Figure out overloads automatically

```py
def f(value: str, optional=False):
if value.startswith("a"):
if optional:
return None
raise ValueError
return value
```
This should find the two overloads automatically.

No files, tests, or entry points are named. Start by reproducing the supplied Python example and determine how the checker currently handles the two behaviors; done should mean it automatically recognizes both overloads with clear expected typing results.

Written by the indexing model from the issue text.