foo.cc: In function ‘void fib(long long int)’:
foo.cc:13:13: warning: ‘d’ may be used uninitialized [-Wmaybe-uninitialized]
   13 |     cout << d << endl;
      |             ^
foo.cc:5:26: note: ‘d’ was declared here
    5 |     int a = 1, b = 1, c, d;
      |                          ^