C++ :: Use T As Parameter Type In Order For Compiler To Deduce It?

Do I have to use T as a parameter type in order for the compiler to deduce it?

For example this:

template<typename T> T max(T x[], const int& len) {
T maximum(x[0]);
for(int i = 1; i < len; i++)
if(maximum < x[i])
maximum = x[i];
return maximum;
}

If I take out the T as type of x[] it will not compile because it can't deduce T right?

Like so

template<typename T> T max(long x[], const int& len) {
T maximum(x[0]);
for(int i = 1; i < len; i++)
if(maximum < x[i])
maximum = x[i];
return maximum;
}

I tried and it wouldn't compile but I just wanna make sure that it is necessary that I always have to use my template parameter as a parameter type in order for my compiler to deduce the type?