rustc has a stack overflow when using `impl Trait` of a trait with a generic parameter with a default type of Self

[quininer]

https://play.rust-lang.org/?gist=a827cb4bc9dd29324227e3ac676dd47a&version=nightly

#![crate_type = "rlib"] 
#![feature(conservative_impl_trait)]

struct Bar {}
trait Foo<T = Self> {}
impl Foo for Bar {}

fn foo() -> impl Foo {
    Bar {}
}

[matthewjasper]

I guess this should be erroring with E0393 or E0666 (nested impl trait is not allowed).

[shepmaster]

Continues crashing in 1.26 stable.

[shepmaster]

This does not crash:

struct Bar {}
trait Foo<T = i32> {}
impl Foo for Bar {}

fn foo() -> impl Foo {
    Bar {}
}

fn main() {}

[leoyvens]

Particularly unfortunate because of PartialEq and PartialOrd.

What is going on here: There is a syntactical check to forbid nested impl Trait, but the default sneaks past that check, so we probably have to check for that substitution after lowering from HIR in type collection. Though it can also be done in the syntax check with some contortion.

I was thinking that this shouldn't be allowed because it's like nested impl Trait, but impl PartialEq is not at all like impl PartialEq<impl PartialEq> because the latter allows the inner type to be different from the outer one, while the former doesn't. So it might just be a case of fixing the overflow itself here.

I can take this myself but someone looking for mentorship or who is just faster is welcome.