autoconst.log

random:~/devel/local/test$ gcc -Og -std=gnu2x -Wall -Wextra -Werror -c -o autoconst.o autoconst.c
autoconst.c: In function ‘test’:
autoconst.c:12:18: error: passing argument 1 of ‘potemkin’ from incompatible pointer type [-Werror=incompatible-pointer-types]
   12 |         potemkin(data0);
      |                  ^~~~~
      |                  |
      |                  char **
autoconst.c:6:35: note: expected ‘const char **’ but argument is of type ‘char **’
    6 | extern void potemkin(const char **data);
      |                      ~~~~~~~~~~~~~^~~~
autoconst.c:15:18: error: passing argument 1 of ‘potemkin’ from incompatible pointer type [-Werror=incompatible-pointer-types]
   15 |         potemkin(data1);
      |                  ^~~~~
      |                  |
      |                  char **
autoconst.c:6:35: note: expected ‘const char **’ but argument is of type ‘char **’
    6 | extern void potemkin(const char **data);
      |                      ~~~~~~~~~~~~~^~~~
autoconst.c:17:42: error: invalid initializer
   17 |         const typeof(data->data) data2 = data->data;
      |                                          ^~~~
autoconst.c:18:18: error: passing argument 1 of ‘potemkin’ from incompatible pointer type [-Werror=incompatible-pointer-types]
   18 |         potemkin(data2);
      |                  ^~~~~
      |                  |
      |                  char * const*
autoconst.c:6:35: note: expected ‘const char **’ but argument is of type ‘char * const*’
    6 | extern void potemkin(const char **data);
      |                      ~~~~~~~~~~~~~^~~~
autoconst.c:20:42: error: invalid initializer
   20 |         typeof(data->data) const data3 = data->data;
      |                                          ^~~~
autoconst.c:21:18: error: passing argument 1 of ‘potemkin’ from incompatible pointer type [-Werror=incompatible-pointer-types]
   21 |         potemkin(data3);
      |                  ^~~~~
      |                  |
      |                  char * const*
autoconst.c:6:35: note: expected ‘const char **’ but argument is of type ‘char * const*’
    6 | extern void potemkin(const char **data);
      |                      ~~~~~~~~~~~~~^~~~
cc1: all warnings being treated as errors

typeofconst.c

struct data {
	void *list;
	char *data[0];
};

extern void potemkin(const char **data);

void
test(struct data *data)
{
	const __auto_type data0 = data->data;
	potemkin(data0);

	__auto_type const data1 = data->data;
	potemkin(data1);

	const typeof(data->data) data2 = data->data;
	potemkin(data2);

	typeof(data->data) const data3 = data->data;
	potemkin(data3);
}