fix: update enum+default vars after interpolation

This doesn't recursively apply interpolation, as discussed in the issue,
but that's a much bigger change.

Fixes #337

kong.go

@@ -249,19 +249,19 @@ func (k *Kong) interpolateValue(value *Value, vars Vars) (err error) {
 		return fmt.Errorf("enum for %s: %s", value.Summary(), err)
 	}
 
-	updatedVars := map[string]string{
-		"default": value.Default,
-		"enum":    value.Enum,
-	}
 	if value.Default, err = interpolate(value.Default, vars, nil); err != nil {
 		return fmt.Errorf("default value for %s: %s", value.Summary(), err)
 	}
 	if value.Enum, err = interpolate(value.Enum, vars, nil); err != nil {
 		return fmt.Errorf("enum value for %s: %s", value.Summary(), err)
 	}
+	updatedVars := map[string]string{
+		"default": value.Default,
+		"enum":    value.Enum,
+	}
 	if value.Flag != nil {
 		for i, env := range value.Flag.Envs {
-			if value.Flag.Envs[i], err = interpolate(env, vars, nil); err != nil {
+			if value.Flag.Envs[i], err = interpolate(env, vars, updatedVars); err != nil {
 				return fmt.Errorf("env value for %s: %s", value.Summary(), err)
 			}
 		}

kong_test.go

@@ -2314,3 +2314,16 @@ func TestIntEnum(t *testing.T) {
 	_, err = k.Parse([]string{"--enum=123"})
 	assert.EqualError(t, err, `--enum must be one of "1","2","3" but got "123"`)
 }
+
+func TestRecursiveVariableExpansion(t *testing.T) {
+	var cli struct {
+		Config string `type:"path" default:"${config_file}" help:"Default: ${default}"`
+	}
+	k := mustNew(t, &cli, kong.Vars{"config_file": "/etc/config"}, kong.Exit(func(int) {}))
+	w := &strings.Builder{}
+	k.Stderr = w
+	k.Stdout = w
+	_, err := k.Parse([]string{"--help"})
+	assert.NoError(t, err)
+	assert.Contains(t, w.String(), "Default: /etc/config")
+}