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  1. #include <stdio.h>
  2.  
  3. constexpr int f()
  4. {
  5. return printf("a side effect!\n");
  6. }
  7.  
  8. int main()
  9. {
  10. char a[f()];
  11. printf("%zd\n", sizeof a);
  12. }
  13.  
Success #stdin #stdout 0s 3340KB
stdin
Standard input is empty
stdout
a side effect!
15