Cloning an Arc with a Trait-Object tries to Clone the Trait-Object

[reem]

This will compile:

use std::sync::Arc;

trait TestTrait {}

pub struct Shared {
    pub thing: Arc<Box<TestTrait + Send + Share>>
}

impl Clone for Shared {
    fn clone(&self) -> Shared {
        Shared {
            thing: self.thing.clone()
        }
    }
}

But adding a :Clone bound to the TestTrait definition like so:

use std::sync::Arc;

trait TestTrait: Clone {}

pub struct Shared {
    pub thing: Arc<Box<TestTrait + Send + Share>>
}

impl Clone for Shared {
    fn clone(&self) -> Shared {
        Shared {
            thing: self.thing.clone()
        }
    }
}

Fails with:

src/shared.rs:12:20: 12:38 error: cannot call a method whose type contains a self-type through an object
src/shared.rs:12             thing: self.thing.clone()
                                    ^~~~~~~~~~~~~~~~~~

I think this is just a name resolution issue, but I'm not sure why this happens.

[alexcrichton]

This appears to be a problem with autoderef. For example, this code compiles just fine:

use std::sync::Arc;

trait TestTrait: Clone {}

pub struct Shared {
    pub thing: Arc<Box<TestTrait + Send + Share>>
}

fn clone<T: Clone>(t: &T) -> T { t.clone() }

impl Clone for Shared {
    fn clone(&self) -> Shared {
        Shared {
            thing: clone(&self.thing),
        }
    }
}

In other words, the error is legitimate, you're just not calling the method you expecting. It appears the invocation of clone() is jumping to Clone::<Self>::clone rather than Clone::<Arc<Self>>::clone.

[reem]

That's what I guessed, and in fact I used that same workaround to make this code work. Is this the expected behavior?

[alexcrichton]

I would suspect that this is a bug, I would also suspect that those such as @pnkfelix, @nick29581, or @nikomatsakis would know more than me.

[reem]

This is closed under new auto-deref semantics.