associated_type_bounds: errors with ambiguous associated type

[RustyYato]

I think I found a bug with associated type bounds! #52662

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=28d566e7244a0b3ad21b73e31418894d

#![feature(associated_type_bounds)]

trait Foo {
    type Bar;
}

fn works<F: Foo>() where F::Bar: Copy {}

// This should be fine, but I get `error[E0221]` instead
fn breaks<F: Foo<Bar: Foo>>() where F::Bar: Copy {}

This is the exact same error you get when you do this

trait Foo<T> {
    type Bar;
}

fn breaks<F: Foo<String> + Foo<u32>>() where F::Bar: Copy {}

Because Rust can't tell which Foo::<_>::Bar you mean, but in the case of associated_type_bounds, there is no ambiguity, which makes this error confusing.

[Centril]

Seems like a bug indeed!

cc @alexreg @nikomatsakis

[Centril]

Same error with:

fn breaks2<F>() where F: Foo<Bar: Foo>, F::Bar: Copy {}

Although the following results in the same problem:

fn breaks3_no_atb<F>() where F: Foo, Foo::Bar: Foo, F::Bar: Copy {}

so it may not be a bug after all?

[RustyYato]

Ignore this, I got myself confused.

Ok, maybe just the wrong error in the desugaring? I guess this never came up because people tend to group up the bounds.

So, they would do this instead

fn breaks3_no_atb<F>() where F: Foo, Foo::Bar: Foo + Copy {}

Which works fine
edit: this should say

fn breaks3_no_atb<F>() where F: Foo, F::Bar: Foo + Copy {}

Which does work fine, but I copy pasted from Centril's comment earlier.

[Centril]

@KrishnaSannasi Oops; I wrote Foo::Bar: Foo; should be F::Bar: Foo. Sorry!

So it is a bug after all...

[alexreg]

It's a very weird bug yes, confirmed. For now, just avoid splitting up your associated type bounds like this, but I'll try to get it fixed soon.

[alexreg]

@KrishnaSannasi Hopefully you can see above a fix PR has been submitted now. It seems to do the job, but will need review then merge...