gistfile1.txt

sub f[T <: Gist, U >: X](o: Option[T], z: U) =
    g(o, z, T.new());

gistfile2.js

function f(o, z, $$typeList) {
    var T, U;
    $lasagnascript$assert($lasagnascript$isOfClass(o, Option));
    if ($$typeList !== undefined) {
        T = $$typeList.types[0];
        U = $$typeList.types[1];
        $lasagnascript$assert($lasagnascript$isOfType(z, U));
    } else {
        T = o.$lasagnascript$types[0];
        U = $lasagnascript$typeOf(z);
    }
    $lasagnascript$assert($lasagnascript$isSubtype(T, Gist));
    $lasagnascript$assert($lasagnascript$isSubtype(X, U));

    return g(o, z, $lasagnascript$new(T));
}