use std::ops::Deref;

pub trait Trait1 { }
pub trait Trait2 { }

impl<T> Trait2 for T where T: Trait1 { }

fn foo<D, T>(obj: D)
where
    D: Deref<Target=T>,
    T: Trait2 + ?Sized,
{
}

fn main() {
    let a: Option<Box<dyn Trait1>> = None;
    if let Some(obj) = a {
        foo(obj);
    }
}