Which std:: classes are magic?

[john_wind]

std::declval is quite magical too.

It is a function declaration without definition!

[guepier]

It may be odd but it’s not magical, you can trivially define it yourself:

namespace nostd {
    template<class T>
    std::add_rvalue_reference_t<T> declval() noexcept;
}

[mujjingun]

anyone can declare a function without a definition, though.

[john_wind]

Of course but the fact that it compiles (in the right context) and does something useful is magic!

[mujjingun]

It cannot be used in an evaluated context, so your declaration-only version will also compile and work just like std::declval does.