[amluto]

> You are also missing the context of C++ defining operator overloading, so you can call `std::byteswap(0ull)` and get an `unsigned long long` and you can call `std::byteswap(std::uint16_t{0})` and get a 16 bit unsigned integer.

I can believe this is useful in explicitly-typed form, i.e. using std::byteswap<T> with T specified. But letting T be inferred seems quite dangerous: C++ loves changing integer types around all by itself (via type promotion, for example), and byteswap<int> and byteswap<long> are (on UNIXy systems) simply not the same operation. For that matter, byteswap should really only be used on uintN_t.

byteswap(a+b) is just asking for trouble.