How do I declare a function whose return type is deduced?

Consider this C++1y code (LIVE EXAMPLE):

#include <iostream>

auto foo();

int main() {
    std::cout << foo();   // ERROR!
}

auto foo() {
    return 1234;
}

The compiler (GCC 4.8.1) generously shoots out this error:

main.cpp: In function ‘int main()’:
main.cpp:8:18: error: use of ‘auto foo()’ before deduction of ‘auto’
std::cout << foo();
                   ^

How do I forward-declare foo() here? Or maybe more appropriately, is it possible to forward-declare foo()?

I've also tried compiling code where I tried to declare foo() in the .h file, defined foo() just like the one above in a .cpp file, included the .h in my main.cpp file containing int main() and the call to foo(), and built them.

The same error occurred.