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// https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=edd0ab90ff3c482b86a21ff9e06277d1
 
mod ext {
    use std::fmt::Debug;
    use std::fmt;
    
    pub trait Trait: Debug {
        fn write_t(&self, _f: &mut fmt::Formatter) -> Result<(), fmt::Error> { Ok(()) }
    }
    
    #[derive(Debug, Clone, Copy)]
    pub struct T0;
    impl Trait for T0 {
        fn write_t(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
            f.write_str("T0")
        }
    }
    
    #[derive(Debug, Clone, Copy)]
    pub struct T1;
    impl Trait for T1 {
        fn write_t(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
            f.write_str("T1")
        }
    }
    
    #[derive(Debug, Clone, Copy)]
    pub struct Wrap<E: Trait>(pub E);
}
 
mod main {
    use crate::ext;
    use std::fmt;
    
    impl<T> ext::Trait for Box<T> where T: ext::Trait + ?Sized {
        fn write_t(&self, _f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
            // self.write_t(f)
            // TODO
            Ok(())
        }
    }
    pub fn test(flag: bool) {
        let b: Box<dyn ext::Trait> = if flag {
            Box::new(ext::T0)
        } else {
            Box::new(ext::T1)
        };
        let w: ext::Wrap<Box<dyn ext::Trait>> = ext::Wrap(b);
        println!("{:?}", w);
    }
}
 
fn main() {
    main::test(false);
    main::test(true);
}