Fix deserializing externally tagged enums from indefinite maps

src/de.rs

@@ -863,6 +863,13 @@ where
                 self.consume();
                 self.parse_enum_map(visitor)
             }
+            Some(0xbf) => {
+                if !self.accept_standard_enums {
+                    return Err(self.error(ErrorCode::WrongEnumFormat));
+                }
+                self.consume();
+                self.parse_indefinite_enum(visitor)
+            }
             None => Err(self.error(ErrorCode::EofWhileParsingValue)),
             _ => {
                 if !self.accept_standard_enums && !self.accept_legacy_enums {

tests/de.rs

@@ -573,6 +573,17 @@ mod std_tests {
             value.unwrap_err().classify(),
             serde_cbor::error::Category::Syntax
         );
+
+        // Externally tagged enum with a variable length map
+        let v: Vec<u8> = vec![
+            0xbf, // map
+            0x67, 0x4e, 0x65, 0x77, 0x54, 0x79, 0x70, 0x65, // utf8 string: NewType
+            0x1a, // u32
+            0x00, 0x00, 0x00, 0x0a, // 10 (dec)
+            0xff, // stop
+        ];
+        let (_rest, value): (&[u8], Enum) = from_slice_stream(&v[..]).unwrap();
+        assert_eq!(value, Enum::NewType(10));
     }
 
     #[test]