C++ compile code depending on the concrete type of auto

Question

I have a C++ piece of code

auto returnvalue = m_func(x, y, z); 

where the type of m_func depends on a template parameter. Afterwards I deal with the returnvalue which works fine until m_func is a function returning void. But I need a mechanism which calls

m_func(x,y,z)

if the return value of m_func is void and the above version is not. Overall in pseudo code it need to look like

if ( returnvalue of m_func is void ) 
     call m_func directly
else 
     auto retval = m_func(...) 
     handle the return value

How can these be done with C++11/14?

Edit:

m_func is either:

void func(type1 arg1, type2 arg, ...) 

or

std::tuple<...> func(type1 arg1, type2 arg, ...) 

Answer 1

Whereas C++17 has if constexpr to handle it simply, C++11/C++14 has to use some overload to handle it via SFINAE or tag dispatching or specialization, follow up a tag dispatching version:

void internal_impl(std::true_type/*, ... */) {
    m_func(x, y, z);
}
void internal_impl(std::false_type/*, ... */) {
    auto value = m_func(x, y, z);

    foo(value);
}

void internal(/*... */) {
    internal_impl(std::integral_constant<bool,
                                         std::is_void<decltype(m_func(x, y, z))>::value>{}
                  /*, ...*/);
}