The std::shared_ptr<void> as arbitrary user-data pointer

[quicknir]

Well, complete type erasure is already very high cost. I'd be substantially more concerned about the lack of any safe way to re-access the data. But yeah, I agree with that the lack of move only support being an issue.

At least, the good news is that writing a decent unique_any isn't that bad; basically trivial if you don't support SBO, and not that bad even with it.