int (*arrayp)[len] = malloc(sizeof(*arrayp));
/* init *arrayp */
qsort_typed(*arrayp, cmp, data);

or, what could be more useful, say you have a struct type:

struct int_array { size_t len; int *data };
#define to_vla(arr) ((int (*)[arr.len]) arr.data)

struct int_array my_ints = ...;
qsort_typed(*to_vla(my_ints), compare, data);

and it’s the reason i really wish we add a way to specify VLA syntax in structures, so we could end up with something similar to:

struct int_array { size_t len; int (*data)[len]; };

struct int_array my_ints = ...;
qsort_typed(*my_ints.data, compare, data);