In this example, I get an (incorrect) compilation error:

#![feature(never_type)]

struct Foo { 
    field1: !,
    field2: Option<&'static Bar>,
}

struct Bar {
    field1: &'static Foo
}

fn test_a() {
  let x: Option<Foo> = None;
  match x { None => () }
}

fn test_b() {
  let x: Option<Bar> = None;
  match x { None => () }
}
    
fn main() { }

But if you comment out test_a, you don't. This is because the inhabitedness code, if it starts from Foo, incorrectly caches a result for Bar -- the result is incorrect because Bar did not fully explore Foo, since we found a cycle.

Encountered while fixing #44137 -- I will fix en passante.