Can't write default methods that depend on a bound of an associated type of their traits

[renato-zannon]

That's a mouthful! To exemplify:

trait DigitCollection: Sized {
    type Iter: Iterator<Item = u8>;
    fn digit_iter(self) -> Self::Iter;

    #[cfg(on_trait)]
    fn digit_sum(self) -> u32 {
        self.digit_iter()
            .map(|digit: u8| digit as u32)
            .fold(0, |sum, digit| sum + digit)
    }
}

#[cfg(not(on_trait))]
fn digit_sum<T: DigitCollection>(collection: T) -> u32 {
    collection.digit_iter()
        .map(|digit: u8| digit as u32)
        .fold(0, |sum, digit| sum + digit)
}

fn main() {
}

This doesn't compile with --cfg on_trait, even though it does without it. Note that the only difference between the two configurations is that the function on not(on_trait) is a free function instead of a default method.

With on_trait, this fails with:

trait.rs:7:14: 7:26 error: type annotations required: cannot resolve `<<Self as DigitCollection>::Iter as core::iter::Iterator>::Item == u8` [E0284]
trait.rs:7         self.digit_iter()
                        ^~~~~~~~~~~~
error: aborting due to previous error

Tested on rustc 1.0.0-dev (f3573aa83 2015-02-06 05:52:20 +0000)

[edwardw]

cc me

[mitchmindtree]

I've been running into very similar errors all day while updating my libs to the latest nightly. Here's an example of a method that works fine along with one that does not (simplest example I could come up with).

use std::num::Float;

trait Foo {
    type A: Float;

    // This does not work.
    fn sub_a(&self, a: <Self as Foo>::A, b: <Self as Foo>::A) -> <Self as Foo>::A {
        a - b
    }

    // However this does.
    fn sub_b(&self, a: <Self as Foo>::A, b: <Self as Foo>::A) -> <Self as Foo>::A {
        fn sub<T: Float>(a: T, b: T) -> T { a - b }
        sub(a, b)
    }
}

fn main() {}

rustc 1.0.0-nightly (74b874071 2015-02-08 00:24:03 +0000)

<anon>:9:9: 9:14 error: type annotations required: cannot resolve `<<Self as Foo>::A as core::ops::Sub>::Output == <Self as Foo>::A` [E0284]
<anon>:9         a - b
                 ^~~~~

[nikomatsakis]

This example works now. I think this was a dup of #21965

[mitchmindtree]

@nikomatsakis I just updated to master and I'm still running into the same error as above - perhaps my error is a separate issue?

rustc 1.0.0-dev (bc87efef2 2015-02-10 19:52:04 +0000)

[mitchmindtree]

Edwardw's PR #22055 does seem to fix my issue however, perhaps it addresses something extra?

[renato-zannon]

@nikomatsakis The original issue is still present if the example on the first post is compiled with --cfg on_trait. I tested on rustc 1.0.0-dev (de8bc4475 2015-02-10 09:10:37 +0000), which contains #21968

Edit: just compiled rustc 1.0.0-dev (1500df893 2015-02-11 08:54:30 +0000). Still reproducible