enum_fuse seems to generate duplicate values

[bshm]

Hi,

The following code fails with both static assertion indicating that enum_fuse does not work as expected:
If you uncomment the assertions you see that both value1 and value2 evaluate to 4;
Expected behavior: enum_fuse generates distinct values for every combination of input parameters
Actual behavior: enum_fuse sometimes returns the identical values

Seen on latest master and on v0.9.6
https://godbolt.org/z/PP5v76Pdq

#include <iostream>
#include "magic_enum.hpp"
#include "magic_enum_fuse.hpp"

enum class Continent
{
    EUROPE,
    ASIA,
    AFRICA,
    AMERICA,
    AUSTRALIA,
};

enum class Color
{
    RED,
    GREEN,
};

static_assert(magic_enum::enum_fuse(Continent::AUSTRALIA, Continent::EUROPE).value() !=
              magic_enum::enum_fuse(Continent::EUROPE, Continent::ASIA).value());

static_assert(magic_enum::enum_fuse(Continent::AUSTRALIA, Color::RED).value() !=
              magic_enum::enum_fuse(Continent::EUROPE, Color::GREEN).value());

int main() {

    auto value1 = magic_enum::enum_integer(magic_enum::enum_fuse(Continent::AUSTRALIA, Color::RED).value());
    auto value2 = magic_enum::enum_integer(magic_enum::enum_fuse(Continent::EUROPE, Color::GREEN).value());
    std::cout << "value1: " << value1 << std::endl;
    std::cout << "value2: " << value2 << std::endl;

    return 0;
}

CMakeLists.txt for minimal example

cmake_minimum_required(VERSION 3.14)
project(MagicEnumExample)
set(CMAKE_CXX_STANDARD 17)
set(CMAKE_CXX_STANDARD_REQUIRED True)
include(FetchContent)
FetchContent_Declare(
    magic_enum
    GIT_REPOSITORY https://github.com/Neargye/magic_enum.git
    GIT_TAG        master
)
FetchContent_MakeAvailable(magic_enum)
include_directories(${magic_enum_SOURCE_DIR}/include/magic_enum)
add_executable(MagicEnumExample main.cpp)
target_link_libraries(MagicEnumExample PRIVATE magic_enum::magic_enum)

[bshm]

Ok, likely the shift value is slightly miscalculated.

log2(enum_count<E>() + 1)) returns 2 (instead of 3) e.g. for enum of size 5 and 6.
So I suggest to use $log_2(2*count - 1)$.
log2((enum_count<E>() << 1) - 1))
instead

template <typename E>
constexpr optional<std::uintmax_t> fuse_one_enum(optional<std::uintmax_t> hash, E value) noexcept {
  if (hash) {
    if (const auto index = enum_index(value)) {
      return (*hash << log2(enum_count<E>() + 1)) | *index;
    }
  }
  return {};
}

needs to be

template <typename E>
constexpr optional<std::uintmax_t> fuse_one_enum(optional<std::uintmax_t> hash, E value) noexcept {
  if (hash) {
    if (const auto index = enum_index(value)) {
      return (*hash << log2((enum_count<E>() << 1) - 1)) | *index;
    }
  }
  return {};
}