Error(s):

source_file.cpp:11:9: error: no function template matches function template specialization 'insert'
void S::insert<char*>(const char * &) {}
        ^
source_file.cpp:7:10: note: candidate template ignored: could not match 'void (char *const &)' against 'void (const char *&)'
    void insert(const T& x);
         ^
1 error generated.