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

[Jonathan Wakely <jwakely@redhat.com>]

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