Return type in demangled member function name

What is the reason for the g++ abi::__cxa_demangle function to not return the return value for member functions?

Here's a working example of this behavior

#include <execinfo.h>
#include <cxxabi.h>
#include <iostream>

struct Foo {
    void operator()() const
    {
        constexpr int buf_size = 100;
        static void *buffer[buf_size];
        int nptrs = backtrace(buffer, buf_size);
        char **strings = backtrace_symbols(buffer, nptrs);
        for(int i = 0; i < nptrs; ++i) {

            auto str = std::string(strings[i]);
            auto first = str.find_last_of('(') + 1;
            auto last = str.find_last_of(')');
            auto mas = str.find_last_of('+');

            int status;
            char* result = abi::__cxa_demangle(str.substr(first, mas-first).c_str(), nullptr, nullptr, &status);
            if (status == 0) std::cout << result << std::endl;
        }
    }
};

int main () {
    Foo f;
    f();
}

and after compiling with g++ foo.cpp -std=c++11 -rdynamic the output is Foo::operator()() const

Is there a way to get a consistent behavior, this is, getting also the return value for the member functions?