Awful callable types

[brunocodutra]

Have you considered adding callable types that overload the operator () on combinations of lr-value-ness vs const-ness?

E.g.

struct void_ {};

struct rvalueconstcallable {
    constexpr void_ operator ()(...) const&& {
        return {};
    }
};

This would be useful to check whether generic algorithms forward callable objects correctly up to the point they are invoked.

[ldionne]

Very interesting. So for example, one would test a generic algorithm by sending it a function object that would only be callable with a (possibly-const) rvalue, and see whether the algorithm is forwarding up to the point of instantiation? That seems like a good idea.

[brunocodutra]

Precisely.

E.g. given the example above, a library that makes the naive assumption that function objects should be callable through lvalue const references would break for instances of rvalueconstcallable.