[libc++] Improve error message for invalid allocators

libcxx/include/__memory/allocator_traits.h

 _LIBCPP_SUPPRESS_DEPRECATED_PUSH
 template <class _Tp, class _Up, class = void>
 struct __has_rebind_other : false_type { };
 template <class _Tp, class _Up>
 struct __has_rebind_other<_Tp, _Up, __void_t<typename _Tp::template rebind<_Up>::other> > : true_type { };
 
 template <class _Tp, class _Up, bool = __has_rebind_other<_Tp, _Up>::value>
 struct __allocator_traits_rebind {
-    using type _LIBCPP_NODEBUG = typename _Tp::template rebind<_Up>::other;
+  static_assert(__has_rebind_other<_Tp, _Up>::value, "This allocator has to implement rebind");
 };
 template <template <class, class...> class _Alloc, class _Tp, class ..._Args, class _Up>
 struct __allocator_traits_rebind<_Alloc<_Tp, _Args...>, _Up, true> {
     using type _LIBCPP_NODEBUG = typename _Alloc<_Tp, _Args...>::template rebind<_Up>::other;
 };
 template <template <class, class...> class _Alloc, class _Tp, class ..._Args, class _Up>
 struct __allocator_traits_rebind<_Alloc<_Tp, _Args...>, _Up, false> {
     using type _LIBCPP_NODEBUG = _Alloc<_Up, _Args...>;

libcxx/test/libcxx/containers/sequences/vector/invalid_allocator.verify.cpp

+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// Check that vector diagnoses an allocator which has to implement rebind with an appropriate error message
+
+#include <vector>
+
+class FooAllocator {
+ public:
+  using value_type = int;
+  FooAllocator() = default;
+
+  int* allocate(int num_objects);
+
+  void deallocate(int* ptr, int num_objects);
+
+  bool operator==(const FooAllocator&) const { return true; }
+  bool operator!=(const FooAllocator&) const { return false; }
+};
+
+void func() {
+  std::vector<int, FooAllocator> v; //expected-error-re@*:* {{{{(static_assert|static assertion)}} failed {{.*}}This allocator has to implement rebind}}
+}