C / C++ / MFC

template<class T> struct Foo;
template<class T> Foo<T> operator +(T const a1, Foo<T> const &a2);

template<class T = double>
struct Foo {
private:
    T m_Data;
    // The following all result in the specified error when attempting to use this friend
    // LNK2019: unresolved external symbol "struct Foo<double> __cdecl operator+(
    //          double, struct Foo<double> const &)" ...
    friend Foo<T> operator +(T const a1, Foo<T> const &a2);
    // C2143: syntax error : missing ';' before '<'
    // C2460: '+' : uses 'Foo<T>', which is being defined
    // C2433: '+' : 'friend' not permitted on data declarations
    // C2365: 'operator +' : redefinition; previous definition was 'function'
    // C2238: unexpected token(s) preceding ';'
    friend Foo<T> operator + <>(T const a1, Foo<T> const &a2);
    // Same errors as above (C2143, C2460, C2433, C2365, C2238)
    friend Foo<T> operator + <T>(T const a1, Foo<T> const &a2);
    // C3637: 'operator +' : a friend function definition cannot be a specialization of a function template
    // C3412: 'operator +' : cannot specialize template in current scope
    template<>
    friend Foo<T> operator +(T const a1, Foo<T> const &a2);

public:
    Foo operator +(T const addend);
};

template<class T>
Foo<T> operator +(T const a1, Foo<T> const &a2) {
    return (a2 + a1);
}