Can “sizeof(arr[0])” lead to undefined behavior?

Question

There is a well known pattern of figuring out array length:

int arr[10]; 
size_t len = sizeof(arr) / sizeof(arr[0]); 
assert(len == 10); 

T

Answer 1

In general case, if I'm missing something, dereferencing a null pointer under sizeof can lead to undefined behavior. Since C99, sizeof is not a purely compile time construct. The operand of sizeof is evaluated at run-time if the operand type is a VLA.

Consider the following example

unsigned n = 10;
int (*a)[n] = NULL; // `a` is a pointer to a VLA 

unsigned i = 0;
sizeof a[i++];      // applying `sizeof` to a VLA

According to the C99 standard, the argument of sizeof is supposed to be evaluated (i.e. i is supposed to get incremented). Yet I'm not entirely sure that the null-point dereference in a[0] is supposed to produce undefined behavior here.