Fix redefinition errors when compiling as module (regression fix)

[ChristopherCanfield]

Hello,

This trivial pull request fixes type redefinition errors when compiling magic_enum as a module while the preprocessor variable MAGIC_ENUM_USE_STD_MODULE is defined.

Issue

Compiling magic_enum modules with the preprocessor variable MAGIC_ENUM_USE_STD_MODULE defined causes type redefinition errors in the Microsoft Compiler. This occurs because the header is #include'd on line 62 of magic_enum_format.hpp. This line should be wrapped in #ifndef MAGIC_ENUM_USE_STD_MODULE / #endif.

This regression was introduced in commit a0db691, on 2025-11-21.

Steps to Reproduce

1. Include current development version (master / https://github.com/Neargye/magic_enum/tree/2e6e8bfff361c2c40990426c462b44af6c27820b) of magic_enum in a project.
2. Enable modules.
3. Define preprocessor variable MAGIC_ENUM_USE_STD_MODULE.
4. Compile and receive many type redefinition errors.

Fix

On line 62 of magic_enum_format.hpp, change

#include <format>

to

#ifndef MAGIC_ENUM_USE_STD_MODULE  
#include <format>  
#endif

Environment

• Compiler: Microsoft C/C++ Optimizing Compiler Version 19.50.35719
• Windows 11 Version 10.0.26200