Re: [PATCH 1/3, 11 backport] libstdc++: Implement P2325 changes to default-constructibility of views

[Patrick Palka <ppalka@redhat.com>]

On Thu, 10 Feb 2022, Patrick Palka wrote:

> Tested on x86_64-pc-linux-gnu, does this look OK for the 11 branch?
> The backport to the 10 branch hasn't been started yet, I figured it'd
> be good to first get the 11 backport right then base the 10 backport
> on the 11 one.
> 
> NB: This backport of r12-1606 to the 11 branch deliberately omits parts
> of P2325R3 so as to maximize backward compatibility with pre-P2325R3 code.
> In particular, we don't remove the default ctors for back_insert_iterator,
> front_insert_iterator, ostream_iterator, ref_view and basic_istream_view.

FWIW here's a diff of the changes in this backport relative to r12-1606:

 libstdc++-v3/include/bits/stl_iterator.h           | 13 ++++++++++-
 libstdc++-v3/include/bits/stream_iterator.h        |  5 +++++
 libstdc++-v3/include/std/ranges                    | 24 +++++++++++++++------
 .../24_iterators/back_insert_iterator/constexpr.cc |  3 ++-
 .../front_insert_iterator/constexpr.cc             |  3 ++-
 .../ostream_iterator/requirements/constexpr.cc     | 24 +++++++++++++++++++++
 libstdc++-v3/testsuite/std/ranges/97600.cc         |  3 ++-
 libstdc++-v3/testsuite/std/ranges/p2325.cc         | 25 ++++++++++++++++++++++
 8 files changed, 89 insertions(+), 11 deletions(-)

diff --git a/libstdc++-v3/include/bits/stl_iterator.h b/libstdc++-v3/include/bits/stl_iterator.h
index 7fe727d8093..549bc26dee5 100644
--- a/libstdc++-v3/include/bits/stl_iterator.h
+++ b/libstdc++-v3/include/bits/stl_iterator.h
@@ -639,6 +639,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       typedef _Container          container_type;
 #if __cplusplus > 201703L
       using difference_type = ptrdiff_t;
+
+      constexpr back_insert_iterator() noexcept : container(nullptr) { }
 #endif
 
       /// The only way to create this %iterator is with a container.
@@ -740,6 +742,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       typedef _Container          container_type;
 #if __cplusplus > 201703L
       using difference_type = ptrdiff_t;
+
+      constexpr front_insert_iterator() noexcept : container(nullptr) { }
 #endif
 
       /// The only way to create this %iterator is with a container.
@@ -839,12 +843,17 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     {
 #if __cplusplus > 201703L && defined __cpp_lib_concepts
       using _Iter = std::__detail::__range_iter_t<_Container>;
+
+    protected:
+      _Container* container = nullptr;
+      _Iter iter = _Iter();
 #else
       typedef typename _Container::iterator		_Iter;
-#endif
+
     protected:
       _Container* container;
       _Iter iter;
+#endif
 
     public:
       /// A nested typedef for the type of whatever container you used.
@@ -852,6 +861,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
 #if __cplusplus > 201703L && defined __cpp_lib_concepts
       using difference_type = ptrdiff_t;
+
+      insert_iterator() = default;
 #endif
 
       /**
diff --git a/libstdc++-v3/include/bits/stream_iterator.h b/libstdc++-v3/include/bits/stream_iterator.h
index d07474d4996..fd8920b8d01 100644
--- a/libstdc++-v3/include/bits/stream_iterator.h
+++ b/libstdc++-v3/include/bits/stream_iterator.h
@@ -192,6 +192,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       const _CharT*	_M_string;
 
     public:
+#if __cplusplus > 201703L
+      constexpr ostream_iterator() noexcept
+      : _M_stream(nullptr), _M_string(nullptr) { }
+#endif
+
       /// Construct from an ostream.
       ostream_iterator(ostream_type& __s)
       : _M_stream(std::__addressof(__s)), _M_string(0) {}
diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
index a01b5e79f1f..bf6cfae2a6e 100644
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -680,6 +680,8 @@ namespace views
     : public view_interface<basic_istream_view<_Val, _CharT, _Traits>>
     {
     public:
+      basic_istream_view() = default;
+
       constexpr explicit
       basic_istream_view(basic_istream<_CharT, _Traits>& __stream)
 	: _M_stream(std::__addressof(__stream))
@@ -688,7 +690,8 @@ namespace views
       constexpr auto
       begin()
       {
-	*_M_stream >> _M_object;
+	if (_M_stream != nullptr)
+	  *_M_stream >> _M_object;
 	return _Iterator{this};
       }
 
@@ -697,8 +700,8 @@ namespace views
       { return default_sentinel; }
 
     private:
-      basic_istream<_CharT, _Traits>* _M_stream;
-      _Val _M_object;
+      basic_istream<_CharT, _Traits>* _M_stream = nullptr;
+      _Val _M_object = _Val();
 
       struct _Iterator
       {
@@ -720,6 +723,7 @@ namespace views
 	_Iterator&
 	operator++()
 	{
+	  __glibcxx_assert(_M_parent->_M_stream != nullptr);
 	  *_M_parent->_M_stream >> _M_parent->_M_object;
 	  return *this;
 	}
@@ -730,18 +734,21 @@ namespace views
 
 	_Val&
 	operator*() const
-	{ return _M_parent->_M_object; }
+	{
+	  __glibcxx_assert(_M_parent->_M_stream != nullptr);
+	  return _M_parent->_M_object;
+	}
 
 	friend bool
 	operator==(const _Iterator& __x, default_sentinel_t)
 	{ return __x._M_at_end(); }
 
       private:
-	basic_istream_view* _M_parent;
+	basic_istream_view* _M_parent = nullptr;
 
 	bool
 	_M_at_end() const
-	{ return !*_M_parent->_M_stream; }
+	{ return _M_parent == nullptr || !*_M_parent->_M_stream; }
       };
 
       friend _Iterator;
@@ -1045,12 +1052,15 @@ namespace views::__adaptor
     class ref_view : public view_interface<ref_view<_Range>>
     {
     private:
-      _Range* _M_r;
+      _Range* _M_r = nullptr;
 
       static void _S_fun(_Range&); // not defined
       static void _S_fun(_Range&&) = delete;
 
     public:
+      constexpr
+      ref_view() noexcept = default;
+
       template<__detail::__not_same_as<ref_view> _Tp>
 	requires convertible_to<_Tp, _Range&>
 	  && requires { _S_fun(declval<_Tp>()); }
diff --git a/libstdc++-v3/testsuite/24_iterators/back_insert_iterator/constexpr.cc b/libstdc++-v3/testsuite/24_iterators/back_insert_iterator/constexpr.cc
index 27acd071df1..bef2289ba79 100644
--- a/libstdc++-v3/testsuite/24_iterators/back_insert_iterator/constexpr.cc
+++ b/libstdc++-v3/testsuite/24_iterators/back_insert_iterator/constexpr.cc
@@ -42,7 +42,8 @@ constexpr bool
 test01()
 {
   container c;
-  std::back_insert_iterator<container> iter = std::back_inserter(c);
+  std::back_insert_iterator<container> iter;
+  iter = std::back_inserter(c);
   *iter++ = 1;
   int i = 2;
   *iter = i;
diff --git a/libstdc++-v3/testsuite/24_iterators/front_insert_iterator/constexpr.cc b/libstdc++-v3/testsuite/24_iterators/front_insert_iterator/constexpr.cc
index cff7f6a4524..7b4c990b107 100644
--- a/libstdc++-v3/testsuite/24_iterators/front_insert_iterator/constexpr.cc
+++ b/libstdc++-v3/testsuite/24_iterators/front_insert_iterator/constexpr.cc
@@ -42,7 +42,8 @@ constexpr bool
 test01()
 {
   container c;
-  std::front_insert_iterator<container> iter = std::front_inserter(c);
+  std::front_insert_iterator<container> iter;
+  iter = std::front_inserter(c);
   *iter++ = 1;
   int i = 2;
   *iter = i;
diff --git a/libstdc++-v3/testsuite/24_iterators/ostream_iterator/requirements/constexpr.cc b/libstdc++-v3/testsuite/24_iterators/ostream_iterator/requirements/constexpr.cc
new file mode 100644
index 00000000000..4edaaa8aebb
--- /dev/null
+++ b/libstdc++-v3/testsuite/24_iterators/ostream_iterator/requirements/constexpr.cc
@@ -0,0 +1,24 @@
+// Copyright (C) 2019-2021 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 compile { target c++2a } }
+
+#include <iterator>
+
+constexpr std::ostream_iterator<int> iter1;
+constexpr std::ostream_iterator<int> iter2{};
diff --git a/libstdc++-v3/testsuite/std/ranges/97600.cc b/libstdc++-v3/testsuite/std/ranges/97600.cc
index c642b9d22d0..7435de022cd 100644
--- a/libstdc++-v3/testsuite/std/ranges/97600.cc
+++ b/libstdc++-v3/testsuite/std/ranges/97600.cc
@@ -24,8 +24,9 @@
 #include <ranges>
 
 void
-test01(std::ranges::basic_istream_view<int, char, std::char_traits<char>> v)
+test01()
 {
+  std::ranges::basic_istream_view<int, char, std::char_traits<char>> v;
   v.begin();
   static_assert(std::ranges::range<decltype(v)>);
 }
diff --git a/libstdc++-v3/testsuite/std/ranges/p2325.cc b/libstdc++-v3/testsuite/std/ranges/p2325.cc
index df6cde29e4d..72ba5c4a35d 100644
--- a/libstdc++-v3/testsuite/std/ranges/p2325.cc
+++ b/libstdc++-v3/testsuite/std/ranges/p2325.cc
@@ -2,6 +2,12 @@
 // { dg-do compile { target c++20 } }
 // P2325R3 "Views should not be required to be default constructible"
 
+// Parts of P2325R3 are deliberately omitted in libstdc++ 11, in particular the
+// removal of default ctors for back_/front_insert_iterator, ostream_iterator,
+// ref_view and basic_istream_view/::iterator, so as to maximize backward
+// compatibility with pre-P2325R3 code.  So most static_asserts in this test fail,
+// see the xfails at the end of this file.
+
 #include <ranges>
 #include <iterator>
 #include <span>
@@ -153,3 +159,22 @@ test11()
   using type2 = ranges::elements_view<ranges::single_view<pair<int,int>>, 0>;
   static_assert(default_initializable<type2>);
 }
+
+// { dg-bogus "static assertion failed" "" { xfail *-*-* } 35 }
+// { dg-bogus "static assertion failed" "" { xfail *-*-* } 36 }
+// { dg-bogus "static assertion failed" "" { xfail *-*-* } 37 }
+// { dg-bogus "static assertion failed" "" { xfail *-*-* } 38 }
+// { dg-bogus "static assertion failed" "" { xfail *-*-* } 43 }
+// { dg-bogus "static assertion failed" "" { xfail *-*-* } 47 }
+// { dg-bogus "static assertion failed" "" { xfail *-*-* } 57 }
+// { dg-bogus "static assertion failed" "" { xfail *-*-* } 58 }
+// { dg-bogus "static assertion failed" "" { xfail *-*-* } 63 }
+// { dg-bogus "static assertion failed" "" { xfail *-*-* } 67 }
+// { dg-bogus "static assertion failed" "" { xfail *-*-* } 76 }
+// { dg-bogus "static assertion failed" "" { xfail *-*-* } 84 }
+// { dg-bogus "static assertion failed" "" { xfail *-*-* } 124 }
+// { dg-bogus "static assertion failed" "" { xfail *-*-* } 126 }
+// { dg-bogus "static assertion failed" "" { xfail *-*-* } 128 }
+// { dg-bogus "static assertion failed" "" { xfail *-*-* } 138 }
+// { dg-bogus "static assertion failed" "" { xfail *-*-* } 148 }
+// { dg-bogus "static assertion failed" "" { xfail *-*-* } 158 }