Restore length check for required on empty array/map/slice

required.go

@@ -24,13 +24,21 @@ func Required[T any](value T) (T, error) {
 	return value, err
 }
 
-// reflectRequired checks if the given value is its zero value. It
-// returns an error if the value is zero.
+// reflectRequired checks if the given value is its zero value, or an
+// empty array, map, or slice. It returns an error if so.
 func reflectRequired(value reflect.Value) (reflect.Value, error) {
 	var err error
 
-	if value.IsZero() {
-		err = ErrRequired
+	switch value.Kind() {
+	case reflect.Array, reflect.Map, reflect.Slice:
+		if value.Len() == 0 {
+			err = ErrRequired
+		}
+
+	default:
+		if value.IsZero() {
+			err = ErrRequired
+		}
 	}
 
 	return value, err

required_test.go

@@ -39,3 +39,43 @@ func TestRequiredMissing(t *testing.T) {
 		t.Fatalf("got unexpected error %v", err)
 	}
 }
+
+func TestRequiredEmptySlice(t *testing.T) {
+	value := []string{}
+
+	_, err := v2.Required(value)
+
+	if !errors.Is(err, v2.ErrRequired) {
+		t.Fatalf("got unexpected error %v", err)
+	}
+}
+
+func TestRequiredNonEmptySlice(t *testing.T) {
+	value := []string{"test"}
+
+	_, err := v2.Required(value)
+
+	if err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestRequiredEmptyMap(t *testing.T) {
+	value := map[string]string{}
+
+	_, err := v2.Required(value)
+
+	if !errors.Is(err, v2.ErrRequired) {
+		t.Fatalf("got unexpected error %v", err)
+	}
+}
+
+func TestRequiredNonEmptyMap(t *testing.T) {
+	value := map[string]string{"key": "value"}
+
+	_, err := v2.Required(value)
+
+	if err != nil {
+		t.Fatal(err)
+	}
+}