use std::filesystem instead of std::experimental::filesystem

include/nana/filesystem/filesystem.hpp

@@ -175,7 +175,7 @@ namespace nana  { namespace experimental { namespace filesystem
 		unknown = 0xFFFF	///<  not known, such as when a file_status object is created without specifying the permissions
 	};
     //enum class copy_options;
-    
+
     enum class directory_options
     {
     	none,
@@ -538,13 +538,20 @@ namespace std {
 	namespace filesystem {
 		using namespace std::experimental::filesystem;
 
-#if (defined(_MSC_VER) && ((!defined(_MSVC_LANG)) || (_MSVC_LANG < 201703)))
+#if defined(NANA_FILESYSTEM_FORCE) || \
+    (defined(_MSC_VER) && ((!defined(_MSVC_LANG)) || (_MSVC_LANG < 201703)))
 		path absolute(const path& p);
 		path absolute(const path& p, std::error_code& err);
 
 		path canonical(const path& p);
 		path canonical(const path& p, std::error_code& err);
 #endif
+
+#if defined(NANA_FILESYSTEM_FORCE) || defined(NANA_MINGW)
+        bool exists( std::filesystem::file_status s ) noexcept;
+        bool exists( const std::filesystem::path& p );
+        bool exists( const std::filesystem::path& p, std::error_code& ec ) noexcept;
+#endif
 	}
 } // std
 

include/nana/paint/detail/image_impl_interface.hpp

@@ -16,7 +16,7 @@ namespace nana{	namespace paint{
 	public:
 		using graph_reference = nana::paint::graphics&;
 		virtual ~image_impl_interface() = 0;	//The destructor is defined in ../image.cpp
-		virtual bool open(const std::experimental::filesystem::path& file) = 0;
+		virtual bool open(const std::filesystem::path& file) = 0;
 		virtual bool open(const void* data, std::size_t bytes) = 0; // reads image from memory
 		virtual bool alpha_channel() const = 0;
 		virtual bool empty() const = 0;

include/nana/paint/graphics.hpp

@@ -33,7 +33,7 @@ namespace nana
 		{
 			friend class graphics;
 		public:
-			using path_type = ::std::experimental::filesystem::path;
+			using path_type = ::std::filesystem::path;
 
 			using font_style = ::nana::detail::font_style;