[libc++] Define insert_iterator::iter with ranges::iterator_t.

The `insert_iterator::iter` member is defined as `Container::iterator` but
the standard requires `iter` to be defined in terms of `ranges::iterator_t` as
of C++20. So, if in C++20 or later, define the `iter` member as
`ranges::iterator_t`.

Original patch by Joe Loser!

Differential Revision: https://reviews.llvm.org/D108575

Author: Quuxplusone

libcxx/include/__iterator/insert_iterator.h

@@ -14,6 +14,7 @@
 #include <__iterator/iterator.h>
 #include <__iterator/iterator_traits.h>
 #include <__memory/addressof.h>
+#include <__ranges/access.h>
 #include <__utility/move.h>
 #include <cstddef>
 
@@ -23,6 +24,14 @@
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
+#if _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_RANGES)
+template <class _Container>
+using __insert_iterator_iter_t = ranges::iterator_t<_Container>;
+#else
+template <class _Container>
+using __insert_iterator_iter_t = typename _Container::iterator;
+#endif
+
 _LIBCPP_SUPPRESS_DEPRECATED_PUSH
 template <class _Container>
 class _LIBCPP_TEMPLATE_VIS insert_iterator
@@ -33,7 +42,7 @@ class _LIBCPP_TEMPLATE_VIS insert_iterator
 _LIBCPP_SUPPRESS_DEPRECATED_POP
 protected:
     _Container* container;
-    typename _Container::iterator iter; // FIXME: `ranges::iterator_t<Container>` in C++20 mode
+    __insert_iterator_iter_t<_Container> iter;
 public:
     typedef output_iterator_tag iterator_category;
     typedef void value_type;
@@ -46,7 +55,7 @@ _LIBCPP_SUPPRESS_DEPRECATED_POP
     typedef void reference;
     typedef _Container container_type;
 
-    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17 insert_iterator(_Container& __x, typename _Container::iterator __i)
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17 insert_iterator(_Container& __x, __insert_iterator_iter_t<_Container> __i)
         : container(_VSTD::addressof(__x)), iter(__i) {}
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17 insert_iterator& operator=(const typename _Container::value_type& __value_)
         {iter = container->insert(iter, __value_); ++iter; return *this;}
@@ -62,7 +71,7 @@ _LIBCPP_SUPPRESS_DEPRECATED_POP
 template <class _Container>
 inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
 insert_iterator<_Container>
-inserter(_Container& __x, typename _Container::iterator __i)
+inserter(_Container& __x, __insert_iterator_iter_t<_Container> __i)
 {
     return insert_iterator<_Container>(__x, __i);
 }

libcxx/test/std/iterators/predef.iterators/insert.iterators/insert.iterator/cxx20_iter_member.pass.cpp

@@ -0,0 +1,54 @@
+//===----------------------------------------------------------------------===//
+//
+// 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, c++11, c++14, c++17
+// UNSUPPORTED: libcpp-no-concepts
+// UNSUPPORTED: libcpp-has-no-incomplete-ranges
+
+// insert_iterator
+// C++20 and above use ranges::iterator_t<Container> instead of Container::iterator.
+
+#include <iterator>
+
+#include <cassert>
+#include <type_traits>
+
+#include "test_macros.h"
+
+struct NoIteratorAlias {
+    double data_[3] = {};
+    using value_type = int;
+    double *begin();
+    constexpr double *insert(double *pos, int value) {
+        *pos = value;
+        return pos;
+    }
+};
+
+static_assert(std::is_constructible_v<std::insert_iterator<NoIteratorAlias>, NoIteratorAlias&, double*>);
+static_assert(!std::is_constructible_v<std::insert_iterator<NoIteratorAlias>, NoIteratorAlias&, int*>);
+
+constexpr bool test() {
+    NoIteratorAlias c;
+    double half = 0.5;
+    auto it = std::insert_iterator<NoIteratorAlias>(c, c.data_);
+    ASSERT_SAME_TYPE(decltype(std::inserter(c, c.data_)), std::insert_iterator<NoIteratorAlias>);
+    *it++ = 1 + half;  // test that RHS is still implicitly converted to _Container::value_type
+    *it++ = 2 + half;
+    assert(c.data_[0] == 1.0);
+    assert(c.data_[1] == 2.0);
+    assert(c.data_[2] == 0.0);
+    return true;
+}
+
+int main(int, char**) {
+    test();
+    static_assert(test());
+
+    return 0;
+}