Conformance: test @override checks on __init__ and __new__ by ashishpatel26 · Pull Request #2297 · python/typing

Closes #2222.

The @override section of the spec states that a method decorated with
@override "should [be treated] as a type error unless that method is
overriding a method or attribute in some ancestor class, and the type of the
overriding method is assignable to the type of the overridden method." There is
no exemption for __init__/__new__, so although constructors are normally
not checked against their parents, an explicit @override should still trigger
the assignability check.

This adds cases to tests/classes_override.py:

• ChildC1: @override __init__/__new__ that are assignable to the
  parent — no error.
• ChildC2: @override __init__/__new__ with an incompatible parameter
  type — error (# E[init], # E[new]).
• ChildC3: incompatible __init__/__new__ without @override — allowed,
  to document the normal constructor exemption.

Scoring

Ran the conformance tool against the locked checker versions:

• pyrefly flags the incompatible constructor overrides → conformant.
• mypy, pyright, pycroscope, ty, zuban do not currently apply the
  assignability check to __init__/__new__ even with @override → scored
  Partial with a note.

I marked the incompatible cases as mandatory errors (# E) since the spec text
is unconditional. If maintainers feel constructor override-checking should be
left optional, these could be relaxed to # E? and the five Partial scores
returned to Pass — happy to adjust.