Neither the Empty Base Class nor [[no_unique_address]] give C++ Zero Size Types. The [[no_unique_address]] attribute is a way to achieve something empty base classes were useful for without the accompanying problems, so that's nice, but it's not ZSTs.

Can you say whether you genuinely thought C++ had ZSTs? And if so, how you came to that conclusion ?

I'm not saying that C++ has zero size types. I'm saying that no_unique_address and EBO are a way to store a stateless object without it occupying any space, which is all you need to implement a zero space overhead Mutext<T> for stateless types.

I think the complexity to deliver an equivalent of Mutex<T> which also works via no_unique_address to deliver no-space-overhead for deliberately stateless types that would otherwise add 1 byte to the type size is probably a bit much to ask.

Thanks for pointing me to Boost synchronized_example<T> showing that this does exist, at least as an experimental library feature.

It is not exactly rocket science: https://gcc.godbolt.org/z/6Kz53bs7x. Bonus it supports visiting multiple synchronized at the same time, deadlock free.

Huh. I was expecting that providing access to the no_unique_address value despite it not having an address would be much trickier than that.