func (t *Type) SetUnderlying(underlying *Type) {
	if underlying.kind == TFORW {
		// This type isn't computed yet; when it is, update n.
		underlying.forwardType().Copyto = append(underlying.forwardType().Copyto, t)
		return
	}

	ft := t.forwardType()

	// TODO(mdempsky): Fix Type rekinding.
	t.kind = underlying.kind
	t.extra = underlying.extra
	t.width = underlying.width