C++: implicit casting in templates, why it does not work?

Question

It seems strange but this simple code works with int instead of T, and does not work with template T.

template <typename T>
class Polynomial {

public:
    Polynomial (T i) {}
    Polynomial& operator+= (const Polynomial& rhs) {
        return *this;
    }
};

template <typename T>
const Polynomial<T> operator+ (Polynomial<T> lhs_copy, const Polynomial<T>& rhs) {
    return lhs_copy += rhs;
}

Polynomial<int> x (1), y = x + 2; // no match for 'operator+' in 'x + 2'

Answer 1

Implicit conversion don't apply during template argument deduction, you might render your function friend(so that the type is known):

template <typename T>
class Polynomial {
public:
    Polynomial (T i) {};
    Polynomial& operator+= (const Polynomial& rhs) { return *this; };

    friend Polynomial operator+ (Polynomial lhs, const Polynomial& rhs) {
        return lhs+=rhs;
    }
};

Also related: C++ addition overload ambiguity