How to fix 'wrong number of template arguments' when C++11 enabled'?

I have some complex third-party code that uses templates. With gcc 7.3.0 it builds ok if gcc flag -std=gnu++98 is specified, but gives a compilation error otherwise (i.e. with C++11 compilation). I need to fix the C++11 compilation. Here is the code (sorry it is not complete but the header files are complex):

#define CPP11 (__cplusplus > 199711L)

namespace csl
{
namespace PostExec
{

struct Complement
{
    static int64_t Execute(int64_t v)
    {
        return ~v;
    }
};

template<size_t bitWidth, bool maskOutput>
struct Mask
{
    static int64_t Execute(int64_t v)
    {
        return 0;
    }
};

#if CPP11
template<typename P, typename... PS>
struct Compound
{
    template<typename T>
    static T&& Execute(T&& v)
    {
        if (sizeof...(PS) > 0)
        {
            return Compound<PS...>::Execute(P::Execute(std::forward<T>(v)));    <<<< COMPILER ERROR HERE
        }
        return P::Execute(std::forward<T>(v));
    }
};
#else
template<typename P1, typename P2>
struct Compound
{
    static int64_t Execute(int64_t v)
    {
        return P1::Execute(P2::Execute(v));
    }
};
#endif

}
}

using namespace csl;
class CModel
{
public:

    void f1();

private:
    void Execute() { }

    static const size_t PAGE_COUNT = 1;

    static size_t CurrentPage;
    static CPage<1, 0> State[PAGE_COUNT];
};

void CModel::f1()
{
    int64_t n[5];
    StepLogicalNXOr<1, false>::Execute(Page.N[1], n[0], n[1]);
}

size_t CModel::CurrentPage = 0;

CPage <1919, 0> CModel::State[PAGE_COUNT] = {
    CPage <1, 0>()    
};

The compiler error (when CPP11 is defined) is:

<snip>: error: wrong number of template arguments (0, should be at least 1)
  return Compound<PS...>::Execute(P::Execute(std::forward<T>(v)));
         ~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
<snip>: note: provided for ‘template<class P, class ... PS> struct csl::PostExec::Compound’
struct Compound
       ^~~~~~~~

I realise this is complicated but if anyone can help I will be grateful.