Function propagation_sides 

pub fn propagation_sides(
    t: &DataType,
    state: &State<'_>,
) -> Option<(DataType, u16)>

What a ? reads out of an enum shaped like Option or Result.

Option is Some(T) and None, Result is Ok(T) and Err(E). Answers the type ? reads out of the success variant and the index of the failure variant, which is the one it hands back to the caller; None for any other type.