Include <filesystem> only if FMT_CPP_LIB_FILESYSTEM is set

[W4RH4WK]

This change results out of necessity since the Nintendo Switch console SDK does not support std::filesystem. The SDK still provides the <filesystem> header, but with an #error directive, effectively breaking any build that includes <filesystem>

Because <filesystem> is present, FMT_HAS_INCLUDE is insufficient here. With this change and FMT_CPP_LIB_FILESYSTEM in place, one can define FMT_CPP_LIB_FILESYSTEM=0 to work around this issue.

This assumes that <filesystem> can be included (without warnings) if FMT_CPP_LIB_FILESYSTEM is set. If this is not the case, fmt would be broken even before this change as std::filesystem::path is used without the accompanying header.

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I've tested this locally using:

• MSVC Version 19.42.34435 (Win10)
• GCC 12.2.0 (WSL)
• Clang 15.0.6 (WSL)
• Clang Nintendo version 1.16.4 (Clang 16.0.6)

[phprus]

This PR is break stantart compatible implementations, because __cpp_lib_filesystem is defined in <filesystem>.

https://en.cppreference.com/w/cpp/utility/feature_test

[vitaut]

Feature-test macros are also defined in <version> so it should be fine.

[vitaut]

Merged, thanks!

[phprus]

Feature-test macros are also defined in <version> so it should be fine.

Only in C++20, but <filesystem> is C++17.

[vitaut]

Hmm, I guess we could define FMT_CPP_LIB_FILESYSTEM to nonzero if FMT_CPLUSPLUS >= 201703L and FMT_HAS_INCLUDE(<filesystem>) is true to handle this case.

[phprus]

Hmm, I guess we could define FMT_CPP_LIB_FILESYSTEM to nonzero if FMT_CPLUSPLUS >= 201703L and FMT_HAS_INCLUDE(<filesystem>) is true to handle this case.

Fix: #4259