Disallow wrapping input iterators.

Iterator::Next() has this line:

  if (it_ != end(*self_)) return &*it_++;

The reference returned by *it_ doesn't have to be valid after the iterator is
incremented; input iterators might create these values on the fly (and don't
store them).

TGP: https://test.corp.google.com/ui#id=OCL:387627910:BASE:391752438:1629382590029:3d364c60

See also: http://g/c-users/xvDy7spjpeM

PiperOrigin-RevId: 392401203

clif/python/stltypes.h

@@ -27,6 +27,7 @@ headers are included.
 #include "Python.h"
 #include <functional>
 #include <deque>
+#include <iterator>
 #include <list>
 #include <queue>
 #include <stack>
@@ -284,6 +285,11 @@ class Iterator {
   static_assert(std::is_same<T&,
                 typename std::iterator_traits<ContainerIter>::reference>::value,
       "Iterators returning proxy refererence not currently supported.");
+  static_assert(
+      std::is_base_of_v<
+          std::forward_iterator_tag,
+          typename std::iterator_traits<ContainerIter>::iterator_category>,
+      "Input iterators are not currently supported.");
 
   const T* Next() noexcept {
     if (!self_) return nullptr;

clif/testing/iterator.h

@@ -22,14 +22,15 @@
 // It's using exceptions to demonstrate they are handled correctly by CLIF.
 
 #include <array>
+#include <iterator>
 
 namespace clif_iterator_test {
 
 template<typename T, size_t N>
 class Ring {
  public:
   // Mimic std containers, so that Ring::const_iterator is a real thing.
-  class const_iterator: public std::iterator<std::input_iterator_tag, T> {
+  class const_iterator: public std::iterator<std::forward_iterator_tag, T> {
    public:
     // CLIF requires both or neither of copy/move constructors and copy/move
     // copy assignment operators.
@@ -59,6 +60,7 @@ class Ring {
       return start_ != o.start_ || offset_ != o.offset_;
     }
     const T& operator*() const { return *(start_+offset_); }
+    const T* operator->() const { return start_ + offset_; }
 
    private:
     size_t offset_;  // Ring<T>::data_ current index.