ICE: Default type parameter that references `Self` in an object type

[nikomatsakis]

This example causes an ICE:

#![feature(default_type_params)]

trait Foo<T=Self> {
    fn method(&self);
}

fn foo(x: &Foo) { }

fn main() { }

yields

<anon>:7:12: 7:15 error: internal compiler error: Type parameter `Self/SelfSpace.0` (Self/SelfSpace/0) out of range when substituting (root type=Self) substs=Substs[types=[[];[];[];[]], regions=[[];[];[];[]]]

This is because, in an object type, the self parameter slot is empty, so the substitution of the default fails. I'm not 100% sure what's the best way to fix this. We could make the substitution report a proper error rather than ICE; perhaps that's best.

It is debatable whether such a trait ought to be object safe, but it seems harmless, if pointless, since the Self type winds up being revealed.

[oli-obk]

not sure if this is the same or a new bug with the same symptom:

use std::ops::Add;

fn main() {
    let x = 5;
    let y = x as Add;
}

yields

<anon>:5:18: 5:21 error: internal compiler error: Type parameter `Self/SelfSpace.0` (Self/SelfSpace/0) out of range when substituting (root type=Self) substs=Substs[types=[[];[];[]], regions=[[];[];[]]]

And even funkier:

use std::ops::Add;

fn main() {
    let x = 5;
    let y = x as Add<i32>;
}

says that the error is in the use line, not in the let y line