Implement LWG#2353: std::next is over-constrained

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@252407 91177308-0d34-0410-b5e6-96231b3b80d8

include/iterator

@@ -513,12 +513,12 @@ distance(_InputIter __first, _InputIter __last)
     return __distance(__first, __last, typename iterator_traits<_InputIter>::iterator_category());
 }
 
-template <class _ForwardIter>
+template <class _InputIter>
 inline _LIBCPP_INLINE_VISIBILITY
-_ForwardIter
-next(_ForwardIter __x,
-     typename iterator_traits<_ForwardIter>::difference_type __n = 1,
-     typename enable_if<__is_forward_iterator<_ForwardIter>::value>::type* = 0)
+_InputIter
+next(_InputIter __x,
+     typename iterator_traits<_InputIter>::difference_type __n = 1,
+     typename enable_if<__is_input_iterator<_InputIter>::value>::type* = 0)
 {
     _VSTD::advance(__x, __n);
     return __x;