fix: defer to os.Open for windows

See: golang/go#44279

merge.go

@@ -187,10 +187,35 @@ func MergeDir(dir string, conditions Conditions, opts *MergeDirOptions) ([]*File
 	if opts.AcceptFile == nil {
 		opts.AcceptFile = DefaultFileAcceptor
 	}
+	fmt.Printf("BEFORE: dir=%v  opts.FS=%#v\n", dir, opts.FS) //nolint:forbidigo
+	// if runtime.GOOS == "windows" {
+	// 	// Go running on windows does not support os.DirFS properly
+	// 	// See: https://github.com/golang/go/issues/44279
+	// 	fmt.Println("  windows") //nolint:forbidigo
+	// 	opts.FS = &osFilesystem{}
+	// 	dir = filepath.Join(filepath.VolumeName(dir), dir)
+	// 	fmt.Printf("MIDDLE2: dir=%v opts.FS=%#v\n", dir, opts.FS) //nolint:forbidigo
+	// }
 	if opts.FS == nil {
+		fmt.Println("  other") //nolint:forbidigo
 		opts.FS = os.DirFS(dir)
 		dir = "."
+		fmt.Printf("MIDDLE1: dir=%v  opts.FS=%#v\n", dir, opts.FS) //nolint:forbidigo
+	} else {
+		var err error
+		fmt.Printf("MIDDLE2A: dir=%v opts.FS=%#v\n", dir, opts.FS) //nolint:forbidigo
+		opts.FS, err = fs.Sub(opts.FS, dir)
+		if err != nil {
+			return nil, fmt.Errorf("fs.Sub of %v and %v failed: %w", opts.FS, dir, err)
+		}
+		dir = "."
+		fmt.Printf("MIDDLE2B: dir=%v opts.FS=%#v\n", dir, opts.FS) //nolint:forbidigo
 	}
+	fmt.Printf("AFTER: dir=%v  opts.FS=%#v\n", dir, opts.FS) //nolint:forbidigo
+
+	// dir + sub: "C:\\Users\\RUNNER~1\\AppData\\Local\\Temp\\TestMergeDir_SubFS959700692\\001\\a\\b\\c"
+	// BEFORE: dir="a\\b\\c"  opts.FS="C:\\Users\\RUNNER~1\\AppData\\Local\\Temp\\TestMergeDir_SubFS959700692\\001"
+	// AFTER: dir="a\\b\\c"  opts.FS="C:\\Users\\RUNNER~1\\AppData\\Local\\Temp\\TestMergeDir_SubFS959700692\\001"
 
 	sorted := &outFile{}
 	var setup sync.Once
@@ -278,7 +303,20 @@ func MergeDir(dir string, conditions Conditions, opts *MergeDirOptions) ([]*File
 	return convertToFiles(sorted, conditions)
 }
 
+// osFilesystem is an io/fs.FS which defers to the os package for opening files
+// See: https://github.com/golang/go/issues/44279
+type osFilesystem struct{}
+
+func (*osFilesystem) Open(name string) (fs.File, error) {
+	fmt.Printf("osFilesystem.Open(%v)\n", name) //nolint:forbidigo
+	return os.Open(name)
+}
+
+var _ fs.FS = new(osFilesystem)
+
 func walkDir(fsys fs.FS, dir string, discoveredPaths chan string) error {
+	fmt.Printf("walkDir: dir=%v  fsys=%#v\n", dir, fsys) //nolint:forbidigo
+
 	reader, ok := fsys.(fs.ReadDirFS)
 	if !ok {
 		return fmt.Errorf("unexpected %T wanted fs.ReadDirFS", fsys)

merge_test.go

@@ -585,9 +585,10 @@ func TestMergeDir__DefaultFileAcceptor(t *testing.T) {
 	require.Equal(t, SkipFile, output)
 }
 
-func TestMergeDir_SubFS(t *testing.T) {
+func TestMergeDir_WithFS(t *testing.T) {
 	dir := t.TempDir()
 	sub := filepath.Join("a", "b", "c")
+	fmt.Printf("dir + sub: %v\n", filepath.Join(dir, sub)) //nolint:forbidigo
 	require.NoError(t, os.MkdirAll(filepath.Join(dir, sub), 0777))
 
 	src, err := os.Open(filepath.Join("test", "testdata", "ppd-debit.ach"))