Fix for "cannot infer type" from Deserialize derive macro with simple

variants and untagged variants

serde_derive/src/de.rs

@@ -1737,7 +1737,6 @@ fn deserialize_untagged_enum_after(
                 quote!(__deserializer),
             ))
         });
-    let attempts = first_attempt.into_iter().chain(attempts);
     // TODO this message could be better by saving the errors from the failed
     // attempts. The heuristic used by TOML was to count the number of fields
     // processed before an error, and use the error that happened after the
@@ -1750,10 +1749,19 @@ fn deserialize_untagged_enum_after(
     );
     let fallthrough_msg = cattrs.expecting().unwrap_or(&fallthrough_msg);
 
+    // This may be infallible so we need to provide the error type.
+    let first_attempt = first_attempt.map(|expr| {
+        quote! {
+            if let _serde::__private::Result::<_, __D::Error>::Ok(__ok) = #expr {
+                return _serde::__private::Ok(__ok);
+            }
+        }
+    });
     quote_block! {
         let __content = <_serde::__private::de::Content as _serde::Deserialize>::deserialize(__deserializer)?;
         let __deserializer = _serde::__private::de::ContentRefDeserializer::<__D::Error>::new(&__content);
 
+        #first_attempt
         #(
             if let _serde::__private::Ok(__ok) = #attempts {
                 return _serde::__private::Ok(__ok);

test_suite/tests/test_annotations.rs

@@ -2383,7 +2383,7 @@ fn test_partially_untagged_enum_desugared() {
 #[test]
 fn test_partially_untagged_simple_enum() {
     #[derive(Serialize, Deserialize, PartialEq, Debug)]
-    #[serde(tag = "tag")]
+    #[serde(tag = "t")]
     enum Data {
         A,
         #[serde(untagged)]
@@ -2391,7 +2391,8 @@ fn test_partially_untagged_simple_enum() {
     }
 
     let data = Data::A;
-    assert_tokens(
+
+    assert_de_tokens(
         &data,
         &[
             Token::Map { len: None },