[PATCH 1/2] libstdc++: Implement LWG 3391 changes to move/counted_iterator::base

[Jonathan Wakely]

On 05/05/21 16:23 -0400, Patrick Palka via Libstdc++ wrote:
>Tested on x86_64-pc-linux-gnu, does this look OK for trunk/10/11?

Yes, thanks.

>libstdc++-v3/ChangeLog:
>
>	* include/bits/stl_iterator.h (move_iterator::base): Make the
>	const& overload return a const reference and remove its
>	constraint as per LWG 3391.  Make unconditionally noexcept.
>	(counted_iterator::base): Likewise.
>	* testsuite/24_iterators/move_iterator/lwg3391.cc: New test.
>	* testsuite/24_iterators/move_iterator/move_only.cc: Adjust
>	has_member_base concept to decay-copy the result of base().
>---
> libstdc++-v3/include/bits/stl_iterator.h      | 13 ++-----
> .../24_iterators/move_iterator/lwg3391.cc     | 37 +++++++++++++++++++
> .../24_iterators/move_iterator/move_only.cc   |  8 +++-
> 3 files changed, 48 insertions(+), 10 deletions(-)
> create mode 100644 libstdc++-v3/testsuite/24_iterators/move_iterator/lwg3391.cc
>
>diff --git a/libstdc++-v3/include/bits/stl_iterator.h b/libstdc++-v3/include/bits/stl_iterator.h
>index 049f83cff90..2409cd71f86 100644
>--- a/libstdc++-v3/include/bits/stl_iterator.h
>+++ b/libstdc++-v3/include/bits/stl_iterator.h
>@@ -1409,11 +1409,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>       base() const
>       { return _M_current; }
> #else
>-      constexpr iterator_type
>-      base() const &
>-#if __cpp_lib_concepts
>-	requires copy_constructible<iterator_type>
>-#endif
>+      constexpr const iterator_type&
>+      base() const & noexcept
>       { return _M_current; }
>
>       constexpr iterator_type
>@@ -2141,10 +2138,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> 	  return *this;
> 	}
>
>-      constexpr _It
>-      base() const &
>-      noexcept(is_nothrow_copy_constructible_v<_It>)
>-      requires copy_constructible<_It>
>+      constexpr const _It&
>+      base() const & noexcept
>       { return _M_current; }
>
>       constexpr _It
>diff --git a/libstdc++-v3/testsuite/24_iterators/move_iterator/lwg3391.cc b/libstdc++-v3/testsuite/24_iterators/move_iterator/lwg3391.cc
>new file mode 100644
>index 00000000000..18e015777cd
>--- /dev/null
>+++ b/libstdc++-v3/testsuite/24_iterators/move_iterator/lwg3391.cc
>@@ -0,0 +1,37 @@
>+// Copyright (C) 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 } }
>+
>+// Verify LWG 3391 changes.
>+
>+#include <iterator>
>+#include <ranges>
>+
>+#include <testsuite_iterators.h>
>+
>+using __gnu_test::test_range;
>+using __gnu_test::input_iterator_wrapper_nocopy;
>+
>+void
>+test01()
>+{
>+  extern test_range<int, input_iterator_wrapper_nocopy> rx;
>+  auto v = rx | std::views::take(5);
>+  std::ranges::begin(v) != std::ranges::end(v);
>+}
>diff --git a/libstdc++-v3/testsuite/24_iterators/move_iterator/move_only.cc b/libstdc++-v3/testsuite/24_iterators/move_iterator/move_only.cc
>index 639adfab0e2..5537dfbf3cd 100644
>--- a/libstdc++-v3/testsuite/24_iterators/move_iterator/move_only.cc
>+++ b/libstdc++-v3/testsuite/24_iterators/move_iterator/move_only.cc
>@@ -43,7 +43,13 @@ template<> struct std::iterator_traits<move_only_iterator>
> static_assert(std::input_iterator<move_only_iterator>);
>
> template<typename T>
>-  concept has_member_base = requires (T t) { std::forward<T>(t).base(); };
>+  concept has_member_base = requires (T t) {
>+    // LWG 3391 made the const& overload of move_iterator::base()
>+    // unconstrained and return a const reference.  So rather than checking
>+    // whether base() is valid (which is now trivially true in an unevaluated
>+    // context), the below now checks whether decay-copying base() is valid.
>+    [](auto){}(std::forward<T>(t).base());
>+  };
>
> using move_only_move_iterator = std::move_iterator<move_only_iterator>;
>
>-- 
>2.31.1.442.g7e39198978
>