'non-exhaustive match failure' in fold_ty (inner constraint)

[jruderman]

tag maybe_ordered_pair {
    yes({low: int, high: int} : less_than(*.low, *.high));
    no;
}
pure fn less_than(x: int, y: int) -> bool { ret x < y; }
fn main() { }

upcall fail 'non-exhaustive match failure', ../src/comp/middle/ty.rs:671

I guess the alt in fold_ty is missing ty_constr