,12 +644,12 @@ public:
     static_assert((is_same<typename allocator_type::value_type, value_type>::value),
                   "Allocator::value_type must be same type as value_type");
 
-    typedef value_type&                                                reference;
-    typedef const value_type&                                          const_reference;
-    typedef typename allocator_traits<allocator_type>::pointer         pointer;
-    typedef typename allocator_traits<allocator_type>::const_pointer   const_pointer;
-    typedef typename __allocator_traits<allocator_type>::size_type     size_type;
-    typedef typename allocator_traits<allocator_type>::difference_type difference_type;
+    typedef value_type&                                                 reference;
+    typedef const value_type&                                           const_reference;
+    typedef typename allocator_traits<allocator_type>::pointer          pointer;
+    typedef typename allocator_traits<allocator_type>::const_pointer    const_pointer;
+    typedef typename allocator_traits<allocator_type>::size_type        size_type;
+    typedef typename allocator_traits<allocator_type>::difference_type  difference_type;
 
     typedef typename base::iterator       iterator;
     typedef typename base::const_iterator const_iterator;
@@ -669,7 +670,13 @@ public:
     explicit forward_list(size_type __n, const allocator_type& __a);
 #endif
     forward_list(size_type __n, const value_type& __v);
-    forward_list(size_type __n, const value_type& __v, const allocator_type& __a);
+
+    template <class = __enable_if_t<__is_allocator<_Alloc>::value> >
+    forward_list(size_type __n, const value_type& __v, const allocator_type& __a) : base(__a)
+    {
+        insert_after(cbefore_begin(), __n, __v);
+    }
+
     template <class _InputIterator>
         forward_list(_InputIterator __f, _InputIterator __l,
                      typename enable_if<
@@ -943,14 +950,6 @@ forward_list<_Tp, _Alloc>::forward_list(size_type __n, const value_type& __v)
     insert_after(cbefore_begin(), __n, __v);
 }
 
-template <class _Tp, class _Alloc>
-forward_list<_Tp, _Alloc>::forward_list(size_type __n, const value_type& __v,
-                                        const allocator_type& __a)
-    : base(__a)
-{
-    insert_after(cbefore_begin(), __n, __v);
-}
-
 template <class _Tp, class _Alloc>
 template <class _InputIterator>
 forward_list<_Tp, _Alloc>::forward_list(_InputIterator __f, _InputIterator __l,

libcxx/include/list

@@ -845,24 +845,24 @@ class _LIBCPP_TEMPLATE_VIS list
     typedef typename base::__link_pointer __link_pointer;
 
 public:
-    typedef _Tp                                                      value_type;
-    typedef _Alloc                                                   allocator_type;
+    typedef _Tp                                            value_type;
+    typedef _Alloc                                         allocator_type;
     static_assert((is_same<value_type, typename allocator_type::value_type>::value),
                   "Invalid allocator::value_type");
-    typedef value_type&                                              reference;
-    typedef const value_type&                                        const_reference;
-    typedef typename base::pointer                                   pointer;
-    typedef typename base::const_pointer                             const_pointer;
-    typedef typename __allocator_traits<allocator_type>::size_type   size_type;
-    typedef typename base::difference_type                           difference_type;
-    typedef typename base::iterator                                  iterator;
-    typedef typename base::const_iterator                            const_iterator;
-    typedef _VSTD::reverse_iterator<iterator>                        reverse_iterator;
-    typedef _VSTD::reverse_iterator<const_iterator>                  const_reverse_iterator;
+    typedef value_type&                                    reference;
+    typedef const value_type&                              const_reference;
+    typedef typename base::pointer                         pointer;
+    typedef typename base::const_pointer                   const_pointer;
+    typedef typename base::size_type                       size_type;
+    typedef typename base::difference_type                 difference_type;
+    typedef typename base::iterator                        iterator;
+    typedef typename base::const_iterator                  const_iterator;
+    typedef _VSTD::reverse_iterator<iterator>              reverse_iterator;
+    typedef _VSTD::reverse_iterator<const_iterator>        const_reverse_iterator;
 #if _LIBCPP_STD_VER > 17
-    typedef size_type                                                __remove_return_type;
+    typedef size_type                                      __remove_return_type;
 #else
-    typedef void                                                     __remove_return_type;
+    typedef void                                           __remove_return_type;
 #endif
 
     _LIBCPP_INLINE_VISIBILITY
@@ -885,7 +885,16 @@ public:
     explicit list(size_type __n, const allocator_type& __a);
 #endif
     list(size_type __n, const value_type& __x);
-    list(size_type __n, const value_type& __x, const allocator_type& __a);
+    template <class = __enable_if_t<__is_allocator<_Alloc>::value> >
+    list(size_type __n, const value_type& __x, const allocator_type& __a) : base(__a)
+    {
+#if _LIBCPP_DEBUG_LEVEL == 2
+        __get_db()->__insert_c(this);
+#endif
+        for (; __n > 0; --__n)
+            push_back(__x);
+    }
+
     template <class _InpIter>
         list(_InpIter __f, _InpIter __l,
              typename enable_if<__is_cpp17_input_iterator<_InpIter>::value>::type* = 0);
@@ -1241,17 +1250,6 @@ list<_Tp, _Alloc>::list(size_type __n, const value_type& __x)
         push_back(__x);
 }
 
-template <class _Tp, class _Alloc>
-list<_Tp, _Alloc>::list(size_type __n, const value_type& __x, const allocator_type& __a)
-    : base(__a)
-{
-#if _LIBCPP_DEBUG_LEVEL == 2
-    __get_db()->__insert_c(this);
-#endif
-    for (; __n > 0; --__n)
-        push_back(__x);
-}
-
 template <class _Tp, class _Alloc>
 template <class _InpIter>
 list<_Tp, _Alloc>::list(_InpIter __f, _InpIter __l,

libcxx/include/vector

@@ -350,23 +350,23 @@ class _LIBCPP_TEMPLATE_VIS vector
     : private __vector_base<_Tp, _Allocator>
 {
 private:
-    typedef __vector_base<_Tp, _Allocator>                           __base;
-    typedef allocator<_Tp>                                           __default_allocator_type;
+    typedef __vector_base<_Tp, _Allocator>                  __base;
+    typedef allocator<_Tp>                                  __default_allocator_type;
 public:
-    typedef vector                                                   __self;
-    typedef _Tp                                                      value_type;
-    typedef _Allocator                                               allocator_type;
-    typedef allocator_traits<allocator_type>                         __alloc_traits;
-    typedef value_type&                                              reference;
-    typedef const value_type&                                        const_reference;
-    typedef typename __allocator_traits<allocator_type>::size_type   size_type;
-    typedef typename __alloc_traits::difference_type                 difference_type;
-    typedef typename __alloc_traits::pointer                         pointer;
-    typedef typename __alloc_traits::const_pointer                   const_pointer;
-    typedef __wrap_iter<pointer>                                     iterator;
-    typedef __wrap_iter<const_pointer>                               const_iterator;
-    typedef _VSTD::reverse_iterator<iterator>                        reverse_iterator;
-    typedef _VSTD::reverse_iterator<const_iterator>                  const_reverse_iterator;
+    typedef vector                                          __self;
+    typedef _Tp                                             value_type;
+    typedef _Allocator                                      allocator_type;
+    typedef allocator_traits<allocator_type>                __alloc_traits;
+    typedef value_type&                                     reference;
+    typedef const value_type&                               const_reference;
+    typedef typename __alloc_traits::size_type              size_type;
+    typedef typename __alloc_traits::difference_type        difference_type;
+    typedef typename __alloc_traits::pointer                pointer;
+    typedef typename __alloc_traits::const_pointer          const_pointer;
+    typedef __wrap_iter<pointer>                            iterator;
+    typedef __wrap_iter<const_pointer>                      const_iterator;
+    typedef _VSTD::reverse_iterator<iterator>               reverse_iterator;
+    typedef _VSTD::reverse_iterator<const_iterator>         const_reverse_iterator;
 
     static_assert((is_same<typename allocator_type::value_type, value_type>::value),
                   "Allocator::value_type must be same type as value_type");
@@ -395,7 +395,21 @@ public:
     explicit vector(size_type __n, const allocator_type& __a);
 #endif
     vector(size_type __n, const value_type& __x);
-    vector(size_type __n, const value_type& __x, const allocator_type& __a);
+
+    template <class = __enable_if_t<__is_allocator<_Allocator>::value> >
+    vector(size_type __n, const value_type& __x, const allocator_type& __a)
+        : __base(__a)
+    {
+#if _LIBCPP_DEBUG_LEVEL == 2
+      __get_db()->__insert_c(this);
+#endif
+      if (__n > 0)
+      {
+          __vallocate(__n);
+          __construct_at_end(__n, __x);
+      }
+    }
+
     template <class _InputIterator>
         vector(_InputIterator __first,
                typename enable_if<__is_cpp17_input_iterator  <_InputIterator>::value &&
@@ -1126,20 +1140,6 @@ vector<_Tp, _Allocator>::vector(size_type __n, const value_type& __x)
     }
 }
 
-template <class _Tp, class _Allocator>
-vector<_Tp, _Allocator>::vector(size_type __n, const value_type& __x, const allocator_type& __a)
-    : __base(__a)
-{
-#if _LIBCPP_DEBUG_LEVEL == 2
-    __get_db()->__insert_c(this);
-#endif
-    if (__n > 0)
-    {
-        __vallocate(__n);
-        __construct_at_end(__n, __x);
-    }
-}
-
 template <class _Tp, class _Allocator>
 template <class _InputIterator>
 vector<_Tp, _Allocator>::vector(_InputIterator __first,

libcxx/test/std/containers/sequences/deque/deque.cons/default_noexcept.pass.cpp

@@ -27,6 +27,7 @@ struct some_alloc
 {
     typedef T value_type;
     some_alloc(const some_alloc&);
+    void allocate(size_t);
 };
 
 int main(int, char**)

libcxx/test/std/containers/sequences/deque/deque.cons/dtor_noexcept.pass.cpp

@@ -25,6 +25,7 @@ struct some_alloc
     typedef T value_type;
     some_alloc(const some_alloc&);
     ~some_alloc() noexcept(false);
+    void allocate(size_t);
 };
 
 int main(int, char**)

libcxx/test/std/containers/sequences/deque/deque.cons/move_assign_noexcept.pass.cpp

@@ -29,6 +29,7 @@ struct some_alloc
 {
     typedef T value_type;
     some_alloc(const some_alloc&);
+    void allocate(size_t);
 };
 
 int main(int, char**)

libcxx/test/std/containers/sequences/deque/deque.cons/move_noexcept.pass.cpp

@@ -27,6 +27,7 @@ struct some_alloc
 {
     typedef T value_type;
     some_alloc(const some_alloc&);
+    void allocate(size_t);
 };
 
 int main(int, char**)

libcxx/test/std/containers/sequences/deque/deque.special/swap_noexcept.pass.cpp

@@ -34,6 +34,7 @@ struct some_alloc
 
     some_alloc() {}
     some_alloc(const some_alloc&);
+    void allocate(size_t);
     void deallocate(void*, unsigned) {}
 
     typedef std::true_type propagate_on_container_swap;
@@ -46,6 +47,7 @@ struct some_alloc2
 
     some_alloc2() {}
     some_alloc2(const some_alloc2&);
+    void allocate(size_t);
     void deallocate(void*, unsigned) {}
 
     typedef std::false_type propagate_on_container_swap;

libcxx/test/std/containers/sequences/forwardlist/forwardlist.cons/default_noexcept.pass.cpp

@@ -27,6 +27,7 @@ struct some_alloc
 {
     typedef T value_type;
     some_alloc(const some_alloc&);
+    void allocate(size_t);
 };
 
 int main(int, char**)

libcxx/test/std/containers/sequences/forwardlist/forwardlist.cons/dtor_noexcept.pass.cpp

@@ -25,6 +25,7 @@ struct some_alloc
     typedef T value_type;
     some_alloc(const some_alloc&);
     ~some_alloc() noexcept(false);
+    void allocate(size_t);
 };
 
 int main(int, char**)

libcxx/test/std/containers/sequences/forwardlist/forwardlist.cons/move_assign_noexcept.pass.cpp

@@ -29,6 +29,7 @@ struct some_alloc
 {
     typedef T value_type;
     some_alloc(const some_alloc&);
+    void allocate(size_t);
 };
 
 int main(int, char**)

libcxx/test/std/containers/sequences/forwardlist/forwardlist.cons/move_noexcept.pass.cpp

@@ -27,6 +27,7 @@ struct some_alloc
 {
     typedef T value_type;
     some_alloc(const some_alloc&);
+    void allocate(size_t);
 };
 
 int main(int, char**)

libcxx/test/std/containers/sequences/forwardlist/forwardlist.spec/swap_noexcept.pass.cpp

@@ -35,6 +35,7 @@ struct some_alloc
 
     some_alloc() {}
     some_alloc(const some_alloc&);
+    void allocate(size_t);
     void deallocate(void*, unsigned) {}
 
     typedef std::true_type propagate_on_container_swap;
@@ -47,6 +48,7 @@ struct some_alloc2
 
     some_alloc2() {}
     some_alloc2(const some_alloc2&);
+    void allocate(size_t);
     void deallocate(void*, unsigned) {}
 
     typedef std::false_type propagate_on_container_swap;

libcxx/test/std/containers/sequences/list/list.cons/default_noexcept.pass.cpp

@@ -27,6 +27,7 @@ struct some_alloc
 {
     typedef T value_type;
     some_alloc(const some_alloc&);
+    void allocate(size_t);
 };
 
 int main(int, char**)

libcxx/test/std/containers/sequences/list/list.cons/dtor_noexcept.pass.cpp

@@ -25,6 +25,7 @@ struct some_alloc
     typedef T value_type;
     some_alloc(const some_alloc&);
     ~some_alloc() noexcept(false);
+    void allocate(size_t);
 };
 
 int main(int, char**)

libcxx/test/std/containers/sequences/list/list.cons/move_assign_noexcept.pass.cpp

@@ -29,6 +29,7 @@ struct some_alloc
 {
     typedef T value_type;
     some_alloc(const some_alloc&);
+    void allocate(size_t);
 };
 
 int main(int, char**)

libcxx/test/std/containers/sequences/list/list.cons/move_noexcept.pass.cpp

@@ -27,6 +27,7 @@ struct some_alloc
 {
     typedef T value_type;
     some_alloc(const some_alloc&);
+    void allocate(size_t);
 };
 
 int main(int, char**)

libcxx/test/std/containers/sequences/list/list.special/swap_noexcept.pass.cpp

@@ -34,6 +34,7 @@ struct some_alloc
 
     some_alloc() {}
     some_alloc(const some_alloc&);
+    void allocate(size_t);
     void deallocate(void*, unsigned) {}
 
     typedef std::true_type propagate_on_container_swap;
@@ -46,6 +47,7 @@ struct some_alloc2
 
     some_alloc2() {}
     some_alloc2(const some_alloc2&);
+    void allocate(size_t);
     void deallocate(void*, unsigned) {}
 
     typedef std::false_type propagate_on_container_swap;

libcxx/test/std/containers/sequences/vector/vector.cons/default_noexcept.pass.cpp

@@ -27,6 +27,7 @@ struct some_alloc
 {
     typedef T value_type;
     some_alloc(const some_alloc&);
+    void allocate(size_t);
 };
 
 int main(int, char**)

libcxx/test/std/containers/sequences/vector/vector.cons/dtor_noexcept.pass.cpp

@@ -25,6 +25,7 @@ struct some_alloc
     typedef T value_type;
     some_alloc(const some_alloc&);
     ~some_alloc() noexcept(false);
+    void allocate(size_t);
 };
 
 int main(int, char**)

libcxx/test/std/containers/sequences/vector/vector.cons/move_assign_noexcept.pass.cpp

@@ -29,6 +29,7 @@ struct some_alloc
 {
     typedef T value_type;
     some_alloc(const some_alloc&);
+    void allocate(size_t);
 };
 
 template <class T>
@@ -38,6 +39,7 @@ struct some_alloc2
 
     some_alloc2() {}
     some_alloc2(const some_alloc2&);
+    void allocate(size_t);
     void deallocate(void*, unsigned) {}
 
     typedef std::false_type propagate_on_container_move_assignment;
@@ -51,6 +53,7 @@ struct some_alloc3
 
     some_alloc3() {}
     some_alloc3(const some_alloc3&);
+    void allocate(size_t);
     void deallocate(void*, unsigned) {}
 
     typedef std::false_type propagate_on_container_move_assignment;

libcxx/test/std/containers/sequences/vector/vector.cons/move_noexcept.pass.cpp

@@ -27,6 +27,7 @@ struct some_alloc
 {
     typedef T value_type;
     some_alloc(const some_alloc&);
+    void allocate(size_t);
 };
 
 int main(int, char**)

libcxx/test/std/containers/sequences/vector/vector.special/swap_noexcept.pass.cpp

@@ -35,6 +35,7 @@ struct some_alloc
 
     some_alloc() {}
     some_alloc(const some_alloc&);
+    void allocate(size_t);
     void deallocate(void*, unsigned) {}
 
     typedef std::true_type propagate_on_container_swap;
@@ -47,6 +48,7 @@ struct some_alloc2
 
     some_alloc2() {}
     some_alloc2(const some_alloc2&);
+    void allocate(size_t);
     void deallocate(void*, unsigned) {}
 
     typedef std::false_type propagate_on_container_swap;