update: mark group mirror as constexpr

src/core/group/group.h

@@ -188,9 +188,9 @@ public:
 
     // TODO: add `is_xxx_mirror` interface
 
-    [[nodiscard]] Group to_vertical_mirror() const;
+    [[nodiscard]] constexpr Group to_vertical_mirror() const;
 
-    [[nodiscard]] Group to_horizontal_mirror() const;
+    [[nodiscard]] constexpr Group to_horizontal_mirror() const;
 
 private:
     uint32_t type_id_;

src/core/group/internal/group.inl

@@ -24,11 +24,11 @@ constexpr uint32_t Group::flat_id() const {
 
 // ----------------------------------------------------------------------------------------- //
 
-Group Group::to_vertical_mirror() const {
+constexpr Group Group::to_vertical_mirror() const {
     return Group::unsafe_create(type_id_, GROUP_MIRROR_V[flat_id()]);
 }
 
-Group Group::to_horizontal_mirror() const {
+constexpr Group Group::to_horizontal_mirror() const {
     return Group::unsafe_create(type_id_, GROUP_MIRROR_H[flat_id()]);
 }