How do we test if an expression of a certain type can be invoked with a prvalue?

Question

With c++17 we have fancy new is_invocable and fancy new prvalues that aren\'t really values.

This permits you to create an object without having to first logically const

Answer 1

You are misusing the Invocable concept. This concept means nothing more than the ability to use std::invoke on the given function and the provided arguments.

You can't do std::invoke(f, no_move(1)), as this would provoke a copy/move of the forwarded argument. It is impossible for a prvalue to be used as a parameter through a forwarded call like invoke. You can pass a prvalue to the forwarding call, but the eventual call to the given function will get an xvalue.

This is a good reason to avoid using immobile types as value parameters in functions. Take them by const& instead.

C++ does not have a type trait to see if a function can be called with specific parameters in the way that you want.