[committed] libstdc++: Add missing constraint to operator+ for std::move_iterator

[Jonathan Wakely]

This constraint was added by the One Ranges proposal (P0896R4) and
then fixed by LWG 3293, but it was missing from libstdc++.

libstdc++-v3/ChangeLog:

        * include/bits/stl_iterator.h (operator+): Add constraint to
        move_iterator operator.
        * testsuite/24_iterators/move_iterator/rel_ops_c++20.cc:
---

Tested x86_64-linux. Pushed to trunk.

 libstdc++-v3/include/bits/stl_iterator.h             |  4 ++++
 .../24_iterators/move_iterator/rel_ops_c++20.cc      | 12 ++++++++++++
 2 files changed, 16 insertions(+)

diff --git a/libstdc++-v3/include/bits/stl_iterator.h 
b/libstdc++-v3/include/bits/stl_iterator.h
index 5ea901bf1f6..e872598d7d8 100644
--- a/libstdc++-v3/include/bits/stl_iterator.h
+++ b/libstdc++-v3/include/bits/stl_iterator.h
@@ -1731,6 +1731,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     inline _GLIBCXX17_CONSTEXPR bool
     operator==(const move_iterator<_Iterator>& __x,
               const move_iterator<_Iterator>& __y)
+    // N.B. No contraints, x.base() == y.base() is always well-formed.
     { return __x.base() == __y.base(); }
 
 #ifdef __cpp_lib_three_way_comparison
@@ -1791,6 +1792,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     inline _GLIBCXX17_CONSTEXPR move_iterator<_Iterator>
     operator+(typename move_iterator<_Iterator>::difference_type __n,
              const move_iterator<_Iterator>& __x)
+#ifdef __glibcxx_concepts
+    requires requires { { __x.base() + __n } -> same_as<_Iterator>; }
+#endif
     { return __x + __n; }
 
   template<typename _Iterator>
diff --git a/libstdc++-v3/testsuite/24_iterators/move_iterator/rel_ops_c++20.cc 
b/libstdc++-v3/testsuite/24_iterators/move_iterator/rel_ops_c++20.cc
index b2c1fe0e9f8..deb17722257 100644
--- a/libstdc++-v3/testsuite/24_iterators/move_iterator/rel_ops_c++20.cc
+++ b/libstdc++-v3/testsuite/24_iterators/move_iterator/rel_ops_c++20.cc
@@ -18,6 +18,7 @@
 // { dg-do compile { target c++20 } }
 
 #include <iterator>
+#include <testsuite_iterators.h>
 
 template<int>
 struct Iter
@@ -141,3 +142,14 @@ static_assert( cend > beg );
 static_assert( beg <= cend );
 static_assert( cend >= beg );
 static_assert( std::is_lt(beg <=> cend) );
+
+template<typename I>
+  concept has_plus = requires(std::iter_difference_t<I> n, I i) {
+       { n + i } -> std::same_as<I>;
+  };
+
+using namespace __gnu_test;
+using MBI = std::move_iterator<bidirectional_iterator_wrapper<int>>;
+static_assert( ! has_plus<MBI> );
+using MRI = std::move_iterator<random_access_iterator_wrapper<int>>;
+static_assert( has_plus<MRI> );
-- 
2.47.0