Allow internal tag field in untagged variant

serde_derive/src/internals/check.rs

@@ -318,6 +318,9 @@ fn check_internal_tag_field_name_conflict(cx: &Ctxt, cont: &Container) {
     for variant in variants {
         match variant.style {
             Style::Struct => {
+                if variant.attrs.untagged() {
+                    continue;
+                }
                 for field in &variant.fields {
                     let check_ser =
                         !(field.attrs.skip_serializing() || variant.attrs.skip_serializing());