In stdlib_checked.h the function calloc is defined thus:

extern _Itype_for_any(T) void *calloc(size_t nmemb, size_t size) : itype(_Array_ptr<T>) byte_count(nmemb * size);

but this type fails to recognize that since the returned memory is zeroed out, we can treat the buffer as an _Nt_array_ptr instead. Would the following type be acceptable?

extern _Itype_for_any(T) void *calloc(size_t nmemb, size_t size) : itype(_Nt_array_ptr<T>) byte_count((nmemb-1) * size);

I tried to use this type and the compiler rejected it, saying

x.c:5:74: error: 'type name' declared as _Nt_array_ptr of type 'T' (aka
      '(0, 0) __BoundsInterface'); only integer and pointer types are allowed
  ...void *calloc(size_t nmemb, size_t size) : itype(_Nt_array_ptr<T>) byte_c...

Another problem I can see is that a normal array pointer would be allocated as an NT array pointer first, which could then be cast to the normal one. But doing so would cause the length to drop by one. I.e., a _Nt_array_ptr<char> : count(3) actually represents a buffer of size 4, but you can only cast it to _Array_ptr<char> : count(3) so that the alias cannot be used to destroy the 0 terminator.