message: Check for enum variant explicitly for error message
  Summary: Error message says "union", so let's check it is union.
diff: @@ -100,7 +100,7 @@ fn derive_coerce_params(input: DeriveInput) -> syn::Result<proc_macro2::TokenStr
  let fields: Vec<_> = match &input.data {
  Data::Struct(x) => x.fields.iter().collect(),
  Data::Enum(x) => x.variants.iter().flat_map(|x| &x.fields).collect(),
  - _ => {
  + Data::Union(..) => {
  return Err(syn::Error::new_spanned(
  input,
  "Type-parameter cannot be a union",
  