[libc++] Fix `_IterOps::__iter_move` to support proxy iterators.

The return type was specified incorrectly for proxy iterators that
define `reference` to be a class that implicitly converts to
`value_type`. `__iter_move` would end up returning an object of type
`reference` which would then implicitly convert to `value_type`; thus,
the function will return a `value_type&&` rvalue reference to the local
temporary.

Differential Revision: https://reviews.llvm.org/D130197

1 files changed, 4 insertions, 2 deletions

diff --git a/libcxx/include/__algorithm/iterator_operations.h b/libcxx/include/__algorithm/iterator_operations.h
index 03dd2c41e94a..a3840594b8ae 100644
--- a/libcxx/include/__algorithm/iterator_operations.h
+++ b/libcxx/include/__algorithm/iterator_operations.h
@@ -66,8 +66,10 @@ struct _IterOps<_ClassicAlgPolicy> {
   // iter_move
   template <class _Iter>
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_AFTER_CXX11
-  // Declaring the return type is necessary for the C++03 mode (which doesn't support placeholder return types).
-  static typename iterator_traits<__uncvref_t<_Iter> >::value_type&& __iter_move(_Iter&& __i) {
+  // Declaring the return type is necessary for C++03, so we basically mirror what `decltype(auto)` would deduce.
+  static typename remove_reference<
+      typename iterator_traits<__uncvref_t<_Iter> >::reference
+  >::type&& __iter_move(_Iter&& __i) {
     return std::move(*std::forward<_Iter>(__i));
   }