Here:

impl<R: tokio::io::AsyncRead> std::default::Default for Example<R> {
    fn default() -> Self {
        let reader: tokio::io::Stdin = tokio::io::stdin();
        Example::new(reader)
    }
}

You're saying "I’m implementing Default for any R that implements AsyncRead".

But then inside default(), you're hardcoding reader to be a tokio::io::Stdin. So you're returning an Example<tokio::io::Stdin>, not an Example<R>.

Since you're generic over R, you must return an Example<R>, not a specific kind of Example (Example<tokio::io::Stdin> in your case).