Visual C++ - decltype as a return type of a class

Question

The code

template <typename T>
struct Foo
{
    int x_;

    int x() const;
};

int a;

template <typename T>
decltype(a) Foo<T>::x() const
{
    return x_;
}

int main()
{
}

produces the following error in Visual Studio 2013:

1>...\source.cpp(15): error C2244: 'Foo<T>::x' : unable to match function 
definition to an existing declaration
1>          ...\source.cpp(6) : see declaration of 'Foo<T>::x'
1>          definition
1>          'unknown-type Foo<T>::x(void) const'
1>          existing declarations
1>          'int Foo<T>::x(void) const'

but compiles fine on Coliru's GCC. Also, it compiles in VS as a non-template class. I guess this is due to Visual Studio's incompleteness and if it is indeed, I'm sure the cause of this already has its name. I just can't find it with my sloppy terminology. What's going on?