fix deprecation of std::iterator (#7332)

When compiling cvc5 with clang-13, it will emit lots of warnings of usages of `std::iterator` as it's deprecated since C++17. 
The recommended implementation of iterators is to manually define the following five types: 
```
template< class Iter >
struct iterator_traits;

difference_type	        Iter::difference_type
value_type	        Iter::value_type
pointer	                Iter::pointer
reference	        Iter::reference
iterator_category	Iter::iterator_category

```
And the iterator-related types could be accessed by for example `typename std::iterator_traits<Iter>::value_type value`. 


This pull request performs the fix, and the program is semantically equivalent before and after the fix. 

References: 
https://en.cppreference.com/w/cpp/iterator/iterator_traits 
https://en.cppreference.com/w/cpp/iterator/iterator 

1 files changed, 20 insertions, 1 deletions

diff --git a/src/expr/node_self_iterator.h b/src/expr/node_self_iterator.h
index de6082ead..945038fbf 100644
--- a/src/expr/node_self_iterator.h
+++ b/src/expr/node_self_iterator.h
@@ -26,11 +26,30 @@
 namespace cvc5 {
 namespace expr {
 
-class NodeSelfIterator : public std::iterator<std::input_iterator_tag, Node> {
+class NodeSelfIterator {
   Node d_node;
   Node::const_iterator d_child;
 
 public:
+  /* The following types are required by trait std::iterator_traits */
+
+  /** Iterator tag */
+  using iterator_category = std::forward_iterator_tag;
+
+  /** The type of the item */
+  using value_type = Node;
+
+  /** The pointer type of the item */
+  using pointer = Node*;
+
+  /** The reference type of the item */
+  using reference = Node&;
+
+  /** The type returned when two iterators are subtracted */
+  using difference_type = std::ptrdiff_t;
+
+  /* End of std::iterator_traits required types */
+
   static NodeSelfIterator self(TNode n);
   static NodeSelfIterator selfEnd(TNode n);