[menaerus]

> but then you can't rely on having a toolchain that actually supports it.

C++20 is still a bleeding edge so I'd advise you to pick C++17 or even C++14 if you want to have pleasant cross-platform coverage. That's what most companies with the goal of true cross-platform support will do. 3-year window time for compiler and library devs is really hard, especially given how many new features were introduced to C++20. And now consider how many different vendors there are ... https://en.cppreference.com/w/cpp/compiler_support numers them 12 but it doesn't count the ones from embedded space for example and probably some other are missing too, so 20? What we want to see on the paper hardly can match the much more complex reality.

> The most recent xcode/Apple Clang do not

It's a PITA, as usually found with Apple, especially considering that all other major compilers including GCC, Clang, MSVC and ICC work just fine: https://godbolt.org/z/Mj6ehq57v

> so on Mac you need to find your lib(std)c++ elsewhere.

This wouldn't work because concepts aren't a library feature but a compiler feature so perhaps your best bet would be to see if you can use vanilla clang or GCC to compile the code on Apple machines. I am not an expert here.