Error(s):

source_file.cpp:19:8: error: class template partial specialization does not specialize any template argument; to define the primary template, remove the template argument list
struct Abstract<T,C> : Abstract<T>
       ^       ~~~~~
source_file.cpp:18:22: error: too few template parameters in template template parameter redeclaration
template<typename T, template<bool> class C>
                     ^~~~~~~~~~~~~~
source_file.cpp:9:11: note: previous template template parameter is here
          template<bool,typename ...> class = Default>
          ^~~~~~~~~~~~~~~~~~~~~~~~~~~
2 errors generated.