[libc++] Adds back_insert_iterator::__get_container.

[Mordante]

Summary

Adds a __get_container member as suggested by @Quuxplusone in D110497.

Includes s/_LIBCPP_INLINE_VISIBILITY/_LIBCPP_HIDE_FROM_ABI/.

[• Quuxplusone]

Presumably you should wait to land this until it's actually needed by <format>, so we don't have a loose wire hanging around for too long without being hooked up to something.
The _LIBCPP_HIDE_FROM_ABI refactoring could just be landed ASAP, AFAIconcerned.

libcxx/include/__iterator/back_insert_iterator.h

59

Making the function a const member function has little use

Agreed, but it has non-negative use, and it's conventional (in that it doesn't modify any of *this's members), so I think you should make it const just for consistency.

Otherwise LGTM!

[Mordante]

Make the new member const.
Format the new unit test.

[Mordante]

Rebased, hopefully fixes CI patch application.

[ldionne]

Let's rebase and ship this since <format> is about to need it.

[Mordante]

Rebase to trigger CI.

[Mordante]

Rebase to trigger CI.

libcxx/include/__iterator/back_insert_iterator.h

_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_AFTER_CXX17 back_insert_iterator& operator=(const typename _Container::value_type& __value_)
         {container->push_back(__value_); return *this;}
 #ifndef _LIBCPP_CXX03_LANG
     _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_AFTER_CXX17 back_insert_iterator& operator=(typename _Container::value_type&& __value_)
         {container->push_back(_VSTD::move(__value_)); return *this;}
 #endif // _LIBCPP_CXX03_LANG
     _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_AFTER_CXX17 back_insert_iterator& operator*()     {return *this;}
     _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_AFTER_CXX17 back_insert_iterator& operator++()    {return *this;}
     _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_AFTER_CXX17 back_insert_iterator  operator++(int) {return *this;}
+
+    _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_AFTER_CXX17 _Container* __get_container() const { return container; }
 };
 
 template <class _Container>
 inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_AFTER_CXX17
 back_insert_iterator<_Container>
 back_inserter(_Container& __x)
 {
     return back_insert_iterator<_Container>(__x);
 }
 
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP___ITERATOR_BACK_INSERT_ITERATOR_H

libcxx/test/libcxx/iterators/predef.iterators/insert.iterators/back.insert.iter.ops/get_container.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// <iterator>
+
+// back_insert_iterator
+
+// _Container* __get_container(); // constexpr in C++20
+
+#include <iterator>
+#include <vector>
+
+#include "test_macros.h"
+#include "nasty_containers.h"
+#include "test_constexpr_container.h"
+
+template <class C>
+TEST_CONSTEXPR_CXX20 bool test(C c) {
+  const std::back_insert_iterator<C> i(c);
+  assert(i.__get_container() == std::addressof(c));
+  return true;
+}
+
+int main(int, char**) {
+  test(std::vector<int>());
+  test(nasty_vector<int>());
+#if TEST_STD_VER >= 20
+  test(ConstexprFixedCapacityDeque<int, 10>());
+  static_assert(test(ConstexprFixedCapacityDeque<int, 10>()));
+#endif
+  return 0;
+}