How do I correct this friend template function link error, implemented in .cpp file?

Question

I would like to implement an Array<T> template class. I defined the class, overloaded operator << () operator function for printing. Thinks seems not hard.

However, when implementing subtraction by overloading operator - () for two Array object, there is link error.

Both the operator << () and operator - () are declared in array.h, declared as friend in Array class, implemented in Array.cpp, and "declared with specified double type"(I don't really know the correct phrase to describe it..).

Here's the code: array.h:

#ifndef MY_ARRAY_H
#define MY_ARRAY_H

#include <stdio.h>
#include <iostream>

template<class T>
class Array;

template<class T>
std::ostream& operator << (std::ostream& os, const Array<T>& mat);

template<class T>
Array<T> operator - (const Array<T>& A, const Array<T>& B);

template<class T>
class Array {
public:
    Array(): len(0), data(NULL) {}
    Array(int _len): len(_len), data(new T[len]) {
        for (int i=0; i<len; i++) {
            data[i] = 0;
        }
    }
    Array(const Array& arr) = delete;
    const Array& operator = (const Array&) = delete;
    ~Array() {
        if (!data) delete[] data;
    }

public:
    int len;
    T* data;

private:
    friend std::ostream& operator << <T>(std::ostream& os, const Array<T>& arr);
    friend Array<T> operator - <T>(const Array<T>& A, const Array<T>& B);
};


#endif // MY_ARRAY

array.cpp:

#include "array.h"
#include <iostream>

template<class T>
std::ostream& operator << (std::ostream& os, const Array<T>& arr)
{
    for (int i=0; i<arr.len; i++) {
        os << arr.data[i] << ", ";
    }
    os << "\n";
    return os;
}


// T=double
template
std::ostream& operator << (std::ostream& os, const Array<double>& arr);




template<class T>
Array<T> operator - (const Array<T>& A, const Array<T>& B)
{
    assert(A.len==B.len && "dimension mismatch(rows)");

    Array<T> C(A.rows, A.cols);
    for (int i=0; i<C.len; i++) {
        C.data[i] = A.data[i] - B.data[i];
    }

    return C;
}

// // T=double
Array<double> operator - (const Array<double>& A, const Array<double>& B);

array_test.cpp

#include "array.h"


int main()
{
    Array<double> arr(4);
    for (int i=0; i<arr.len; i++) {
        arr.data[i] = i;
    }
    
    std::cout << arr << std::endl;

    Array<double> arr2(4);
    for (int i=0; i<arr2.len; i++) {
        arr2.data[i] = 1;
    }

    std::cout << (arr2-arr) << std::endl;

    return 0;
}

To compile:

clang++ array.cpp array_test.cpp

Suffers link error:

/usr/bin/ld: /tmp/array_test-c0515e.o: in function `main':
array_test.cpp:(.text+0xdc): undefined reference to `Array<double> operator-<double>(Array<double> const&, Array<double> const&)'
clang: error: linker command failed with exit code 1 (use -v to see invocation)

How do I change my code, to solve this link error?