Should a friend operator in a class with enum parameters be found by Koenig lookup?

Question

Consider the following code:

enum A : unsigned { a = 1, b = 2, c = 4 };

class B
{
  friend constexpr A operator|(A a1, A a2)
  { return A(unsigned(a1) | unsigned(a2)); }
};

template <A a>
class C
{
};

int main()
{
  C<a|c> c;
}

When compiled with g++ (4.8.1, -std=c++1), I get the following error:

test.C: In function ‘int main()’:
test.C:16:12: error: invalid conversion from ‘unsigned int’ to ‘A’ [-fpermissive]
       C<a|c> c;

Which tells me that the operator is not being found. However, if the code is changed to:

enum A : unsigned { a = 1, b = 2, c = 4 };

constexpr A operator|(A a1, A a2)
{ return A(unsigned(a1) | unsigned(a2)); }

template <A a>
class C
{
};

int main()
{
  C<a|c> c;
}

Then it compiles and runs fine. Is the error in the first case a compiler error, or I do misunderstand something? I would like to use the method of Boost.Operators to easily define operators by declaring a class with base classes from templates which define the operators, but this result precludes that in this case.

Thanks for any help.

Answer 1

Another way to solve it (other than jogojapan already mentioned), is to simply declare the friend function outside the scope.

enum A : unsigned { a = 1, b = 2, c = 4 };

//declaration
constexpr A operator|(A a1, A a2);


class B
{
  //definition inside, ok.
  friend constexpr A operator|(A a1, A a2)
  { return A(unsigned(a1) | unsigned(a2)); }
};

template <A a>
class C
{
};

int main()
{
  C<a|c> c;
}