Improve error messages when comparing btree iterators.

- Add assertions that the iterators are either (a) from the same container or (b) both default constructed. Standard says: "The domain of == for forward iterators is that of iterators over the same underlying sequence. However, value-initialized iterators may be compared and shall compare equal to other value-initialized iterators of the same type." - [reference](https://eel.is/c++draft/forward.iterators#2).
- Also optimize IsEndIterator a bit.

PiperOrigin-RevId: 487617518
Change-Id: Iefba5c3bc8caa93954671793e6929e22f419c298

absl/container/btree_test.cc

@@ -3358,6 +3358,31 @@ TEST(Btree, DereferencingEndIterator) {
   EXPECT_DEATH(*set.end(), R"regex(Dereferencing end\(\) iterator)regex");
 }
 
+TEST(Btree, InvalidIteratorComparison) {
+  if (!IsAssertEnabled()) GTEST_SKIP() << "Assertions not enabled.";
+
+  absl::btree_set<int> set1, set2;
+  for (int i = 0; i < 1000; ++i) {
+    set1.insert(i);
+    set2.insert(i);
+  }
+
+  constexpr const char *kValueInitDeathMessage =
+      "Comparing default-constructed iterator with .*non-default-constructed "
+      "iterator";
+  typename absl::btree_set<int>::iterator iter1, iter2;
+  EXPECT_EQ(iter1, iter2);
+  EXPECT_DEATH(void(set1.begin() == iter1), kValueInitDeathMessage);
+  EXPECT_DEATH(void(iter1 == set1.begin()), kValueInitDeathMessage);
+
+  constexpr const char *kDifferentContainerDeathMessage =
+      "Comparing iterators from different containers";
+  iter1 = set1.begin();
+  iter2 = set2.begin();
+  EXPECT_DEATH(void(iter1 == iter2), kDifferentContainerDeathMessage);
+  EXPECT_DEATH(void(iter2 == iter1), kDifferentContainerDeathMessage);
+}
+
 }  // namespace
 }  // namespace container_internal
 ABSL_NAMESPACE_END

absl/container/internal/btree.h

@@ -1017,6 +1017,17 @@ class btree_node {
   friend struct btree_access;
 };
 
+template <typename Node>
+bool AreNodesFromSameContainer(const Node *node_a, const Node *node_b) {
+  // If either node is null, then give up on checking whether they're from the
+  // same container. (If exactly one is null, then we'll trigger the
+  // default-constructed assert in Equals.)
+  if (node_a == nullptr || node_b == nullptr) return true;
+  while (!node_a->is_root()) node_a = node_a->parent();
+  while (!node_b->is_root()) node_b = node_b->parent();
+  return node_a == node_b;
+}
+
 template <typename Node, typename Reference, typename Pointer>
 class btree_iterator {
   using field_type = typename Node::field_type;
@@ -1073,16 +1084,16 @@ class btree_iterator {
   }
 
   bool operator==(const iterator &other) const {
-    return node_ == other.node_ && position_ == other.position_;
+    return Equals(other.node_, other.position_);
   }
   bool operator==(const const_iterator &other) const {
-    return node_ == other.node_ && position_ == other.position_;
+    return Equals(other.node_, other.position_);
   }
   bool operator!=(const iterator &other) const {
-    return node_ != other.node_ || position_ != other.position_;
+    return !Equals(other.node_, other.position_);
   }
   bool operator!=(const const_iterator &other) const {
-    return node_ != other.node_ || position_ != other.position_;
+    return !Equals(other.node_, other.position_);
   }
 
   // Returns n such that n calls to ++other yields *this.
@@ -1161,13 +1172,27 @@ class btree_iterator {
 #endif
   }
 
+  bool Equals(const node_type *other_node, int other_position) const {
+    ABSL_HARDENING_ASSERT(((node_ == nullptr && other_node == nullptr) ||
+                           (node_ != nullptr && other_node != nullptr)) &&
+                          "Comparing default-constructed iterator with "
+                          "non-default-constructed iterator.");
+    // Note: we use assert instead of ABSL_HARDENING_ASSERT here because this
+    // changes the complexity of Equals from O(1) to O(log(N) + log(M)) where
+    // N/M are sizes of the containers containing node_/other_node.
+    assert(AreNodesFromSameContainer(node_, other_node) &&
+           "Comparing iterators from different containers.");
+    return node_ == other_node && position_ == other_position;
+  }
+
   bool IsEndIterator() const {
     if (position_ != node_->finish()) return false;
-    // Navigate to the rightmost node.
     node_type *node = node_;
-    while (!node->is_root()) node = node->parent();
-    while (node->is_internal()) node = node->child(node->finish());
-    return node == node_;
+    while (!node->is_root()) {
+      if (node->position() != node->parent()->finish()) return false;
+      node = node->parent();
+    }
+    return true;
   }
 
   // Returns n such that n calls to ++other yields *this.