The following code shows the problem, which only arises when the struct sockaddr_in definition happens to be included. Otherwise the workaround is an explicit cast.

void main()
{
//    foo f = new foo(cast(AddressFamily)AddressFamily.INET);
    foo f = new foo(AddressFamily.INET);
}

class foo
{
    this(AddressFamily f)
    {
    }
}

// from std.c.linux.socket
enum: int
{
        AF_INET =       2
}

// from std.c.linux.socket
struct sockaddr_in
{
        short sin_family = AF_INET;
}

// from std.socket
enum AddressFamily: int
{
        INET =       AF_INET
}