Add `DynamicCastMessage` overloads for `std::shared_ptr` to replace uses of

`std::dynamic_pointer_cast`.

PiperOrigin-RevId: 686603280

src/google/protobuf/lite_unittest.cc

@@ -11,6 +11,7 @@
 #include <cstdint>
 #include <iostream>
 #include <limits>
+#include <memory>
 #include <string>
 #include <type_traits>
 #include <utility>
@@ -1381,6 +1382,17 @@ TEST(LiteTest, DynamicCastMessage) {
   const MessageLite& test_type_1_pointer_const_ref = test_type_1;
   EXPECT_EQ(&test_type_1,
             &DynamicCastMessage<CastType1>(test_type_1_pointer_const_ref));
+
+  std::shared_ptr<MessageLite> shared(new CastType1);
+  EXPECT_EQ(1, shared.use_count());
+  std::shared_ptr<CastType1> shared_1 = DynamicCastMessage<CastType1>(shared);
+  // Check that both shared_ptr instances are pointing to the same control
+  // block by checking use_count().
+  EXPECT_EQ(2, shared.use_count());
+  EXPECT_EQ(shared_1.get(), shared.get());
+  std::shared_ptr<CastType2> shared_2 = DynamicCastMessage<CastType2>(shared);
+  EXPECT_EQ(2, shared.use_count());
+  EXPECT_EQ(shared_2, nullptr);
 }
 
 #if GTEST_HAS_DEATH_TEST

src/google/protobuf/message_lite.h

@@ -21,9 +21,11 @@
 #include <cstdint>
 #include <cstring>
 #include <iosfwd>
+#include <memory>
 #include <new>
 #include <string>
 #include <type_traits>
+#include <utility>
 
 #include "absl/base/attributes.h"
 #include "absl/base/casts.h"
@@ -1438,6 +1440,28 @@ T& DownCastToGenerated(MessageLite& from) {
   return DownCastMessage<T>(from);
 }
 
+// Overloads for `std::shared_ptr` to substitute `std::dynamic_pointer_cast`
+template <typename T>
+std::shared_ptr<T> DynamicCastMessage(std::shared_ptr<MessageLite> ptr) {
+  if (auto* res = DynamicCastMessage<T>(ptr.get())) {
+    // Use aliasing constructor to keep the same control block.
+    return std::shared_ptr<T>(std::move(ptr), res);
+  } else {
+    return nullptr;
+  }
+}
+
+template <typename T>
+std::shared_ptr<const T> DynamicCastMessage(
+    std::shared_ptr<const MessageLite> ptr) {
+  if (auto* res = DynamicCastMessage<T>(ptr.get())) {
+    // Use aliasing constructor to keep the same control block.
+    return std::shared_ptr<const T>(std::move(ptr), res);
+  } else {
+    return nullptr;
+  }
+}
+
 }  // namespace protobuf
 }  // namespace google