Differentiate between omitted and empty default.

Fixes #250.

build.go

@@ -211,7 +211,7 @@ func buildChild(k *Kong, node *Node, typ NodeType, v reflect.Value, ft reflect.S
 		if child.Help == "" {
 			child.Help = child.Argument.Help
 		}
-	} else if tag.Default != "" {
+	} else if tag.HasDefault {
 		if node.DefaultCmd != nil {
 			return failField(v, ft, "can't have more than one default command under %s", node.Summary())
 		}
@@ -239,6 +239,7 @@ func buildField(k *Kong, node *Node, v reflect.Value, ft reflect.StructField, fv
 		Name:         name,
 		Help:         tag.Help,
 		OrigHelp:     tag.Help,
+		HasDefault:   tag.HasDefault,
 		Default:      tag.Default,
 		DefaultValue: reflect.New(fv.Type()).Elem(),
 		Mapper:       mapper,

context.go

@@ -167,7 +167,7 @@ func (c *Context) Validate() error { // nolint: gocyclo
 		switch node := node.(type) {
 		case *Value:
 			_, ok := os.LookupEnv(node.Tag.Env)
-			if node.Enum != "" && (!node.Required || node.Default != "" || (node.Tag.Env != "" && ok)) {
+			if node.Enum != "" && (!node.Required || node.HasDefault || (node.Tag.Env != "" && ok)) {
 				if err := checkEnum(node, node.Target); err != nil {
 					return err
 				}
@@ -175,7 +175,7 @@ func (c *Context) Validate() error { // nolint: gocyclo
 
 		case *Flag:
 			_, ok := os.LookupEnv(node.Tag.Env)
-			if node.Enum != "" && (!node.Required || node.Default != "" || (node.Tag.Env != "" && ok)) {
+			if node.Enum != "" && (!node.Required || node.HasDefault || (node.Tag.Env != "" && ok)) {
 				if err := checkEnum(node.Value, node.Target); err != nil {
 					return err
 				}

kong.go

@@ -319,7 +319,7 @@ func (k *Kong) applyHookToDefaultFlags(ctx *Context, node *Node, name string) er
 		}
 		binds := k.bindings.clone().add(ctx).add(node.Vars().CloneWith(k.vars))
 		for _, flag := range node.Flags {
-			if flag.Default == "" || ctx.values[flag.Value].IsValid() || !flag.Target.IsValid() {
+			if !flag.HasDefault || ctx.values[flag.Value].IsValid() || !flag.Target.IsValid() {
 				continue
 			}
 			method := getMethod(flag.Target, name)

kong_test.go

@@ -1432,6 +1432,13 @@ func TestEnumValidation(t *testing.T) {
 			}{},
 			false,
 		},
+		{
+			"EnumWithEmptyDefault",
+			&struct {
+				Flag string `enum:"one,two," default:""`
+			}{},
+			false,
+		},
 	}
 	for _, test := range tests {
 		test := test

model.go

@@ -232,6 +232,7 @@ type Value struct {
 	Name         string
 	Help         string
 	OrigHelp     string // Original help string, without interpolated variables.
+	HasDefault   bool
 	Default      string
 	DefaultValue reflect.Value
 	Enum         string
@@ -357,7 +358,7 @@ func (v *Value) Reset() error {
 			return nil
 		}
 	}
-	if v.Default != "" {
+	if v.HasDefault {
 		return v.Parse(ScanFromTokens(Token{Type: FlagValueToken, Value: v.Default}), v.Target)
 	}
 	return nil
@@ -404,7 +405,7 @@ func (f *Flag) FormatPlaceHolder() string {
 	if f.PlaceHolder != "" {
 		return f.PlaceHolder + tail
 	}
-	if f.Default != "" {
+	if f.HasDefault {
 		if f.Value.Target.Kind() == reflect.String {
 			return strconv.Quote(f.Default) + tail
 		}

tag.go

@@ -20,6 +20,7 @@ type Tag struct {
 	Help        string
 	Type        string
 	TypeName    string
+	HasDefault  bool
 	Default     string
 	Format      string
 	PlaceHolder string
@@ -182,9 +183,10 @@ func hydrateTag(t *Tag, typ reflect.Type) error { // nolint: gocyclo
 	}
 	t.Required = required
 	t.Optional = optional
+	t.HasDefault = t.Has("default")
 	t.Default = t.Get("default")
 	// Arguments with defaults are always optional.
-	if t.Arg && t.Default != "" {
+	if t.Arg && t.HasDefault {
 		t.Optional = true
 	} else if t.Arg && !optional { // Arguments are required unless explicitly made optional.
 		t.Required = true
@@ -229,7 +231,7 @@ func hydrateTag(t *Tag, typ reflect.Type) error { // nolint: gocyclo
 	t.PlaceHolder = t.Get("placeholder")
 	t.Enum = t.Get("enum")
 	scalarType := (typ == nil || !(typ.Kind() == reflect.Slice || typ.Kind() == reflect.Map))
-	if t.Enum != "" && !(t.Required || t.Default != "") && scalarType {
+	if t.Enum != "" && !(t.Required || t.HasDefault) && scalarType {
 		return fmt.Errorf("enum value is only valid if it is either required or has a valid default value")
 	}
 	passthrough := t.Has("passthrough")