internal/impl: skip synthetic oneofs in messageInfo

Calling WhichOneof should not be possible for synthetic oneofs;
on the reflection level, these fields should work as if they
were regular fields, not as if they were oneofs:

> Reflection for proto3 optional fields should work properly. For example, a
> method like Reflection::HasField() should know to look for the hasbit for a
> proto3 optional field. It should not be fooled by the synthetic oneof into
> thinking that there is a case member for the oneof.

From the Protobuf docs at:
https://github.com/protocolbuffers/protobuf/blob/main/docs/implementing_proto3_presence.md#updating-reflection

This change was tested Google-internally as CL 701866153.

Change-Id: Id9500d4aa492acf4ebc6d2d84be07ed81201d2aa
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/632735
Reviewed-by: Chressie Himpel <chressie@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

internal/impl/message_reflect.go

@@ -85,7 +85,9 @@ func (mi *MessageInfo) makeKnownFieldsFunc(si structInfo) {
 	mi.oneofs = map[protoreflect.Name]*oneofInfo{}
 	for i := 0; i < md.Oneofs().Len(); i++ {
 		od := md.Oneofs().Get(i)
-		mi.oneofs[od.Name()] = makeOneofInfo(od, si, mi.Exporter)
+		if !od.IsSynthetic() {
+			mi.oneofs[od.Name()] = makeOneofInfo(od, si, mi.Exporter)
+		}
 	}
 
 	mi.denseFields = make([]*fieldInfo, fds.Len()*2)

testing/prototest/message.go

@@ -579,8 +579,13 @@ func testOneof(t testing.TB, m protoreflect.Message, od protoreflect.OneofDescri
 				// Set fields explicitly.
 				m.Set(fda, newValue(m, fda, 1, nil))
 			}
-			if got, want := m.WhichOneof(od), fda; got != want {
-				t.Errorf("after setting oneof field %q:\nWhichOneof(%q) = %v, want %v", fda.FullName(), fda.Name(), got, want)
+			if !od.IsSynthetic() {
+				// Synthetic oneofs are used to represent optional fields in
+				// proto3. While they show up in protoreflect, WhichOneof does
+				// not work on these (only on non-synthetic, explicit oneofs).
+				if got, want := m.WhichOneof(od), fda; got != want {
+					t.Errorf("after setting oneof field %q:\nWhichOneof(%q) = %v, want %v", fda.FullName(), fda.Name(), got, want)
+				}
 			}
 			for j := 0; j < od.Fields().Len(); j++ {
 				fdb := od.Fields().Get(j)