Why is a cast `int**const` to `const int**const` forbidden

The following is allowed:

    int * const p1 = nullptr;
    auto p2 = static_cast< const int * const >( p1 );

In my understanding p1 is a "const-ptr to int", which is casted to a "const-ptr to const-int".

Why is the following forbidden

    int * * const pp1 = nullptr;
    auto pp2 = static_cast< const int * * const >( pp1 );

In my understanding pp1 is a "const-ptr to ptr to int" which shall be casted to a "const-ptr to ptr to const-int". Thus I am adding const, so I would think it is allowed.

Even though it looks like you're only adding const, you can use the resulting pointer to remove constness (a const_cast in disguise). Observe:

int **const pp1 = nullptr;
auto pp2 = (const int **const)pp1;

const int x = 42;
*pp2 = &x;
**pp1 = 43; // Modifying `x` = undefined behavior.

Note that, on the other hand, casting to const int *const * (and const int *const *const) is allowed.