fix: slices/maps of existingfile would fail to work

mapper.go

@@ -540,7 +540,7 @@ func sliceDecoder(r *Registry) MapperFunc {
 		var childScanner *Scanner
 		if ctx.Value.Flag != nil {
 			t := ctx.Scan.Pop()
-			// If decoding a flag, we need an value.
+			// If decoding a flag, we need a value.
 			if t.IsEOL() {
 				return fmt.Errorf("missing value, expecting \"<arg>%c...\"", sep)
 			}
@@ -641,7 +641,7 @@ func existingFileMapper(r *Registry) MapperFunc {
 			return err
 		}
 
-		if !ctx.Value.Active || ctx.Value.Set {
+		if !ctx.Value.Active || (ctx.Value.Set && ctx.Value.Target.Type() == target.Type()) {
 			// early return to avoid checking extra files that may not exist;
 			// this hack only works because the value provided on the cli is
 			// checked before the default value is checked (if default is set).
@@ -677,7 +677,7 @@ func existingDirMapper(r *Registry) MapperFunc {
 			return err
 		}
 
-		if !ctx.Value.Active || ctx.Value.Set {
+		if !ctx.Value.Active || (ctx.Value.Set && ctx.Value.Target.Type() == target.Type()) {
 			// early return to avoid checking extra dirs that may not exist;
 			// this hack only works because the value provided on the cli is
 			// checked before the default value is checked (if default is set).

mapper_test.go

@@ -553,6 +553,20 @@ func TestExistingFileMapper(t *testing.T) {
 	assert.Contains(t, err.Error(), "exists but is a directory")
 }
 
+func TestExistingFileMapperSlice(t *testing.T) {
+	type CLI struct {
+		Files []string `type:"existingfile"`
+	}
+	var cli CLI
+	p := mustNew(t, &cli)
+	_, err := p.Parse([]string{"--files", "testdata/file.txt", "--files", "testdata/file.txt"})
+	assert.NoError(t, err)
+	assert.NotZero(t, cli.Files)
+	pwd, err := os.Getwd()
+	assert.NoError(t, err)
+	assert.Equal(t, []string{filepath.Join(pwd, "testdata", "file.txt"), filepath.Join(pwd, "testdata", "file.txt")}, cli.Files)
+}
+
 func TestExistingFileMapperDefaultMissing(t *testing.T) {
 	type CLI struct {
 		File string `type:"existingfile" default:"testdata/missing.txt"`