fix: set trailing positional arguments to active

This fixes an issue where existingfile et al would not correctly apply
to positional arguments with defaults.

context.go

@@ -520,9 +520,26 @@ func (c *Context) trace(node *Node) (err error) { // nolint: gocyclo
 			return fmt.Errorf("unexpected token %s", token)
 		}
 	}
+	if err := c.traceDefaults(); err != nil {
+		return fmt.Errorf("error tracing defaults: %w", err)
+	}
 	return c.maybeSelectDefault(flags, node)
 }
 
+func (c *Context) traceDefaults() error {
+	tail := c.Path[len(c.Path)-1]
+	for _, positional := range tail.Node().Positional {
+		if positional.DefaultValue.IsValid() {
+			positional.Active = true
+			c.Path = append(c.Path, &Path{
+				Parent:     tail.Node(),
+				Positional: positional,
+			})
+		}
+	}
+	return nil
+}
+
 // End of the line, check for a default command, but only if we're not displaying help,
 // otherwise we'd only ever display the help for the default command.
 func (c *Context) maybeSelectDefault(flags []*Flag, node *Node) error {
@@ -532,6 +549,7 @@ func (c *Context) maybeSelectDefault(flags []*Flag, node *Node) error {
 		}
 	}
 	if node.DefaultCmd != nil {
+		node.Active = true
 		c.Path = append(c.Path, &Path{
 			Parent:  node.DefaultCmd,
 			Command: node.DefaultCmd,

kong_test.go

@@ -4,6 +4,8 @@ import (
 	"bytes"
 	"errors"
 	"fmt"
+	"os"
+	"path/filepath"
 	"strings"
 	"testing"
 
@@ -1984,3 +1986,16 @@ func TestEnumPtrOmittedNoDefault(t *testing.T) {
 	assert.NotZero(t, ctx)
 	assert.Zero(t, cli.X)
 }
+
+func TestTrailingPositionalActive(t *testing.T) {
+	var cli struct {
+		Arg string `arg:"" default:"." existingdir:"testdata"`
+	}
+	pwd, err := os.Getwd()
+	assert.NoError(t, err)
+	k, err := kong.New(&cli)
+	assert.NoError(t, err)
+	_, err = k.Parse([]string{})
+	assert.NoError(t, err)
+	assert.Equal(t, cli.Arg, filepath.Join(pwd, "testdata"))
+}