Add Oneof custom options test

csharp/protos/unittest_custom_options_proto3.proto

@@ -68,10 +68,9 @@ extend google.protobuf.FieldOptions {
   fixed64 field_opt1 = 7740936;
 }
 
-// See https://github.com/google/protobuf/issues/2316
-// extend google.protobuf.OneofOptions {
-//   int32 oneof_opt1 = 7740111;
-// }
+extend google.protobuf.OneofOptions {
+  int32 oneof_opt1 = 7740111;
+}
 
 extend google.protobuf.EnumOptions {
   sfixed32 enum_opt1 = 7753576;
@@ -106,8 +105,7 @@ message TestMessageWithCustomOptions {
                               (field_opt1)=8765432109];
 
   oneof AnOneof {
-// See https://github.com/google/protobuf/issues/2316
-//    option (oneof_opt1) = -99;
+    option (oneof_opt1) = -99;
     int32 oneof_field = 2;
   }
 

csharp/src/Google.Protobuf.Test/Reflection/CustomOptionsTest.cs

@@ -50,6 +50,7 @@ namespace Google.Protobuf.Test.Reflection
         FileOpt1 = 7736974,
         MessageOpt1 = 7739036,
         FieldOpt1 = 7740936,
+        OneofOpt1 = 7740111,
         EnumOpt1 = 7753576,
         EnumValueOpt1 = 1560678,
         ServiceOpt1 = 7887650,
@@ -197,6 +198,9 @@ namespace Google.Protobuf.Test.Reflection
             var fieldOptions = TestMessageWithCustomOptions.Descriptor.Fields["field1"] .CustomOptions;
             AssertOption(8765432109UL, fieldOptions.TryGetFixed64, FieldOpt1);
 
+            var oneofOptions = TestMessageWithCustomOptions.Descriptor.Oneofs[0].CustomOptions;
+            AssertOption(-99, oneofOptions.TryGetInt32, OneofOpt1);
+
             var enumOptions = TestMessageWithCustomOptions.Descriptor.EnumTypes[0].CustomOptions;
             AssertOption(-789, enumOptions.TryGetSFixed32, EnumOpt1);