foo.cc: In function 'void Mul(std::vector<int>&, int)':
foo.cc:10:24: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   10 |         for (int i=0; i<v.size(); i++) {
      |                       ~^~~~~~~~~
foo.cc: At global scope:
foo.cc:19:3: warning: ISO C++ forbids declaration of 'main' with no type [-Wreturn-type]
   19 | } main() {
      |   ^~~~

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