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C++ has no built-in byte type. Use unsigned char to represent a byte value.

This is not true anymore from C++17.

Do I understand correctly that comment to be tongue in cheek?

std::byte is a part of C++17.

In light of Your answer then this part is not true :

Use unsigned char to represent a byte value.

But also please consider that this document http://open-std.org/JTC1/SC22/WG21/docs/papers/2017/p0298r3.pdf says:

Taken altogether, std::byte is just as “builtin” as if it was designated by dedicated keyword

The last draft standard for C++17 http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/n4640.pdf

A search for std::byte reveals many special cases for it to be handled like a builtin type.

There's now a std::byte is in C++20 as part of <cstddef>, in section 21.2.1, along with a handy set of operators. I'd certainly recommend its use in any new code that turns on the /std:c++latest switch. However, my search of the C++17 draft standard you linked reports no hits for std::byte at all. And even Neil's paper that you refer to is basically showing how a library implementation is good enough to act as if it were part of the language - but specifically states that it can't be part the language, or at least, not under that name. Far too much code exists that defines byte either with the preprocessor or as a typedef or alias UDT (usually as unsigned char), and it would all break. That Would Be Bad.

Ah, I must apologize, I now see that std::byte did make it into the final C++17 standard after all. I should have looked at that, instead of the earlier drafts. You are right, it does get mentioned in a few places, but my reading of the standard indicates that these references to std::byte don't require any special compiler behavior, and should work for any UDT defined the same way. It's still a library type, not built-in. And you can get it with /std:c++17 as well as /std:c++latest, which is handy.