Zero value in is_flags enum.

[rpatters1]

In reading #314, it appears that it is not possible to fully iterate an is_flags enum that has a zero value. This seems like an unfortunate limitation. I have frequently encountered bit-sensitive enums that include a 0 value for clearing out the bits. My code simply needs to iterate the names and values to create Lua constants, and bit-sensitive values are resource-intensive if they don't set is_flags.

[rpatters1]

Just to fill this out with a code example:

template <auto Start, auto End, auto Inc, class F>
constexpr void constexpr_for(F&& f)
{
   if constexpr (Start < End)
   {
      f(std::integral_constant<decltype(Start), Start>());
      constexpr_for<Start + Inc, End, Inc>(f);
   }
}

enum APLHABITS
{
   NONE = 0,
   A = 0x0001,
   B = 0x0002,
   C = 0x0004,
   D = 0x0008,
   E = 0x0010,
   F = 0x0020,
   G = 0x0040,
   H = 0x0080,
   I = 0x0100,
   J = 0x0200,
// K = 0x0300, // ignore this one, it's a typo
   L = 0x0400,
   M = 0x0800,
   N = 0x1000,
   O = 0x2000,
   P = 0x4000,
   Q = 0x8000
};

template <>
struct magic_enum::customize::enum_range<APLHABITS> {
   static constexpr bool is_flags = true;
};

void myfunc()
{
   constexpr std::size_t count = magic_enum::enum_count<APLHABITS>();
   printf("%ld ", count);
   constexpr auto names = magic_enum::enum_names<APLHABITS>();
   constexpr_for<0, count, 1>([&](auto i)
   {
      printf("%s:%d ", names[i].data(), magic_enum::enum_value<APLHABITS, i>());
   });
}

When you run my_func it prints

16 A:1 B:2 C:4 D:8 E:16 F:32 G:64 H:128 I:256 J:512 L:1024 M:2048 N:4096 O:8192 P:16384 Q:32768.

I would like it to print

17 NONE:0 A:1 B:2 C:4 D:8 E:16 F:32 G:64 H:128 I:256 J:512 L:1024 M:2048 N:4096 O:8192 P:16384 Q:32768.

while keeping the is_flags set for performance reasons. Is this possible?