Function argument returning void or non-void type

Question

I am in the middle of writing some generic code for a future library. I came across the following problem inside a template function. Consider the code below:

Answer 1

Some specialization, somewhere, is necessary. But the goal here is to avoid specializing the function itself. However, you can specialize a helper class.

Tested with gcc 9.1 with -std=c++17.

#include 
#include 

template
struct return_value {


    T val;

    template
    return_value(F &&f, Args && ...args)
        : val{f(std::forward(args)...)}
    {
    }

    T value() const
    {
        return val;
    }
};

template<>
struct return_value {

    template
    return_value(F &&f, Args && ...args)
    {
        f(std::forward(args)...);
    }

    void value() const
    {
    }
};

template
auto foo(F &&f)
{
    return_value()(2, 4))> r{f, 2, 4};

    // Something

    return r.value();
}

int main()
{
    foo( [](int a, int b) { return; });

    std::cout << foo( [](int a, int b) { return a+b; }) << std::endl;
}