Include <functional>

[ts826848]

This is necessary to avoid compilation errors using recent libc++ versions with C++23 due to libc++ taking steps to remove transitive includes in newer C++ versions 0.

For a more concrete example, this program builds using Clang 16.0.0 and C++20, but fails to build with C++23:

#include <magic_enum.hpp>
enum class ec { RED };
template <>
constexpr auto magic_enum::customize::enum_name(ec val) noexcept
    -> magic_enum::customize::customize_t {
  switch (val) {
  case ec::RED: return "Red";
  };
  return invalid_tag;
}

The compiler hits the "too many errors" threshold when building with C++23, but I believe this is due to compiling failing to recover after the first error. The error message starts with:

include % /usr/local/opt/llvm/bin/clang++ -std=c++2b -c test.cpp
In file included from test.cpp:1:
./magic_enum.hpp:324:61: error: no member named 'equal_to' in namespace 'std'
  return std::is_same_v<std::decay_t<BinaryPredicate>, std::equal_to<string_view::value_type>> ||
                                                       ~~~~~^
./magic_enum.hpp:324:93: error: expected '(' for function-style cast or type construction
  return std::is_same_v<std::decay_t<BinaryPredicate>, std::equal_to<string_view::value_type>> ||
                                                                     ~~~~~~~~~~~~~~~~~~~~~~~^
./magic_enum.hpp:324:96: error: expected expression
  return std::is_same_v<std::decay_t<BinaryPredicate>, std::equal_to<string_view::value_type>> ||
                                                                                               ^

std::equal_to is defined in <functional>, but <functional> is not directly included by magic_enum.hpp. <string> 1 and <string_view> 2 both include <functional> for C++20 and earlier, but they no longer include <functional> in C++23 and later, so a definition for std::equal_to is no longer visible, leading to the above error.

Directly including <functional> fixes this error, but I'm not sure there aren't similar errors.

[Neargye]

Could you check the commit? For me it shows that no files have changed

[ts826848]

Ack, sorry! I put off adding the change to grab error messages for the commit message and forgot that I did that!