Example of weakness of -Werror=return-type in gcc

gistfile1.c

#include <stdio.h>
#include <stdbool.h>

bool f(char achar) {
  switch (achar)
    {
    case 'q':
      return true;
      break;
    case 'w':
      return false;
      break;
    }
  //We shouldn't be here
}

int main() {
  printf("Enter a character\n");
  char input = getchar();

  /*
  if ( !( input == 'q' || input == 'w') ) {
      input = 'w';
    }
  */

  bool output = f(input);

  if ( output )
    { 
      printf("You entered q\n");
    }
  else
    {
      printf("You entered w\n");
    }
  return 0;
}

If compiled with LANG=C && gcc -Werror=return-type gistfile1.c this will always result in

gistfile1.c: In function 'f':
gistfile1.c:15:1: error: control reaches end of non-void function [-Werror=return-type]
cc1: some warnings being treated as errors

no matter if lines 21-25 are commented out or not.

If you leave out the -Werror=return-type, the compiled program will exhibit undefined behaviour for the return value of f() (Chapter 6.9.1, Clause 12, ISO 9899 / C99 standard).

So, what happens when the user enters d?

So, what happens when the user enters d?

Well, undefined behaviour as per my comment from 2014.