Why isn't it legal to convert “pointer to pointer to non-const” to a “pointer to pointer to const”

Question

It is legal to convert a pointer-to-non-const to a pointer-to-const.

Then why isn\'t it legal to convert a pointer to pointer to non-const to a

Answer 1

Just since nobody has posted the solution, here:

char *s1 = 0;
const char *s2 = s1; // OK...
char *a[MAX]; // aka char **
const char * const*ps = a; // no error!

(http://www.parashift.com/c++-faq-lite/const-correctness.html#faq-18.17 for why)