SFINAE to enable nontemplate member function

Question

This is probably a duplicate, but I just can't find one where the OP clearly has the same problem I'm having.
I have a class, and I'm trying to enable operator- only if the class template parameter is not an unsigned type.

#include <type_traits>

template<class T>
struct A {
    typename std::enable_if<!std::is_unsigned<T>::value,A>::type operator-() {return {};}
};

int main() {
    A<unsigned> a=a;
}

Unfortunately, this produces a compiler error any time I instantiate it with an unsigned type as shown.

main.cpp:5:29: error: no type named 'type' in 'std::enable_if<false, A<unsigned int> >'; 'enable_if' cannot be used to disable this declaration
    typename std::enable_if<!std::is_unsigned<T>::value,A>::type operator-() {return {};}
                            ^~~~~~~~~~~~~~~~~~~~~~~~~~~
main.cpp:9:17: note: in instantiation of template class 'A<unsigned int>' requested here
    A<unsigned> a=a;
                ^

Well, I can clearly see that enable_if is not going to work here. One vaguely similar question mentioned I can use inheritance and template specialization to work around this, but... is there really no better way?

Answer 1

I had the same problem once. Turns out there can't be a substitution failure since the default template argument doesn't depend on a template parameter from the function template. You have to have a template argument defaulted to the enclosing template type, like this:

template<typename U = T,
         class = typename std::enable_if<!std::is_unsigned<U>::value, U>::type>
A operator-() { return {}; }

Answer 2

A bit long for a comment: You can also use a free function, even for unary operators.

#include <type_traits>

template<class T>
struct A {
};

template<class T>
typename std::enable_if<!std::is_unsigned<T>::value,A<T>>::type
operator-(A<T>) {return {};}

int main() {
    A<signed> b;
    -b; // ok

    A<unsigned> a;
    -a; // error
}

This doesn't introduce a member function template for each class template.

---

Here's how you can befriend it:

template<class T>
class A {
    int m;

public:
    A(T p) : m(p) {}

    template<class U>
    friend
    typename std::enable_if<!std::is_unsigned<U>::value,A<U>>::type
    operator-(A<U>);
};

template<class T>
typename std::enable_if<!std::is_unsigned<T>::value,A<T>>::type
operator-(A<T> p) {return {p.m};}

int main() {
    A<signed> b(42);
    -b; // ok

    A<unsigned> a(42);
    //-a; // error
}

You can (should) forward-declare that function template, though.

Answer 3

One possible way is introducing a dummy template parameter:

template<class T>
struct A {
    template<
       typename D = int,
       typename = typename std::enable_if<!std::is_unsigned<T>::value, D>::type
    >
    A operator-() {return {};}
};