[gcc(refs/vendors/ARM/heads/arm-perf-staging)] libstdc++: Apply the move_iterator changes described in P1207R4

[Tamar Christina <tnfchris@gcc.gnu.org>]

https://gcc.gnu.org/g:7db12d155ddf4b5d7a5e783a97995f85e19e88dd

commit 7db12d155ddf4b5d7a5e783a97995f85e19e88dd
Author: Patrick Palka <ppalka@redhat.com>
Date:   Wed Jan 22 16:51:19 2020 -0500

    libstdc++: Apply the move_iterator changes described in P1207R4
    
    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 __i
            when initializing _M_current.
            (move_iterator::base): Split into two overloads differing in
            ref-qualifiers as in P1207R4 for C++20.

Diff:
---
 libstdc++-v3/ChangeLog                   |  7 +++++++
 libstdc++-v3/include/bits/stl_iterator.h | 15 ++++++++++++++-
 2 files changed, 21 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/ChangeLog b/libstdc++-v3/ChangeLog
index d925a0b3de8..b54f1f694e4 100644
--- a/libstdc++-v3/ChangeLog
+++ b/libstdc++-v3/ChangeLog
@@ -1,3 +1,10 @@
+2020-02-05  Patrick Palka  <ppalka@redhat.com>
+
+	* include/bits/stl_iterator.h (move_iterator::move_iterator): Move __i
+	when initializing _M_current.
+	(move_iterator::base): Split into two overloads differing in
+	ref-qualifiers as in P1207R4 for C++20.
+
 2020-02-04  Jonathan Wakely  <jwakely@redhat.com>
 
 	* include/std/functional (_GLIBCXX_NOT_FN_CALL_OP): Un-define after
diff --git a/libstdc++-v3/include/bits/stl_iterator.h b/libstdc++-v3/include/bits/stl_iterator.h
index 784d200d22f..c200f7a9d14 100644
--- a/libstdc++-v3/include/bits/stl_iterator.h
+++ b/libstdc++-v3/include/bits/stl_iterator.h
@@ -1166,16 +1166,29 @@ _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
 	move_iterator(const move_iterator<_Iter>& __i)
 	: _M_current(__i.base()) { }
 
+#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
 
       _GLIBCXX17_CONSTEXPR reference
       operator*() const