update fix

src/include/filesystem.hpp

@@ -1,10 +1,71 @@
 #pragma once
 
-#ifndef __cpp_lib_filesystem
-#include <filesystem>
+// We haven't checked which filesystem to include yet
+#ifndef INCLUDE_STD_FILESYSTEM_EXPERIMENTAL
+
+// Check for feature test macro for <filesystem>
+#if defined(__cpp_lib_filesystem)
+#define INCLUDE_STD_FILESYSTEM_EXPERIMENTAL 0
+
+// Check for feature test macro for <experimental/filesystem>
+#elif defined(__cpp_lib_experimental_filesystem)
+#define INCLUDE_STD_FILESYSTEM_EXPERIMENTAL 1
+
+// We can't check if headers exist...
+// Let's assume experimental to be safe
+#elif !defined(__has_include)
+#define INCLUDE_STD_FILESYSTEM_EXPERIMENTAL 1
+
+// Check if the header "<filesystem>" exists
+#elif __has_include(<filesystem>)
+
+// If we're compiling on Visual Studio and are not compiling with C++17, we need to use experimental
+#ifdef _MSC_VER
+
+// Check and include header that defines "_HAS_CXX17"
+#if __has_include(<yvals_core.h>)
+#include <yvals_core.h>
+
+// Check for enabled C++17 support
+#if defined(_HAS_CXX17) && _HAS_CXX17
+// We're using C++17, so let's use the normal version
+#define INCLUDE_STD_FILESYSTEM_EXPERIMENTAL 0
+#endif
+#endif
+
+// If the marco isn't defined yet, that means any of the other VS specific checks failed, so we need to use experimental
+#ifndef INCLUDE_STD_FILESYSTEM_EXPERIMENTAL
+#define INCLUDE_STD_FILESYSTEM_EXPERIMENTAL 1
+#endif
+
+// Not on Visual Studio. Let's use the normal version
+#else // #ifdef _MSC_VER
+#define INCLUDE_STD_FILESYSTEM_EXPERIMENTAL 0
+#endif
+
+// Check if the header "<filesystem>" exists
+#elif __has_include(<experimental/filesystem>)
+#define INCLUDE_STD_FILESYSTEM_EXPERIMENTAL 1
+
+// Fail if neither header is available with a nice error message
 #else
+#error Could not find system header "<filesystem>" or "<experimental/filesystem>"
+#endif
+
+// We priously determined that we need the exprimental version
+#if INCLUDE_STD_FILESYSTEM_EXPERIMENTAL
+// Include it
 #include <experimental/filesystem>
+
+// We need the alias from std::experimental::filesystem to std::filesystem
 namespace std {
 namespace filesystem = experimental::filesystem;
 }
-#endif
+
+// We have a decent compiler and can use the normal version
+#else
+// Include it
+#include <filesystem>
+#endif
+
+#endif // #ifndef INCLUDE_STD_FILESYSTEM_EXPERIMENTAL