FileSystemPath : Work around Windows exFAT bug

MSVC's `std::filesystem` has a bug that causes it to fail to retrieve
file information on exFAT partitions in versions prior to (roughly)
17.2. That version is the first release after merging a fix from
microsoft/STL#2373, but I'm not certain that
version has the fix included.

Using `std::filesystem::status` instead of `symlink_status()` seems
justified because exFAT paritions don't support symlinks. If other
partition types are included in the filter for error code 87, this
may need to be revisited.

Changes.md

@@ -8,6 +8,11 @@ Improvements
   - Popups for string cells and row names are now sized to fit their column.
   - Added "Triple" and "Quadruple" width options to the spreadsheet row name popup menu.
 
+Fixes
+-----
+
+- FileSystemPath : Fixed bug on Windows where paths on an exFAT partition were not considered valid.
+
 1.3.4.0 (relative to 1.3.3.0)
 =======
 

src/Gaffer/FileSystemPath.cpp

@@ -258,7 +258,27 @@ bool FileSystemPath::isValid( const IECore::Canceller *canceller ) const
 
 	std::error_code e;
 
-	const std::filesystem::file_type t = std::filesystem::symlink_status( std::filesystem::path( this->string() ), e ).type();
+	std::filesystem::file_type t = std::filesystem::symlink_status( std::filesystem::path( this->string() ), e ).type();
+
+#if defined(_MSC_VER) && _MSC_VER < 1932
+
+// Fix MSVC bug preventing `symlink_status()` working with exFAT partitions, and possibly FAT.
+// Filtering to error 87 is based on experimentation and backed up by
+// https://github.com/microsoft/STL/issues/233. Using `status()` instead of `symlink_status()`
+// allows exFAT partitions to be used, and because exFAT does not support symlinks, this
+// should be a valid workaround provided filtering to error value `87` doesn't include
+// partitions that do support symlinks.
+if(
+	(
+		t == std::filesystem::file_type::none || t == std::filesystem::file_type::not_found
+	) && e.value() == 87 // "The parameter is incorrect."
+)
+{
+	t = std::filesystem::status( std::filesystem::path( this->string() ), e ).type();
+}
+
+#endif
+
 	return t != std::filesystem::file_type::none && t != std::filesystem::file_type::not_found;
 }