Abstract class needs at least one abstract method to be considered abstract (#2053)

The introduction of the method is needed, for instance, for constructions like std::is_abstract<AbstractBase>::value to be evaluated to true

CppCoreGuidelines.md

@@ -4938,6 +4938,7 @@ defined as defaulted.
 
     class AbstractBase {
     public:
+        virtual void foo() = 0;  // at least one abstract method to make the class abstract
         virtual ~AbstractBase() = default;
         // ...
     };