Use rb_obj_class instead of CLASS_OF in Ruby C to ignore singleton classes (#9342)

Co-authored-by: Adam Cozzette <acozzette@google.com>

ruby/ext/google/protobuf_c/convert.c

@@ -162,9 +162,9 @@ upb_MessageValue Convert_RubyToUpb(VALUE value, const char* name,
     }
     case kUpb_CType_String: {
       VALUE utf8 = rb_enc_from_encoding(rb_utf8_encoding());
-      if (CLASS_OF(value) == rb_cSymbol) {
+      if (rb_obj_class(value) == rb_cSymbol) {
         value = rb_funcall(value, rb_intern("to_s"), 0);
-      } else if (CLASS_OF(value) != rb_cString) {
+      } else if (rb_obj_class(value) != rb_cString) {
         rb_raise(cTypeError,
                  "Invalid argument for string field '%s' (given %s).", name,
                  rb_class2name(CLASS_OF(value)));
@@ -185,7 +185,7 @@ upb_MessageValue Convert_RubyToUpb(VALUE value, const char* name,
     }
     case kUpb_CType_Bytes: {
       VALUE bytes = rb_enc_from_encoding(rb_ascii8bit_encoding());
-      if (CLASS_OF(value) != rb_cString) {
+      if (rb_obj_class(value) != rb_cString) {
         rb_raise(cTypeError,
                  "Invalid argument for bytes field '%s' (given %s).", name,
                  rb_class2name(CLASS_OF(value)));

ruby/tests/basic.rb

@@ -644,5 +644,19 @@ module BasicTest
       assert_equal 2, m.map_string_int32.size
       assert_equal 1, m.map_string_msg.size
     end
+
+    def test_string_with_singleton_class_enabled
+      str = 'foobar'
+      # NOTE: Accessing a singleton class of an object changes its low level class representation
+      #       as far as the C API's CLASS_OF() method concerned, exposing the issue
+      str.singleton_class
+      m = proto_module::TestMessage.new(
+        optional_string: str,
+        optional_bytes: str
+      )
+
+      assert_equal str, m.optional_string
+      assert_equal str, m.optional_bytes
+    end
   end
 end