C is a very "strict" language. It do the right thing only if you write the right code. It has many undefined behaviors. One of the undefined behaviors I found when I studied the compiler course. The purposed grammar from https://www.amazon.com/Programming-Language-Brian-W-Kernigha... allows this kind of structure, there can be statements after the switch but before the case labels. gcc also accepts this, but those statements will not be executed. Note: code modified from https://www.tutorialspoint.com/cprogramming/switch_statement...

#include "stdio.h"

int main () {

   /* local variable definition */
   char grade = 'B';
   int a = 1;

   switch(grade) {
      printf("great!\n" );
      a = a + 1;
      
      case 'A' :
         printf("Excellent!\n" );
         break;
      case 'B' :
      case 'C' :
         printf("Well done\n" );
         printf("a: %d\n", a);
         break;
      case 'D' :
         printf("You passed\n" );
         break;
      case 'F' :
         printf("Better try again\n" );
         break;
      default :
         printf("Invalid grade\n" );
   }
   
   printf("Your grade is  %c\n", grade );
 
   return 0;
}