Compilation failure with std::result_of and SFINAE

[Veena Lakshmana <veenmurthy@xxxxxxxxx>]

Hello,

I'm working on a project to move from GCC 4.9.3 to GCC 6.1
In the process I've a hit a compilation failure which passed with GCC 4.9.3

The reduced test case :
======================================
#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 x;
    return 0;
}
========================================

Error:
unittest/tMetaIsIncomplete.cpp: In function ‘int main()’:
unittest/tMetaIsIncomplete.cpp:38:5: error: ‘type’ is not a member of
‘std::result_of<{anonymous}::foo({anonymous}::FooIncomplete&)>’
     std::result_of<foo(FooIncomplete&)>::type();
     ^~~

The above test case compiles fine with Clang 3.8 and Visual Studio 2015.
Is this a regression with GCC ?

Am I doing something wrong here ?
Any help is appreciated.

Regards,
Veena