extern "C" {
    pub fn a_thing() -> u8;
}

pub trait ExternFn {
    type FnPtr;
}

enum a_thing {}

impl ExternFn for a_thing {
    type FnPtr = extern "C" fn() -> u8;
}

fn main() {
    let _: Option<<a_thing as ExternFn>::FnPtr> = None;
}