Deserialize failed to derive on enums with tuple variant and flatten struct variant

[Mingun]

Instead of thousand of words

This code is failed to compile due to panic in serde derive:

#[derive(Deserialize)]
enum Failed {
    Tuple(f64, String),
    Flatten {
        #[serde(flatten)]
        nested: Nested,
    },
}

Output:

   Compiling playground v0.0.1 (/playground)
error: proc-macro derive panicked
  --> src/lib.rs:30:10
   |
30 | #[derive(Deserialize)]
   |          ^^^^^^^^^^^
   |
   = help: message: assertion failed: !cattrs.has_flatten()

error: aborting due to previous error

error: could not compile `playground`.

To learn more, run the command again with --verbose.

Each variant individually and Serialize derive works well, problem is only in combination of tuple variant and struct variant, when struct variant contains flattened field.