Detect if variable provides a function in template

Question

I have a function template which is called with different values, i.e.

template <typename T>
T do_calculations(const T &var)
{
     T return_variable;
     //Do calculations
     return return_variable;
}

Now the calculations depend on the type of the variable var. T can either be a scalar (such as double or int), a vector (such as std::vector<> or other types) or a matrix. I know that I can differentiate between for example a std::vector and a double by knowing that double does not have a member function called size(). How can I implement that in my code above? My aim is to have similar code to this:

template <typename T>
T do_calculations(const T &var)
{
     T return_variable;
     if(T.has_function_size())
         //Do calculations on vector
     else
         //Do calculations on scalar
     return return_variable;
}

I can not do a simple check if T is a known type due to having a lot of different input types, but I know that all possible types having a function size() behave similar enough for me.

Now my problem is:
My code for the vector calculation loops over all elements, but the code for scalar calculation only over the single element:

template <typename T>
T do_calculations(const T &var)
{
     T return_variable;
     if(T.has_function_size())
         for(size_t i = 0; i < T.size(); ++i)
             return_variable[i] = func(var[i]);
     else
         return_variable = func(var);
     return return_variable;
}

with func() a function taking a scalar.
Compilation fails with the error that I can not apply func() to a vector (or my own type). Do I have to create separate functions for that, or can I tell the compiler that I know what I do?