Allow "-" as a positional or flag argument.

See #70.

There are some more elaborate ideas in that issue, but I think this is
sufficient for now.

context.go

@@ -279,6 +279,12 @@ func (c *Context) trace(node *Node) (err error) { // nolint: gocyclo
 			case string:
 
 				switch {
+				case v == "-":
+					fallthrough
+				default: // nolint
+					c.scan.Pop()
+					c.scan.PushTyped(token.Value, PositionalArgumentToken)
+
 				// Indicates end of parsing. All remaining arguments are treated as positional arguments only.
 				case v == "--":
 					c.scan.Pop()
@@ -305,9 +311,6 @@ func (c *Context) trace(node *Node) (err error) { // nolint: gocyclo
 					}
 					c.scan.PushTyped(parts[0], FlagToken)
 
-				case v == "-":
-					return errors.New("expected short flag")
-
 				// Short flag.
 				case strings.HasPrefix(v, "-"):
 					c.scan.Pop()
@@ -316,10 +319,6 @@ func (c *Context) trace(node *Node) (err error) { // nolint: gocyclo
 						c.scan.PushTyped(tail, ShortFlagTailToken)
 					}
 					c.scan.PushTyped(v[1:2], ShortFlagToken)
-
-				default:
-					c.scan.Pop()
-					c.scan.PushTyped(token.Value, PositionalArgumentToken)
 				}
 			default:
 				c.scan.Pop()

kong_test.go

@@ -863,3 +863,19 @@ func TestMultipleDefaultCommands(t *testing.T) {
 	_, err := p.Parse([]string{})
 	require.EqualError(t, err, "can't have more than one default command under  <command>")
 }
+
+func TestLoneHpyhen(t *testing.T) {
+	var cli struct {
+		Flag string
+		Arg  string `arg:"" optional:""`
+	}
+	p := mustNew(t, &cli)
+
+	_, err := p.Parse([]string{"-"})
+	require.NoError(t, err)
+	require.Equal(t, "-", cli.Arg)
+
+	_, err = p.Parse([]string{"--flag", "-"})
+	require.NoError(t, err)
+	require.Equal(t, "-", cli.Flag)
+}

scanner.go

@@ -78,13 +78,16 @@ func (t TokenType) IsAny(types ...TokenType) bool {
 
 // InferredType tries to infer the type of a token.
 func (t Token) InferredType() TokenType {
-	if t.Type == UntypedToken {
-		if v, ok := t.Value.(string); ok {
-			if strings.HasPrefix(v, "--") {
-				return FlagToken
-			} else if strings.HasPrefix(v, "-") {
-				return ShortFlagToken
-			}
+	if t.Type != UntypedToken {
+		return t.Type
+	}
+	if v, ok := t.Value.(string); ok {
+		if strings.HasPrefix(v, "--") { // nolint: gocritic
+			return FlagToken
+		} else if v == "-" {
+			return PositionalArgumentToken
+		} else if strings.HasPrefix(v, "-") {
+			return ShortFlagToken
 		}
 	}
 	return t.Type

scanner_test.go

@@ -7,11 +7,13 @@ import (
 )
 
 func TestScannerTake(t *testing.T) {
-	s := Scan("a", "b", "c")
-	require.Equal(t, s.Pop().Value, "a")
-	require.Equal(t, s.Pop().Value, "b")
-	require.Equal(t, s.Pop().Value, "c")
-	require.Equal(t, s.Pop().Type, EOLToken)
+	s := Scan("a", "b", "c", "-")
+	require.Equal(t, "a", s.Pop().Value)
+	require.Equal(t, "b", s.Pop().Value)
+	require.Equal(t, "c", s.Pop().Value)
+	hyphen := s.Pop()
+	require.Equal(t, PositionalArgumentToken, hyphen.InferredType())
+	require.Equal(t, EOLToken, s.Pop().Type)
 }
 
 func TestScannerPeek(t *testing.T) {