Clarify the preconditions of AddXXX functions.

Also, add a debug check to verify this precondition.

PiperOrigin-RevId: 574538974

src/google/protobuf/repeated_field_unittest.cc

@@ -1677,6 +1677,14 @@ TEST(RepeatedPtrField, AddAllocated) {
   EXPECT_EQ(moo, &field.Get(index));
 }
 
+TEST(RepeatedPtrField, AddMethodsDontAcceptNull) {
+#if !defined(NDEBUG)
+  RepeatedPtrField<std::string> field;
+  EXPECT_DEATH(field.AddAllocated(nullptr), "nullptr");
+  EXPECT_DEATH(field.UnsafeArenaAddAllocated(nullptr), "nullptr");
+#endif
+}
+
 TEST(RepeatedPtrField, AddAllocatedDifferentArena) {
   RepeatedPtrField<TestAllTypes> field;
   Arena arena;

src/google/protobuf/repeated_ptr_field.h

@@ -443,11 +443,13 @@ class PROTOBUF_EXPORT RepeatedPtrFieldBase {
   template <typename TypeHandler>
   void AddAllocated(Value<TypeHandler>* value) {
     typename TypeImplementsMergeBehavior<Value<TypeHandler>>::type t;
+    ABSL_DCHECK_NE(value, nullptr);
     AddAllocatedInternal<TypeHandler>(value, t);
   }
 
   template <typename TypeHandler>
   void UnsafeArenaAddAllocated(Value<TypeHandler>* value) {
+    ABSL_DCHECK_NE(value, nullptr);
     // Make room for the new pointer.
     if (current_size_ == total_size_) {
       // The array is completely full with no cleared objects, so grow it.
@@ -1179,6 +1181,7 @@ class RepeatedPtrField final : private internal::RepeatedPtrFieldBase {
   //   (ii) if this field holds strings, the passed-in string *must* be
   //   heap-allocated, not arena-allocated. There is no way to dynamically check
   //   this at runtime, so User Beware.
+  // Requires:  value != nullptr
   void AddAllocated(Element* value);
 
   // Removes and returns the last element, passing ownership to the caller.
@@ -1201,6 +1204,7 @@ class RepeatedPtrField final : private internal::RepeatedPtrFieldBase {
   // If you put temp_field on the arena this fails, because the ownership
   // transfers to the arena at the "AddAllocated" call and is not released
   // anymore, causing a double delete. UnsafeArenaAddAllocated prevents this.
+  // Requires:  value != nullptr
   void UnsafeArenaAddAllocated(Element* value);
 
   // Removes and returns the last element.  Unlike ReleaseLast, the returned