decltype of pointer to class accessing its () operator

I have following class

template<class T> class CT
{
  T& operator()(int i,int j) const noexcept
  { return indx[i*N+j]; }
  private:
  T  *indx;
  int N;

};

Now some where in my application I have something as the following

CT<double> arr, (...);
(...)
decltype(arr) *alias;
if(usearr)alias = &arr;
(...)


alias(0,0) = 3.0; // Compiler ERROR!

The compiler complains that the object type I am accessing through decltype is not a function or function pointer.

The problem can be solved if I remove the pointer in declaring alias and replaces the line to an assignment overload

decltype(arr) alias;
if(usearr)alias = arr;

What is wrong with the former approach ?