Client cannot access struct int_stack's member

step3-compile-error2.c

/* main.c */

while (stack->top) {
      printf("`%d` was popped from the stack\n", pop_stack(stack));
}

// Output from the compiler:
main.c:11:17: error: incomplete definition of type 'struct int_stack'
    while (stack->top) {
           ~~~~~^
./int_stack.h:6:16: note: forward declaration of 'struct int_stack'
typedef struct int_stack IntStack;
               ^