Undefined Behavior Sample

test.cpp

#include <iostream>

int foo() {
  std::cout << "foo" << std::endl;
}

void bar() {
  std::cout << "bar" << std::endl;
  foo();
}

int main() {
  foo();
}

$ g++ --version
g++ (Homebrew GCC 8.2.0) 8.2.0
Copyright (C) 2018 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ g++ -O1 test.cpp
test.cpp: In function 'int foo()':
test.cpp:5:1: warning: no return statement in function returning non-void [-Wreturn-type]
 }
 ^

$ ./a.out
foo
bar
(snip)
foo
bar
foo
zsh: segmentation fault  ./a.out