In julia we switched to making method ambiguities a runtime error because you often end up with situations where even though there are ambiguous methods, you don't ever really want to call them (because they don't make much sense semantically or because they came from different packages, etc). Stanza looks a little more static than julia in this regard, so perhaps they do want to make it a compile-time error (or at least make calling it a compile-time error if types are declared).