Failed to find impl for trait, but that trait shouldn't be used anyway

[DaGenix]

The following code fails to compile:

pub trait Even {
    fn say(&self);
}

impl <'a, E: Even> Even for &'a mut E {
    fn say(&self) { }
}

pub trait Odd {
    fn say(&self);
}

fn print_odd<O: Odd>(o: &mut O) {
    o.say();
}

fn main() { }

with the error:

foo9.rs:14:5: 14:12 error: failed to find an implementation of trait Even for O
foo9.rs:14     o.say();
               ^~~~~~~

[DaGenix]

As a note, it compiles fine if Even::say is renamed to anything else or if the impl is removed.

[DaGenix]

This similar code also fails to compile. I'm not sure if its related:

trait Even {
        fn say(&self);
}

impl <'a, E: Even> Even for &'a E { 
        fn say(&self) { self.say() }
}

trait Odd {
        fn say(&self);
}

impl <'a, O: Odd> Odd for &'a O { 
        fn say(&self) { self.say() }
}

fn main() { }

with

foo8.rs:6:25: 6:35 error: multiple applicable methods in scope
foo8.rs:6         fn say(&self) { self.say() }
                                  ^~~~~~~~~~
foo8.rs:6:9: 6:37 note: candidate #1 is `Even$$BP$E::say`
foo8.rs:6         fn say(&self) { self.say() }
                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
foo8.rs:14:9: 14:37 note: candidate #2 is `Odd$$BP$O::say`
foo8.rs:14         fn say(&self) { self.say() }
                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
foo8.rs:14:25: 14:35 error: multiple applicable methods in scope
foo8.rs:14         fn say(&self) { self.say() }
                                   ^~~~~~~~~~
foo8.rs:14:9: 14:37 note: candidate #1 is `Odd$$BP$O::say`
foo8.rs:14         fn say(&self) { self.say() }
                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
foo8.rs:6:9: 6:37 note: candidate #2 is `Even$$BP$E::say`
foo8.rs:6         fn say(&self) { self.say() }
                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
error: aborting due to 2 previous errors

[nikomatsakis]

This is a consequence of the method lookup algorithm's "eager confirmation". Not sure if there is a bug on this yet or not. I would link it to #5527.

[japaric]

The two examples seem to compile now. @nikomatsakis can we close or should we label with needtest?

[nikomatsakis]

I think there are enough tests for this scenario already.

[nikomatsakis]

Specifically, the method-* tests in run-pass and compile-fail