visit() should not pass prvalues to its callable

[ned14]

visit() should not pass prvalues to its callable

[pdimov]

https://godbolt.org/z/5Pdd7nvbz doesn't work, one needs to do https://godbolt.org/z/8YocbodaG.

The workaround is to use auto&&, but it would be more consistent for auto& to work, as nullptr is the only case that passes an rvalue.

[grisumbras]

I've written a fix, but then decided to also write a rvalue reference overload for visit, at which point I've reached a conundrum. We don't have rvalue ref-qualified overloads for e.g. get_bool and get_double, as there's no need for them. Due to that a visitor that has a bunch of operator(T&&) overloads will not work. Which makes me rethink whether it's actually necessary to make visitor::operator()(nullptr_t&) work. Maybe it is better to simply better explain the behaviour in documentation.

On the other hand, a visitor with operator(T) overloads will work with rvalue ref-qualified overload, so maybe that's good enough?