Let's take step by step.

Can we agree that the following is true always?

char*  ptr;

  The least significant nibble of 'ptr' can be any value in {0,1,2,3,4,5,6,7,8}

char**  c_ptr = &ptr;

  The least significant nibble of 'c_ptr' can have only 2 values : {0, alignof(void*)}

The former is to be able to index any item of a char array.

And the later depends on the platform.

Since the later is alway guaranteed, then we can use some N lower bits (which are always 0), to store a bit tag.

This tagging cannot be applied to the former because of pointer arithmetics.

Is there an exception to the above?

And on which platform?