default_type_params bug: `does not implement any method in scope named` error

[carllerche]

I hit some more unexpected behavior with default_type_params which is probably just a bad error message.

Code

#![feature(default_type_params)]

pub trait Foo<A, B = ()> {}

pub struct Wrap<F> {
    foo: F,
}

impl<A: Send, F: Foo<A>> Wrap<F> {
    fn stuff(&self) {
    }
}

struct MyFoo;

impl Foo<uint, uint> for MyFoo {}

pub fn main() {
    let w = Wrap { foo: MyFoo };
    w.stuff();
}

Output

rustc repro2.rs
repro2.rs:20:7: 20:14 error: type `Wrap<MyFoo>` does not implement any method in scope named `stuff`
repro2.rs:20     w.stuff();
                   ^~~~~~~
error: aborting due to previous error

[sfackler]

I may be misreading this but the error seems reasonable. MyFoo implements Foo<uint, uint>, but stuff is only defined for Foo<A, ()>, so the closest you could get is Foo<uint, ()>.

[carllerche]

Perhaps at the very least expand the error message saying that MyFoo implements Foo for X?

[steveklabnik]

Updated code:

#![feature(default_type_params)]

pub trait Foo<A, B = ()> {}

pub struct Wrap<F> {
    foo: F,
}

impl<A: Send, F: Foo<A>> Wrap<F> {
    fn stuff(&self) {
    }
}

struct MyFoo;

impl Foo<usize, usize> for MyFoo {}

pub fn main() {
    let w = Wrap { foo: MyFoo };
    w.stuff();
}

This fails with a completely different error today:

hello.rs:9:6: 9:7 error: the type parameter `A` is not constrained by the impl trait, self type, or predicates [E0207]
hello.rs:9 impl<A: Send, F: Foo<A>> Wrap<F> {
                ^~
hello.rs:9:6: 9:7 help: run `rustc --explain E0207` to see a detailed explanation
error: aborting due to previous error

(the feature flag is also no longer needed)

So, is this bug still relevant?

[Mark-Simulacrum]

The new error can be reproduced with the following, which is a valid error. Closing.

trait Foo<A> {}
struct Wrap<F>(F);

impl<A: Clone, F: Foo<A>> Wrap<F> {}

pub fn main() {}