It's about whether you can put stack allocated VLAs into temporary structs:

   keno@anubis:~$ cat test2.c
   void bar(int *);
   int foo(int n) {
       struct {
           int non_terminal_val[n];
           int some_other_member;
       } val;
       bar(val.non_terminal_val);
       return val.some_other_member;
    }
    keno@anubis:~$ gcc -c test2.c
    keno@anubis:~$ clang test2.c
    test2.c:4:13: error: fields must have a constant size: 
    'variable length array in
          structure' extension will never be supported
            int non_terminal_val[n];
                ^
    1 error generated.