typeck hang with structural recursive type

[jruderman]

tag noption { some(noption); }
fn main() {
    let nop2: noption = "x";
    alt nop2 {
      some(t) { log t.x; }
    }
}

Hangs the compiler in typeck.

sully, will your planned fix for #742 take care of this?

[msullivan]

Yeah, I think this is a straight-up dupe of #742. Closing.