foo.cc: In function 'void Concat(const char*, const char*, char*)':
foo.cc:9:9: warning: unused variable 'len2' [-Wunused-variable]
    9 |     int len2 = strlen(str2);
      |         ^~~~
foo.cc:12:12: warning: 'char* strncpy(char*, const char*, size_t)' specified bound depends on the length of the source argument [-Wstringop-truncation]
   12 |     strncpy(result, str1, len1 - 1);
      |     ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~
foo.cc:8:22: note: length computed here
    8 |     int len1 = strlen(str1);
      |                ~~~~~~^~~~~~