Re: [PATCH 1/3] libstdc++: Apply the move_iterator changes described in P1207R4

[Jonathan Wakely]

On 03/02/20 21:07 -0500, Patrick Palka wrote:

These changes are needed for some of the tests in the constrained algorithm
patch, because they use move_iterator with an uncopyable output_iterator.  The
other changes described in the paper are already applied, it seems.

libstdc++-v3/ChangeLog:

        * include/bits/stl_iterator.h (move_iterator::move_iterator): Move the
        iterator when initializing _M_current.
        (move_iterator::base): Split into two overloads differing in
        ref-qualifiers as in P1207R4.
---
libstdc++-v3/include/bits/stl_iterator.h | 11 +++++++++--
1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/libstdc++-v3/include/bits/stl_iterator.h 
b/libstdc++-v3/include/bits/stl_iterator.h
index 784d200d22f..1a288a5c785 100644
--- a/libstdc++-v3/include/bits/stl_iterator.h
+++ b/libstdc++-v3/include/bits/stl_iterator.h
@@ -1166,7 +1166,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION

      explicit _GLIBCXX17_CONSTEXPR
      move_iterator(iterator_type __i)
-      : _M_current(__i) { }
+      : _M_current(std::move(__i)) { }

      template<typename _Iter>
        _GLIBCXX17_CONSTEXPR
@@ -1174,9 +1174,16 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
        : _M_current(__i.base()) { }

      _GLIBCXX17_CONSTEXPR iterator_type
-      base() const
+      base() const &
+#if __cplusplus > 201703L && __cpp_lib_concepts
+       requires copy_constructible<iterator_type>
+#endif
      { return _M_current; }

+      _GLIBCXX17_CONSTEXPR iterator_type
+      base() &&
+      { return std::move(_M_current); }
+

I think this change has to be restricted to C++20 and later, otherwise
a move_iterator in C++17 can change state so that its _M_current
becomes moved-from, which should not be possible before C++20.

So something like:

#if __cplusplus <= 201703L
      _GLIBCXX17_CONSTEXPR iterator_type
      base() const
      { return _M_current; }
#else
      constexpr iterator_type
      base() const &
#if __cpp_lib_concepts
        requires copy_constructible<iterator_type>
#endif
      { return _M_current; }

      constexpr iterator_type
      base() &&
      { return std::move(_M_current); }
#endif