[SE-0470] Reinstate sendable-metatype model for type checking generics #2763
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Based on discussions in the review thread, reinstate the sendable-metatype model for type checking generic definitions rather than the potentially-isolated-conformance model. While slightly less expressive, the sendable-metatype model is easier to reason about and teach. It's still possible to generalize the model toward the potentially-isolated-conformance model, as is already captured in Future Directions.
Close a potential data race with my attempt to loosen the rules. In essence, if we're allowed to pass a metatype `T.Type` across isolation boundaries when there are no constraints of the form `T: P`, then we will incorrectly allow a dynamic cast `as? any T & P` to bind to an isolated conformance (because `T` is not `SendableMetatype`).
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Based on discussions in the review thread, reinstate the sendable-metatype model for type checking generic definitions rather than the potentially-isolated-conformance model. While slightly less expressive, the sendable-metatype model is easier to reason about and teach.
It's still possible to generalize the model toward the potentially-isolated-conformance model, as is already captured in Future Directions.