#![feature(try_from)]

trait Bar {
    type Inner;
}

pub struct B16(u16);
pub struct B32(u32);

impl Bar for B16 {
    type Inner = u16;
}
impl Bar for B32 {
    type Inner = u32;
}

use std::convert::{TryFrom, TryInto};
impl TryFrom<B32> for B16 {
    type Error = <<B16 as Bar>::Inner as TryFrom<<B32 as Bar>::Inner>>::Error;
//  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
//  warning: private type `<B16 as Bar>::Inner` in public interface (error E0446)
//  (Despite the fact that that actual type is core::num::TryFromIntError)
    fn try_from(x: B32) -> Result<Self, Self::Error> {
       Ok(B16(x.0.try_into()?))
    }
}

fn main() {}