Improve b-tree error messages when dereferencing invalid iterators.

- Check for invalid generation before checking for other types of invalid iterators.
- Check specifically for dereferencing end() iterators.

PiperOrigin-RevId: 483725646
Change-Id: Ibca19c48b1b242384683580145be8fb9ae707bc8

absl/container/btree_test.cc

@@ -3340,6 +3340,14 @@ TEST(Btree, IteratorSubtraction) {
   }
   }
 }
 }
 
 
+#ifndef NDEBUG
+TEST(Btree, DereferencingEndIterator) {
+  absl::btree_set<int> set;
+  for (int i = 0; i < 1000; ++i) set.insert(i);
+  EXPECT_DEATH(*set.end(), R"regex(Dereferencing end\(\) iterator)regex");
+}
+#endif
+
 }  // namespace
 }  // namespace
 }  // namespace container_internal
 }  // namespace container_internal
 ABSL_NAMESPACE_END
 ABSL_NAMESPACE_END

absl/container/internal/btree.h

@@ -1098,9 +1098,12 @@ class btree_iterator {
   // Accessors for the key/value the iterator is pointing at.
   // Accessors for the key/value the iterator is pointing at.
   reference operator*() const {
   reference operator*() const {
     ABSL_HARDENING_ASSERT(node_ != nullptr);
     ABSL_HARDENING_ASSERT(node_ != nullptr);
-    ABSL_HARDENING_ASSERT(node_->start() <= position_);
-    ABSL_HARDENING_ASSERT(node_->finish() > position_);
     assert_valid_generation();
     assert_valid_generation();
+    ABSL_HARDENING_ASSERT(position_ >= node_->start());
+    if (position_ >= node_->finish()) {
+      ABSL_HARDENING_ASSERT(!IsEndIterator() && "Dereferencing end() iterator");
+      ABSL_HARDENING_ASSERT(position_ < node_->finish());
+    }
     return node_->value(static_cast<field_type>(position_));
     return node_->value(static_cast<field_type>(position_));
   }
   }
   pointer operator->() const { return &operator*(); }
   pointer operator->() const { return &operator*(); }
@@ -1158,6 +1161,15 @@ class btree_iterator {
 #endif
 #endif
   }
   }
 
 
+  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_;
+  }
+
   // Returns n such that n calls to ++other yields *this.
   // Returns n such that n calls to ++other yields *this.
   // Precondition: n exists && (this->node_ != other.node_ ||
   // Precondition: n exists && (this->node_ != other.node_ ||
   // !this->node_->is_leaf() || this->position_ != other.position_).
   // !this->node_->is_leaf() || this->position_ != other.position_).