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

Is there an alternative, or do we have to invent our own trait to handle this case?

Yeah, you'd just have to write your own trait that doesn't use declval. Assuming you have std::is_detected lying around (which I know you certainly do):

template  T make();

template 
using invoke_result_t = decltype(std::declval()(make()...));
//                               ^^^^^^^^^^^^^     ^^^^^

template 
using is_invocable = std::is_detected;

This way, std::is_invocable is false_type, but is_invocable is true_type.

I intentionally use declval() for the function instead of make so as to allow using decltype(f) here. Really, invoke_result_t should be more complicated, and "do the right thing" for pointers to members, etc. But this is at least a simple approximation that indicates the viability of this approach.