I use core::string_view in Boost.MySQL, too, because implicit conversion to std::string_view is super important for me. core::string_view has std::format support, too. Not having any of these would have forced me to upgrade to C++17 already. So I'd like to see core::string_view as a public type, as it's already widely used in our interfaces.
For the record, the problem with not having explicit std::format support is specially hostile for users because the problem manifests at runtime rather than compile-time. Since XXX::string_view is a range, it will be formatted as a range of characters instead of a string. You can see what it looks like here: https://godbolt.org/z/W7acqxYWd Copying a snippet here just in case: std::println("{}", std::string_view("hello world")); std::println("{}", boost::core::string_view("hello world")); std::println("{}", boost::string_view("hello world")); Prints: hello world hello world ['h', 'e', 'l', 'l', 'o', ' ', 'w', 'o', 'r', 'l', 'd']