For std::span, these are free functions in namespace std and therefore work for anything implicitly convertible to std::span, incl. QSpan. But they're C++20, QSpan needs something for C++17 builds. By adding them as hidden friends for QSpan, we allow unqualifid calls to transparently resolve to the respective overload, ensuring source-compatibility between std:: and QSpan, and, eventually, a transition back to std::as_*_bytes. I considered the alternative to add these functions in the q20 namespace, but q20::as_bytes() would have to take QSpan, and QSpan is convertible from more types than std::span, so we wouldn't be able to guarantee that std::as_bytes(t) works for all T t for which q20::as_bytes(t) works, the fundamental guarantee for namespace qNN. [ChangeLog][QtCore][QSpan] Added std::span-style as_bytes() and as_writable_bytes() functions. Fixes: QTBUG-125489 Change-Id: Ia9a7560c7843e182892608178433be7349c825ba Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>