Much more thorough checking of enum values.

context.go

@@ -135,13 +135,10 @@ func (c *Context) Empty() bool {
 func (c *Context) Validate() error {
 	err := Visit(c.Model, func(node Visitable, next Next) error {
 		if value, ok := node.(*Value); ok {
-			if value.Enum != "" && !value.EnumMap()[fmt.Sprintf("%v", value.Target.Interface())] {
-				enums := []string{}
-				for enum := range value.EnumMap() {
-					enums = append(enums, fmt.Sprintf("%q", enum))
+			if value.Enum != "" {
+				if err := checkEnum(value, value.Target); err != nil {
+					return err
 				}
-				sort.Strings(enums)
-				return fmt.Errorf("%s must be one of %s but got %q", value.ShortSummary(), strings.Join(enums, ","), value.Target.Interface())
 			}
 		}
 		return next(nil)
@@ -646,6 +643,34 @@ func checkMissingPositionals(positional int, values []*Value) error {
 	return fmt.Errorf("missing positional arguments %s", strings.Join(missing, " "))
 }
 
+func checkEnum(value *Value, target reflect.Value) error {
+	switch target.Kind() {
+	case reflect.Slice, reflect.Array:
+		for i := 0; i < target.Len(); i++ {
+			if err := checkEnum(value, target.Index(i)); err != nil {
+				return err
+			}
+		}
+		return nil
+
+	case reflect.Map, reflect.Struct:
+		return errors.Errorf("enum can only be applied to a slice or value")
+
+	default:
+		enumMap := value.EnumMap()
+		v := fmt.Sprintf("%v", target)
+		if enumMap[v] {
+			return nil
+		}
+		enums := []string{}
+		for enum := range enumMap {
+			enums = append(enums, fmt.Sprintf("%q", enum))
+		}
+		sort.Strings(enums)
+		return fmt.Errorf("%s must be one of %s but got %q", value.ShortSummary(), strings.Join(enums, ","), target.Interface())
+	}
+}
+
 func checkXorDuplicates(paths []*Path) error {
 	for _, path := range paths {
 		seen := map[string]*Flag{}