gcc 6.1 std::result_of compilation error

Consider a small standalone use case wherein I want to determine if a type is a complete or incomplete using

#include <type_traits>
namespace {

struct foo {
    template<class T, std::size_t = sizeof(T)>
    std::false_type  operator()(T&);
    std::true_type operator()(...);
};

struct FooIncomplete;
}


int main() {
    std::result_of<foo(FooIncomplete&)>::type();

    return 0;
}

This compiles fine with gcc 4.9.3 with --std=c++11 flag. However, with gcc 6.1 and --std=c++11 it produces a compilation error as

main.cpp: In function 'int main()':
main.cpp:17:5: error: 'type' is not a member of 'std::result_of<{anonymous}::foo({anonymous}::FooIncomplete&)>'
     std::result_of<foo(FooIncomplete&)>::type();

What am I missing here ? What could be a possible work around ?