[PATCH v2] libstdc++: Constrain move_iterator default constructor.

[Tomasz Kamiński]

This implements LWG4125, "move_iterator's default constructor should
be constrained". As for other operations on move_iterator, the constrain
is only when concepts are available (__glibcxx_concepts).

libstdc++-v3/ChangeLog:

        * include/bits/stl_iterator.h (move_iterator::move_iterator())
        [__glibcxx_concepts]: Constrain with default_initializable.
        * testsuite/24_iterators/move_iterator/default_init.cc: New test.
---
v2 actually includes test file mentioned in ChangeLog.


 libstdc++-v3/include/bits/stl_iterator.h      |  5 ++++
 .../move_iterator/default_init.cc             | 25 +++++++++++++++++++
 2 files changed, 30 insertions(+)
 create mode 100644 
libstdc++-v3/testsuite/24_iterators/move_iterator/default_init.cc

diff --git a/libstdc++-v3/include/bits/stl_iterator.h 
b/libstdc++-v3/include/bits/stl_iterator.h
index bcc22c50de5..c8418266e0c 100644
--- a/libstdc++-v3/include/bits/stl_iterator.h
+++ b/libstdc++-v3/include/bits/stl_iterator.h
@@ -1508,8 +1508,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
                          __base_ref>;
 #endif
 
+      // _GLIBCXX_RESOLVE_LIB_DEFECTS
+      // 4125. move_iterator's default constructor should be constrained
       _GLIBCXX17_CONSTEXPR
       move_iterator()
+#ifdef __glibcxx_concepts
+       requires default_initializable<_Iterator>
+#endif
       : _M_current() { }
 
       explicit _GLIBCXX17_CONSTEXPR
diff --git a/libstdc++-v3/testsuite/24_iterators/move_iterator/default_init.cc 
b/libstdc++-v3/testsuite/24_iterators/move_iterator/default_init.cc
new file mode 100644
index 00000000000..5134ef57009
--- /dev/null
+++ b/libstdc++-v3/testsuite/24_iterators/move_iterator/default_init.cc
@@ -0,0 +1,25 @@
+// { dg-do compile { target c++20 } }
+
+#include <iterator>
+
+struct no_default_init_iterator
+{
+  using value_type = int;
+  using difference_type = std::ptrdiff_t;
+  using iterator_category = std::input_iterator_tag;
+      
+  no_default_init_iterator() = delete;
+  no_default_init_iterator(no_default_init_iterator&&) = default;
+  no_default_init_iterator& operator=(no_default_init_iterator&&) = default;
+
+  no_default_init_iterator& operator++();
+  no_default_init_iterator operator++(int);
+  int& operator*() const;
+
+  bool operator==(const no_default_init_iterator&) const;
+};
+
+static_assert( std::is_default_constructible_v<std::move_iterator<int*>> );
+static_assert( 
!std::is_default_constructible_v<std::move_iterator<no_default_init_iterator>> 
);
+
+
-- 
2.55.0