Self is not recognised as implementing inherited traits in default method implementations

[chris-morgan]

Sample code:

#[allow(default_methods)];

fn require_y<T: Y>(_: T){}

trait Y {
    fn y(self);
}

trait InheritedMethodCallIsOk: Y {
    fn x(self) {
        self.y();  // Observe this carefully; the methods are there
    }
}

trait SelfTypeIsBroken: Y {  // Self must implement Y
    fn x(self) {
        require_y(self);  // ... but it is not recognised as implementing Y
    }
}

I expect this to succeed. It does not:

test.rs:17:8: 18:17 error: failed to find an implementation of trait Y for Self
test.rs:17         require_y(self);  // ... but it is not recognised as implementing Y
                   ^~~~~~~~~

Note that the inheritance works correctly at the impl level:

#[allow(default_methods)];

fn require_y<T: Y>(_: T){}

trait Y {
    fn y(self) {}
}

trait SelfIsOkInImpl: Y {
    fn x(self);
}

struct X;
impl Y for X;
impl SelfIsNotY for X {
    fn x(self) {
        require_y(self);
    }
}

fn main(){}

---

Second example, using Send, a kind, rather than a trait of my own devising:

#[allow(default_methods)];

fn require_send<T: Send>(_: T){}

trait TragicallySelfIsNotSend: Send {
    fn x(self) {
        require_send(self);
    }
}

fn main(){}

This similarly does not compile, though it should:

test.rs:7:8: 7:20 error: instantiating a type parameter with an incompatible type `Self`, which does not fulfill `Send`
test.rs:7         require_send(self);
                  ^~~~~~~~~~~~
error: aborting due to previous error

This, in fact, is how I came across the problem, because I'm wanting to refer to self inside a spawn block, and that will only work if it's Send, and so there I sadly cannot provide a default method implementation of the trait at present.

rust 0.8-pre (a48ca32 2013-07-08 18:49:46 -0700)
host: x86_64-unknown-linux-gnu

[Blei]

Related to #7183, #7320.

[msullivan]

Yeah, the interaction of default methods and inheritance basically doesn't work at all. Part of #2794.

[chris-morgan]

@msullivan Thanks, that fixes the first example, but it doesn't fix the second example (the Send one).

[chris-morgan]

Opened #8171 about the second example (Send).