fix deserialization of untagged variants within internally or adjacently

tagged enums

serde_derive/src/de.rs

@@ -1749,10 +1749,14 @@ 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.
+    // 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 {
+            if let _serde::__private::Result::<_, __D::Error>::Ok(__ok) = (|| {
+                #expr
+            })() {
                 return _serde::__private::Ok(__ok);
             }
         }

test_suite/tests/test_annotations.rs

@@ -2381,11 +2381,12 @@ fn test_partially_untagged_enum_desugared() {
 }
 
 #[test]
-fn test_partially_untagged_simple_enum() {
+fn test_partially_untagged_tagged_enum() {
     #[derive(Serialize, Deserialize, PartialEq, Debug)]
     #[serde(tag = "t")]
     enum Data {
         A,
+        B,
         #[serde(untagged)]
         Var(u32),
     }
@@ -2398,11 +2399,44 @@ fn test_partially_untagged_simple_enum() {
             Token::Map { len: None },
             Token::Str("t"),
             Token::Str("A"),
-            Token::Str("b"),
-            Token::I32(0),
             Token::MapEnd,
         ],
     );
+
+    let data = Data::Var(42);
+
+    assert_de_tokens(&data, &[Token::U32(42)]);
+
+    // TODO test error output
+
+    #[derive(Serialize, Deserialize, PartialEq, Debug)]
+    #[serde(tag = "t", content = "c")]
+    enum Data2 {
+        A(u32),
+        B,
+        #[serde(untagged)]
+        Var(u32),
+    }
+
+    let data = Data2::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 = Data2::Var(42);
+
+    assert_de_tokens(&data, &[Token::U32(42)]);
+
+    // TODO test error output
 }
 
 #[test]