deal with "-" as option value

parse.go

@@ -174,6 +174,8 @@ func NewParser(config Config, dests ...interface{}) (*Parser, error) {
 					}
 
 					switch {
+					case strings.HasPrefix(key, "---"):
+						errs = append(errs, fmt.Sprintf("%s.%s: too many hyphens", t.Name(), field.Name))
 					case strings.HasPrefix(key, "--"):
 						spec.long = key[2:]
 					case strings.HasPrefix(key, "-"):
@@ -285,7 +287,7 @@ func process(specs []*spec, args []string) error {
 			continue
 		}
 
-		if !strings.HasPrefix(arg, "-") || allpositional {
+		if !isFlag(arg) || allpositional {
 			positionals = append(positionals, arg)
 			continue
 		}
@@ -309,7 +311,7 @@ func process(specs []*spec, args []string) error {
 		if spec.multiple {
 			var values []string
 			if value == "" {
-				for i+1 < len(args) && !strings.HasPrefix(args[i+1], "-") {
+				for i+1 < len(args) && !isFlag(args[i+1]) {
 					values = append(values, args[i+1])
 					i++
 				}
@@ -331,7 +333,7 @@ func process(specs []*spec, args []string) error {
 
 		// if we have something like "--foo" then the value is the next argument
 		if value == "" {
-			if i+1 == len(args) || strings.HasPrefix(args[i+1], "-") {
+			if i+1 == len(args) || isFlag(args[i+1]) {
 				return fmt.Errorf("missing value for %s", arg)
 			}
 			value = args[i+1]
@@ -370,6 +372,11 @@ func process(specs []*spec, args []string) error {
 	return nil
 }
 
+// isFlag returns true if a token is a flag such as "-v" or "--user" but not "-" or "--"
+func isFlag(s string) bool {
+	return strings.HasPrefix(s, "-") && strings.TrimLeft(s, "-") != ""
+}
+
 // validate an argument spec after arguments have been parse
 func validate(spec []*spec) error {
 	for _, arg := range spec {

parse_test.go

@@ -693,3 +693,49 @@ func TestEmptyArgs(t *testing.T) {
 	// put the original arguments back
 	os.Args = origArgs
 }
+
+func TestTooManyHyphens(t *testing.T) {
+	var args struct {
+		TooManyHyphens string `arg:"---x"`
+	}
+	err := parse("--foo -", &args)
+	assert.Error(t, err)
+}
+
+func TestHyphenAsOption(t *testing.T) {
+	var args struct {
+		Foo string
+	}
+	err := parse("--foo -", &args)
+	require.NoError(t, err)
+	assert.Equal(t, "-", args.Foo)
+}
+
+func TestHyphenAsPositional(t *testing.T) {
+	var args struct {
+		Foo string `arg:"positional"`
+	}
+	err := parse("-", &args)
+	require.NoError(t, err)
+	assert.Equal(t, "-", args.Foo)
+}
+
+func TestHyphenInMultiOption(t *testing.T) {
+	var args struct {
+		Foo []string
+		Bar int
+	}
+	err := parse("--foo --- x - y --bar 3", &args)
+	require.NoError(t, err)
+	assert.Equal(t, []string{"---", "x", "-", "y"}, args.Foo)
+	assert.Equal(t, 3, args.Bar)
+}
+
+func TestHyphenInMultiPositional(t *testing.T) {
+	var args struct {
+		Foo []string `arg:"positional"`
+	}
+	err := parse("--- x - y", &args)
+	require.NoError(t, err)
+	assert.Equal(t, []string{"---", "x", "-", "y"}, args.Foo)
+}