foo.cc: In function ‘int main()’:
foo.cc:13:37: warning: comparison of integer expressions of different signedness: ‘int’ and ‘const std::__cxx11::basic_string<char>::size_type’ {aka ‘const long unsigned int’} [-Wsign-compare]
   13 |     while ((pos = a1.find(a2, pos)) != string::npos)
      |            ~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~