fix: existing file/dir always checks default

updated existing file/dir mappers to early return if already set;
added tests for existing file/dir mappers;

mapper.go

@@ -612,6 +612,14 @@ func existingFileMapper(r *Registry) MapperFunc {
 		if err != nil {
 			return err
 		}
+
+		if ctx.Value.Set {
+			// 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).
+			return nil
+		}
+
 		if path != "-" {
 			path = ExpandPath(path)
 			stat, err := os.Stat(path)
@@ -640,6 +648,14 @@ func existingDirMapper(r *Registry) MapperFunc {
 		if err != nil {
 			return err
 		}
+
+		if ctx.Value.Set {
+			// 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).
+			return nil
+		}
+
 		path = ExpandPath(path)
 		stat, err := os.Stat(path)
 		if err != nil {

mapper_test.go

@@ -423,6 +423,72 @@ func TestFileMapper(t *testing.T) {
 	require.Equal(t, os.Stdin, cli.File)
 }
 
+//nolint:dupl
+func TestExistingFileMapper(t *testing.T) {
+	type CLI struct {
+		File string `type:"existingfile"`
+	}
+	var cli CLI
+	p := mustNew(t, &cli)
+	_, err := p.Parse([]string{"--file", "testdata/file.txt"})
+	require.NoError(t, err)
+	require.NotNil(t, cli.File)
+	p = mustNew(t, &cli)
+	_, err = p.Parse([]string{"--file", "testdata/missing.txt"})
+	require.Error(t, err)
+	require.Contains(t, err.Error(), "missing.txt: no such file or directory")
+	p = mustNew(t, &cli)
+	_, err = p.Parse([]string{"--file", "testdata/"})
+	require.Error(t, err)
+	require.Contains(t, err.Error(), "exists but is a directory")
+}
+
+func TestExistingFileMapperDefaultMissing(t *testing.T) {
+	type CLI struct {
+		File string `type:"existingfile" default:"testdata/missing.txt"`
+	}
+	var cli CLI
+	p := mustNew(t, &cli)
+	file := "testdata/file.txt"
+	_, err := p.Parse([]string{"--file", file})
+	require.NoError(t, err)
+	require.NotNil(t, cli.File)
+	require.Contains(t, cli.File, file)
+}
+
+//nolint:dupl
+func TestExistingDirMapper(t *testing.T) {
+	type CLI struct {
+		Dir string `type:"existingdir"`
+	}
+	var cli CLI
+	p := mustNew(t, &cli)
+	_, err := p.Parse([]string{"--dir", "testdata/"})
+	require.NoError(t, err)
+	require.NotNil(t, cli.Dir)
+	p = mustNew(t, &cli)
+	_, err = p.Parse([]string{"--dir", "missingdata/"})
+	require.Error(t, err)
+	require.Contains(t, err.Error(), "missingdata: no such file or directory")
+	p = mustNew(t, &cli)
+	_, err = p.Parse([]string{"--dir", "testdata/file.txt"})
+	require.Error(t, err)
+	require.Contains(t, err.Error(), "exists but is not a directory")
+}
+
+func TestExistingDirMapperDefaultMissing(t *testing.T) {
+	type CLI struct {
+		File string `type:"existingdir" default:"missing-dir"`
+	}
+	var cli CLI
+	p := mustNew(t, &cli)
+	file := "testdata"
+	_, err := p.Parse([]string{"--file", file})
+	require.NoError(t, err)
+	require.NotNil(t, cli.File)
+	require.Contains(t, cli.File, file)
+}
+
 func TestMapperPlaceHolder(t *testing.T) {
 	var cli struct {
 		Flag string