"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 +1749,23 @@ fn deserialize_untagged_enum_after(
     );
     let fallthrough_msg = cattrs.expecting().unwrap_or(&fallthrough_msg);
 
+    // Ignore any error associated with non-untagged deserialization so that we
+    // can fall through to the untagged variants. 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

@@ -2380,6 +2380,68 @@ fn test_partially_untagged_enum_desugared() {
     );
 }
 
+#[test]
+fn test_partially_untagged_internally_tagged_enum() {
+    #[derive(Serialize, Deserialize, PartialEq, Debug)]
+    #[serde(tag = "t")]
+    enum Data {
+        A,
+        B,
+        #[serde(untagged)]
+        Var(u32),
+    }
+
+    let data = Data::A;
+
+    assert_de_tokens(
+        &data,
+        &[
+            Token::Map { len: None },
+            Token::Str("t"),
+            Token::Str("A"),
+            Token::MapEnd,
+        ],
+    );
+
+    let data = Data::Var(42);
+
+    assert_de_tokens(&data, &[Token::U32(42)]);
+
+    // TODO test error output
+}
+
+#[test]
+fn test_partially_untagged_adjacently_tagged_enum() {
+    #[derive(Serialize, Deserialize, PartialEq, Debug)]
+    #[serde(tag = "t", content = "c")]
+    enum Data {
+        A(u32),
+        B,
+        #[serde(untagged)]
+        Var(u32),
+    }
+
+    let data = Data::A(7);
+
+    assert_de_tokens(
+        &data,
+        &[
+            Token::Map { len: None },
+            Token::Str("t"),
+            Token::Str("A"),
+            Token::Str("c"),
+            Token::U32(7),
+            Token::MapEnd,
+        ],
+    );
+
+    let data = Data::Var(42);
+
+    assert_de_tokens(&data, &[Token::U32(42)]);
+
+    // TODO test error output
+}
+
 #[test]
 fn test_flatten_option() {
     #[derive(Serialize, Deserialize, PartialEq, Debug)]