> However, I don't think it's doable in languages with structural type systems like Typescript or Go's interface without a massive ergonomic hit for minimal gai

> However, I don't think it's doable in languages with structural type systems like Typescript or Go's interface without a massive ergonomic hit for minimal gain. Go only has structural interfaces, concrete types are nominative, and this sort of patterns tends to be more on the concrete side. Typescript is a lot more broadly structural, but even there a class with a private member is not substitutable with a different class with the same private member.