Understanding "Expression does not compute the number of elements in this array"

Using Apple Clang 12.0.0 to compile this code:

int my_array[10];
int arr_size = sizeof(my_array) / sizeof(decltype(my_array[0]))

And getting this warning/error:

Expression does not compute the number of elements in this array; element type is 'int', not 'decltype(my_array[0])' (aka 'int &')

Note, this is simplified code. In the real code instead of 'int' there's a class type, instead of '10' there's an expression.

Why am I getting this warning, and what is the correct way to calculate the array size without warnings?