I initially avoided chiming in here given that this started with me writing something inflammatory about the type system on github. These sorts of problems are exactly why I felt so incensed by the situation. I shouldn’t have phrased what I did how I did, but the frustration I feel by all of these people claiming to make tools for users, and then intentionally make it so that the system can’t check certain things for users is very real.
next(iter(tuple[*Ts])) returning the result of this is impossible to type. Allowing Union[*Ts] is both the obvious answer here, and not allowed because people picked easy for tooling over possible for users.
In the interactions I’ve had with discussing python’s type system, people seem to think this is good and virtuous. You incrementally support more and that’s supposed to be a good thing right? In reality, that doesn’t work for specifications. You get ossification as implementations block eachother on compatability. How many things are in the typeshed with a comment about a specific type checker not handling something correctly? Special casing that turns into a bigger problem down the road. This becomes self-fulfilling if even one implementation does this if implementations are allowed to be a blocker for the specification of something that makes sense.
At some point if typing is going to keep up with demands from users, it needs to stop being implementations that dictate what the type system supports, and only a matter of if a type checker is compliant with a specification made to support real use.