Remove EndSentinel, not allowed in C++14.

tdutils/td/utils/FlatHashTable.h

@@ -45,7 +45,8 @@ class FlatHashTable {
   using key_type = typename NodeT::public_key_type;
   using value_type = typename NodeT::public_type;
 
-  struct EndSentinel {};
+  // TODO use EndSentinel for end() after switching to C++17
+  // struct EndSentinel {};
 
   struct Iterator {
     using iterator_category = std::forward_iterator_tag;
@@ -84,10 +85,12 @@ class FlatHashTable {
       return it_;
     }
 
-    bool operator==(const EndSentinel &other) const {
+    bool operator==(const Iterator &other) const {
+      DCHECK(other.it_ == nullptr);
       return it_ == nullptr;
     }
-    bool operator!=(const EndSentinel &other) const {
+    bool operator!=(const Iterator &other) const {
+      DCHECK(other.it_ == nullptr);
       return it_ != nullptr;
     }
 
@@ -119,11 +122,11 @@ class FlatHashTable {
     pointer operator->() const {
       return &*it_;
     }
-    bool operator==(const EndSentinel &other) const {
-      return it_ == other;
+    bool operator==(const ConstIterator &other) const {
+      return it_ == other.it_;
     }
-    bool operator!=(const EndSentinel &other) const {
-      return it_ != other;
+    bool operator!=(const ConstIterator &other) const {
+      return it_ != other.it_;
     }
 
     ConstIterator() = default;
@@ -154,10 +157,10 @@ class FlatHashTable {
       return it_;
     }
 
-    bool operator==(const EndSentinel &other) const {
+    bool operator==(const Iterator &) const {
       return it_ == nullptr;
     }
-    bool operator!=(const EndSentinel &other) const {
+    bool operator!=(const Iterator &) const {
       return it_ != nullptr;
     }
 
@@ -176,10 +179,10 @@ class FlatHashTable {
       return &it_->get_public();
     }
 
-    bool operator==(const EndSentinel &other) const {
+    bool operator==(const ConstIterator &) const {
       return it_ == nullptr;
     }
-    bool operator!=(const EndSentinel &other) const {
+    bool operator!=(const ConstIterator &) const {
       return it_ != nullptr;
     }
 
@@ -282,12 +285,14 @@ class FlatHashTable {
   Iterator begin() {
     return create_iterator(begin_impl());
   }
+  Iterator end() {
+    return Iterator();
+  }
   ConstIterator begin() const {
     return ConstIterator(const_cast<FlatHashTable *>(this)->begin());
   }
-
-  EndSentinel end() const {
-    return EndSentinel();
+  ConstIterator end() const {
+    return ConstIterator();
   }
 
   void reserve(size_t size) {