I might have missed here something but what is the point restricting the opaque types to be public? If that would be the ultimate goal of the whole feature, then I don't think I would still support it as I personally want to use some opaque types internally. Making internal API that only requires the opaque feature set public is a no-go for me. Also I would be highly disappointed if this feature would be merged into an access modifier like we did in the past with open.
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