Validate enums (finally).

context.go

@@ -119,6 +119,17 @@ func (c *Context) Empty() bool {
 
 // Validate the current context.
 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())] {
+				return fmt.Errorf("%s must be one of %s but got %q", value.Summary(), value.Enum, value.Target.Interface())
+			}
+		}
+		return next(nil)
+	})
+	if err != nil {
+		return err
+	}
 	for _, resolver := range c.combineResolvers() {
 		if err := resolver.Validate(c.Model); err != nil {
 			return err

kong_test.go

@@ -654,3 +654,11 @@ func TestHooksCalledForDefault(t *testing.T) {
 	require.Equal(t, "default", string(cli.Flag))
 	require.Equal(t, []string{"before:default", "after:default"}, ctx.values)
 }
+
+func TestEnum(t *testing.T) {
+	var cli struct {
+		Flag string `enum:"a,b,c"`
+	}
+	_, err := mustNew(t, &cli).Parse([]string{"--flag", "d"})
+	require.EqualError(t, err, "--flag=STRING must be one of a,b,c but got \"\"")
+}

model.go

@@ -269,6 +269,11 @@ func (v *Value) IsBool() bool {
 
 // Parse tokens into value, parse, and validate, but do not write to the field.
 func (v *Value) Parse(scan *Scanner, target reflect.Value) error {
+	defer func() {
+		if err := recover(); err != nil {
+			panic(fmt.Sprintf("mapper %T failed to apply to %s: %s", v.Mapper, v.Summary(), err))
+		}
+	}()
 	err := v.Mapper.Decode(&DecodeContext{Value: v, Scan: scan}, target)
 	if err != nil {
 		return fmt.Errorf("%s: %s", v.Summary(), err)