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Re: [PATCH] libstdc++: Add missing bits of P0896R4 pertaining to [back|front]_insert_iterator


This should do the same changes for std::insert_iterator, but it
currently crashes the compiler (PR 93907). I am not committing this
yet.


commit 2db20402125e6136d6768598f5974e917b2622c6
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Mon Feb 24 15:50:59 2020 +0000

    libstdc++: Add missing bits of P0896R4 pertaining to insert_iterator (PR 93884)
    
    This completes the fix for PR 93884. In order to use ranges::iterator_t
    in std::insert_iterator the definition of ranges::iterator_t and
    ranges::begin need to be moved, to avoid a circular dependency between
    <bits/stl_iterator.h> and <bits/range_access.h>.
    
    As noted in a FIXME comment, the std::inserter function has the wrong
    signature because LWG issue 561 was never implemented. That is tracked
    by PR 93904.
    
            PR libstdc++/93884
            * include/bits/iterator_concepts.h (__detail::__maybe_borrowed_range)
            (__cust_access::_Begin, ranges::iterator_t): Move from ...
            * include/bits/range_access.h: ... here.
            * include/bits/stl_iterator.h (insert_iterator, inserter): Implement
            C++20 changes from P0896R4.
            * testsuite/24_iterators/insert_iterator/pr93884.cc: New test.

diff --git a/libstdc++-v3/include/bits/iterator_concepts.h b/libstdc++-v3/include/bits/iterator_concepts.h
index 08e622259b4..ae0c207d0d9 100644
--- a/libstdc++-v3/include/bits/iterator_concepts.h
+++ b/libstdc++-v3/include/bits/iterator_concepts.h
@@ -835,6 +835,90 @@ namespace ranges
   struct default_sentinel_t { };
   inline constexpr default_sentinel_t default_sentinel{};
 
+namespace ranges
+{
+  template<typename _Tp>
+    extern const bool enable_borrowed_range;
+
+  namespace __detail
+  {
+    // Part of the constraints of ranges::borrowed_range
+    template<typename _Tp>
+      concept __maybe_borrowed_range
+	= is_lvalue_reference_v<_Tp>
+	  || enable_borrowed_range<remove_cvref_t<_Tp>>;
+  } // namespace __detail
+
+  namespace __cust_access
+  {
+    template<typename _Tp>
+      constexpr decay_t<_Tp>
+      __decay_copy(_Tp&& __t)
+      noexcept(is_nothrow_convertible_v<_Tp, decay_t<_Tp>>)
+      { return std::forward<_Tp>(__t); }
+
+    template<typename _Tp>
+      concept __member_begin = requires(_Tp& __t)
+	{
+	  __t.begin();
+	  requires input_or_output_iterator<decay_t<decltype(__t.begin())>>;
+	};
+
+    void begin(auto&) = delete;
+    void begin(const auto&) = delete;
+
+    template<typename _Tp>
+      concept __adl_begin
+	= std::__detail::__class_or_enum<remove_reference_t<_Tp>>
+	&& requires(_Tp& __t)
+	{
+	  begin(__t);
+	  requires input_or_output_iterator<decay_t<decltype(begin(__t))>>;
+	};
+
+    struct _Begin
+    {
+    private:
+      template<typename _Tp>
+	static constexpr bool
+	_S_noexcept()
+	{
+	  if constexpr (is_array_v<remove_reference_t<_Tp>>)
+	    return true;
+	  else if constexpr (__member_begin<_Tp>)
+	    return noexcept(__decay_copy(std::declval<_Tp&>().begin()));
+	  else
+	    return noexcept(__decay_copy(begin(std::declval<_Tp&>())));
+	}
+
+    public:
+      template<__detail::__maybe_borrowed_range _Tp>
+	requires is_array_v<remove_reference_t<_Tp>> || __member_begin<_Tp>
+	  || __adl_begin<_Tp>
+	constexpr auto
+	operator()(_Tp&& __t) const noexcept(_S_noexcept<_Tp>())
+	{
+	  if constexpr (is_array_v<remove_reference_t<_Tp>>)
+	    {
+	      static_assert(is_lvalue_reference_v<_Tp>);
+	      using _Up = remove_all_extents_t<remove_reference_t<_Tp>>;
+	      static_assert(sizeof(_Up) != 0, "not array of incomplete type");
+	      return __t + 0;
+	    }
+	  else if constexpr (__member_begin<_Tp>)
+	    return __t.begin();
+	  else
+	    return begin(__t);
+	}
+    };
+  } // namespace __cust_access
+
+  template<typename _Tp>
+    using iterator_t
+      = decltype(__cust_access::_Begin{}(std::declval<_Tp&>()));
+
+} // namespace ranges
+
 _GLIBCXX_END_NAMESPACE_VERSION
 } // namespace std
 #endif // C++20 library concepts
diff --git a/libstdc++-v3/include/bits/range_access.h b/libstdc++-v3/include/bits/range_access.h
index 8b276fd6625..7ae08f58f67 100644
--- a/libstdc++-v3/include/bits/range_access.h
+++ b/libstdc++-v3/include/bits/range_access.h
@@ -359,13 +359,6 @@ namespace ranges
     template<typename _Tp, bool _MaxDiff = same_as<_Tp, __max_diff_type>>
       using __make_unsigned_like_t
 	= conditional_t<_MaxDiff, __max_size_type, make_unsigned_t<_Tp>>;
-
-    // Part of the constraints of ranges::borrowed_range
-    template<typename _Tp>
-      concept __maybe_borrowed_range
-	= is_lvalue_reference_v<_Tp>
-	  || enable_borrowed_range<remove_cvref_t<_Tp>>;
-
   } // namespace __detail
 
   namespace __cust_access
@@ -373,64 +366,6 @@ namespace ranges
     using std::ranges::__detail::__maybe_borrowed_range;
     using std::__detail::__class_or_enum;
 
-    template<typename _Tp>
-      constexpr decay_t<_Tp>
-      __decay_copy(_Tp&& __t)
-      noexcept(is_nothrow_convertible_v<_Tp, decay_t<_Tp>>)
-      { return std::forward<_Tp>(__t); }
-
-    template<typename _Tp>
-      concept __member_begin = requires(_Tp& __t)
-	{
-	  { __decay_copy(__t.begin()) } -> input_or_output_iterator;
-	};
-
-    void begin(auto&) = delete;
-    void begin(const auto&) = delete;
-
-    template<typename _Tp>
-      concept __adl_begin = __class_or_enum<remove_reference_t<_Tp>>
-	&& requires(_Tp& __t)
-	{
-	  { __decay_copy(begin(__t)) } -> input_or_output_iterator;
-	};
-
-    struct _Begin
-    {
-    private:
-      template<typename _Tp>
-	static constexpr bool
-	_S_noexcept()
-	{
-	  if constexpr (is_array_v<remove_reference_t<_Tp>>)
-	    return true;
-	  else if constexpr (__member_begin<_Tp>)
-	    return noexcept(__decay_copy(std::declval<_Tp&>().begin()));
-	  else
-	    return noexcept(__decay_copy(begin(std::declval<_Tp&>())));
-	}
-
-    public:
-      template<__maybe_borrowed_range _Tp>
-	requires is_array_v<remove_reference_t<_Tp>> || __member_begin<_Tp>
-	  || __adl_begin<_Tp>
-	constexpr auto
-	operator()(_Tp&& __t) const noexcept(_S_noexcept<_Tp>())
-	{
-	  if constexpr (is_array_v<remove_reference_t<_Tp>>)
-	    {
-	      static_assert(is_lvalue_reference_v<_Tp>);
-	      using _Up = remove_all_extents_t<remove_reference_t<_Tp>>;
-	      static_assert(sizeof(_Up) != 0, "not array of incomplete type");
-	      return __t + 0;
-	    }
-	  else if constexpr (__member_begin<_Tp>)
-	    return __t.begin();
-	  else
-	    return begin(__t);
-	}
-    };
-
     template<typename _Tp>
       concept __member_end = requires(_Tp& __t)
 	{
@@ -888,9 +823,6 @@ namespace ranges
     concept borrowed_range
       = range<_Tp> && __detail::__maybe_borrowed_range<_Tp>;
 
-  template<typename _Tp>
-    using iterator_t = decltype(ranges::begin(std::declval<_Tp&>()));
-
   template<range _Range>
     using sentinel_t = decltype(ranges::end(std::declval<_Range&>()));
 
diff --git a/libstdc++-v3/include/bits/stl_iterator.h b/libstdc++-v3/include/bits/stl_iterator.h
index caaa8c483b8..c332343d11b 100644
--- a/libstdc++-v3/include/bits/stl_iterator.h
+++ b/libstdc++-v3/include/bits/stl_iterator.h
@@ -704,19 +704,36 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     class insert_iterator
     : public iterator<output_iterator_tag, void, void, void, void>
     {
+#if __cplusplus <= 201703L
+      typedef typename _Container::iterator _Iter;
+#else
+      using _Iter = ranges::iterator_t<_Container>;
+#endif
+
     protected:
+#if __cplusplus <= 201703L
       _Container* container;
-      typename _Container::iterator iter;
+      _Iter iter;
+#else
+      _Container* container = nullptr;
+      _Iter iter = _Iter();
+#endif
 
     public:
       /// A nested typedef for the type of whatever container you used.
       typedef _Container          container_type;
 
+#if __cplusplus > 201703L
+      using difference_type = ptrdiff_t;
+
+      insert_iterator() = default;
+#endif
+
       /**
        *  The only way to create this %iterator is with a container and an
        *  initial position (a normal %iterator into the container).
       */
-      insert_iterator(_Container& __x, typename _Container::iterator __i)
+      insert_iterator(_Container& __x, _Iter __i)
       : container(std::__addressof(__x)), iter(__i) {}
 
       /**
@@ -782,6 +799,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       insert_iterator&
       operator++(int)
       { return *this; }
+
+#if __cplusplus >= 201703L
+      template<typename _Cont>
+	friend insert_iterator<_Cont>
+	inserter(_Cont&, typename insert_iterator<_Cont>::_Iter);
+#endif
     };
 
   /**
@@ -796,6 +819,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
    *  template parameter deduction, making the compiler match the correct
    *  types for you.
   */
+#if __cplusplus <= 201703L
+  // FIXME: This version should not exist (PR 93904)
   template<typename _Container, typename _Iterator>
     inline insert_iterator<_Container>
     inserter(_Container& __x, _Iterator __i)
@@ -803,6 +828,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       return insert_iterator<_Container>(__x,
 					 typename _Container::iterator(__i));
     }
+#else
+  template<typename _Container>
+    inline insert_iterator<_Container>
+    inserter(_Container& __x, typename insert_iterator<_Container>::_Iter __i)
+    {
+      return insert_iterator<_Container>(__x, __i);
+    }
+#endif
 
   // @} group iterators
 
diff --git a/libstdc++-v3/testsuite/24_iterators/insert_iterator/pr93884.cc b/libstdc++-v3/testsuite/24_iterators/insert_iterator/pr93884.cc
new file mode 100644
index 00000000000..e0949e64152
--- /dev/null
+++ b/libstdc++-v3/testsuite/24_iterators/insert_iterator/pr93884.cc
@@ -0,0 +1,44 @@
+// Copyright (C) 2020 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-options "-std=gnu++2a" }
+// { dg-do run { target c++2a } }
+
+#include <iterator>
+#include <algorithm>
+#include <set>
+#include <testsuite_hooks.h>
+
+namespace ranges = std::ranges;
+namespace views = std::views;
+
+void
+test01()
+{
+  auto s = std::set<int>{};
+  auto i = views::iota(0, 10);
+  auto o = std::inserter(s, s.begin());
+  static_assert(std::output_iterator<decltype(o), int>);
+  ranges::copy(i, o);
+  VERIFY( ranges::equal(s, i) );
+}
+
+int
+main()
+{
+  test01();
+}

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