Can I write a function type that returns a function?

The following fails to compile on both gcc and clang

#include <type_traits>

int foo();

int main()
{
    using R = std::result_of_t<decltype(foo)()>; // error
}

The error on both compilers deals with the illegality of declaring a function returning a function. But I'm not declaring such a function - I'm just trying to write its type - since that's what result_of expects. Is this really still ill-formed?