Obligation caching allows for unsound coinductive matching

[arielb1]

STR

trait Tweedledum: IntoIterator {}
trait Tweedledee: IntoIterator {}

impl<T: Tweedledum> Tweedledee for T {}
impl<T: Tweedledee> Tweedledum for T {}

trait Combo: IntoIterator {}
impl<T: Tweedledee + Tweedledum> Combo for T {}

fn is_ee<T: Combo>(t: T) {
    t.into_iter();
}

fn main() {
    is_ee(4);
}

Expected Result

This should not compile, as adding where T: Tweedledum to is_ee demonstrates.

Actual Result

This typecks and fails on trans. A variant using : 'static compiles but is unsound.

Analysis

The current scheme for within-tree obligation caching checks in a very deliberate (and slow) way that obligations are not satisfied by their parents, but it allows sibling obligations to satisfy each-other.

[arielb1]

cc @nikomatsakis

I think the best way to fix this would be to move in-fulfillcx caching to the obligation forest and check for cycles in compress.

[nikomatsakis]

triage: P-high