cyber: make DEFINE_TYPE_TRAITS more readale

cyber/base/macros.h

@@ -32,15 +32,17 @@
 
 #define DEFINE_TYPE_TRAIT(name, func)                     \
   template <typename T>                                   \
-  class name {                                                   \
-   private:                                                      \
+  struct name {                                           \
     template <typename Class>                             \
-    static char Test(decltype(&Class::func)*);                   \
+    static constexpr bool Test(decltype(&Class::func)*) { \
+      return true;                                        \
+    }                                                     \
     template <typename>                                   \
-    static int Test(...);                                        \
+    static constexpr bool Test(...) {                     \
+      return false;                                       \
+    }                                                     \
                                                           \
-   public:                                                       \
-    static constexpr bool value = sizeof(Test<T>(nullptr)) == 1; \
+    static constexpr bool value = Test<T>(nullptr);       \
   };                                                      \
                                                           \
   template <typename T>                                   \