Fix open world public types#8792
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In an open world, we have to assume that the environment may cast any public type down to any of its subtypes and expect that to work before and after optimization. We therefore must make public visibility propagate to subtypes. But we can be more precise than this, because some types are public not because they can cross the module boundary, but rather because they are reachable in the definition of some other type that can cross the module boundary. Subtypes of such public types do not need to be public, since their values will never cross the module boundary. In addition, we do not need to make public the subtypes of types that are exposed only as exact references. Allow Unsubtyping to operate in open-world mode and use it in tests that the visibility propagation works correctly. Fixes #8718.
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In an open world, we have to assume that the environment may cast any public type down to any of its subtypes and expect that to work before and after optimization. We therefore must make public visibility propagate to subtypes.
But we can be more precise than this, because some types are public not because they can cross the module boundary, but rather because they are reachable in the definition of some other type that can cross the module boundary. Subtypes of such public types do not need to be public, since their values will never cross the module boundary. In addition, we do not need to make public the subtypes of types that are exposed only as exact references.
Allow Unsubtyping to operate in open-world mode and use it in tests that the visibility propagation works correctly.
Fixes #8718.