How does std::declval() work?

Basically, in a sizeof or decltype expression you can call functions that aren’t implemented anywhere (they need to be declared, not implemented).

E.g.

class Silly { private: Silly( Silly const& ) = delete; };

auto foo() -> Silly&&;

auto main() -> int
{
    sizeof( foo() );
}

The linker should not complain about that.