fn ensure_signatures_on_valid_method(
    fn_type: &FnType,
    signature: Option<&KeywordAttribute<signature, Signature>>,
    text_signature: Option<&KeywordAttribute<text_signature, TextSignatureAttributeValue>>,
) -> Result<()>fn ensure_signatures_on_valid_method(
    fn_type: &FnType,
    signature: Option<&KeywordAttribute<signature, Signature>>,
    text_signature: Option<&KeywordAttribute<text_signature, TextSignatureAttributeValue>>,
) -> Result<()>