Type parameter defaults are not checked correctly

[Tobba]

This typechecks:

pub trait Foo {
    fn foo<E=<Self as Bar>::A>(_: E) { }
}

pub trait Bar {
    type A;
}

fn main() { }

Which doesn't really make a lot of sense and seems like it could be a backwards compatibility footgun.

[jroesch]

Can you clarify what you are expecting here? this looks fine to me. You are allowed to declare that default and it will be manifested at a call site to Foo::foo.

[eddyb]

@jroesch the default depends on Self: Bar but that is implicit (due to a use of <Self as Bar> projection).
I am not sure that is actually a problem, as it will get type-checked at call sites (not in that specific form, though, not yet. would need defaults to be used in inference for that default to be instantiated).

[jroesch]

@eddyb yeah that was my point, currently we don't check implied constraints like that in most places. You can do similar things in trait declarations but will receive a type error at a call site. Thought I agree that we should try to improve the error reporting. I would like to do some work in this area after we finish the current set of internal refactors.

[steveklabnik]

Triage: so is this a problem or not?

[eddyb]

cc @rust-lang/lang I am a bit worried about this.

EDIT: Currently the example hits #36887 in playpen on stable (1.14).
There might still be a way to construct a problematic example.

[Mark-Simulacrum]

@rust-lang/lang didn't comment on this; I'm going to nominate.

[nikomatsakis]

I believe we intentionally deprecated this with #36887 -- we can probably just close this issue, in my opinion.