Proposal: Optional Explicit covariance/contravariance for PEP 695

[randolf-scholz]

Make __replace__ stop interfering with variance inference

Consider: from dataclasses import dataclass from typing import Generic, TypeVar, override if False: # Switch to True is a type error. @dataclass(frozen=True) class C[T]: x: T # T is inferred to be invariant! else: U = TypeVar('U', covariant=True) @dataclass(frozen=True) class C(Generic[U]): x: U class X: def f(self) -> C[object]: return C(1.0) class Y(X): @override def f(self) -> C[float]: return C(1.0) Eric explains the …

This whole debacle could have been prevented with explicit variance.