foo.cc: In function 'int concat(char*, char*, char*)':
foo.cc:10:23: warning: comparison of integer expressions of different signedness: 'int' and 'size_t' {aka 'long unsigned int'} [-Wsign-compare]
   10 |     for (int i = 0; i < strlen(str1) - 1; i++)
      |                     ~~^~~~~~~~~~~~~~~~~~
foo.cc:14:23: warning: comparison of integer expressions of different signedness: 'int' and 'size_t' {aka 'long unsigned int'} [-Wsign-compare]
   14 |     for (int i = 1; i < strlen(str2); i++)
      |                     ~~^~~~~~~~~~~~~~

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