Requirements for early adopters

If you happen to read this book in the earliest days of C++17 and use bleeding- edge compilers to compile the code, you might experience that some recipes do not compile yet. This depends on how much of the C++17 STL has been implemented already in your STL distribution.
While writing this book, it was necessary to add the path prefix experimental/ to the headers <execution_policy> and <filesystem>. There might also be additional includes such as algorithm, numeric, and so on, in the experimental/ folder of your STL distribution, depending on how new and stable it is.

The same applies for the namespace of brand new features. The parts of the library that were included from the experimental part of the STL are usually exported not within the std namespace but the std::experimental namespace.