[hardware.interference] Bad examples of using alignas

Example 1 is the following:

struct keep_apart {
  alignas(hardware_destructive_interference_size) atomic<int> cat;
  alignas(hardware_destructive_interference_size) atomic<int> dog;
};

There is nothing specifying that these must be valid alignments, or that they must even be nonnegative integer powers of two. Would it make sense to use something like bit_ceil here, plus a check to handle the case where that result is not representable? That would fix the latter issue, not sure if it is possible to check if something is a valid alignment.

Example 2 is the following:

struct together {
  atomic<int> dog;
  int puppy;
};
struct kennel {
  // Other data members...
  alignas(sizeof(together)) together pack;
  // Other data members...
};
static_assert(sizeof(together) <= hardware_constructive_interference_size);

Why is alignas(sizeof(together)) being done? The two issues mentioned above are also present here.

Start with the two hardware interference examples quoted in the issue and review the surrounding standard-draft wording for alignas and alignment requirements. Determine whether the examples need valid-alignment handling and whether alignas(sizeof(together)) is justified. Done means the examples and their explanation no longer imply unsupported alignment assumptions, with the rationale for the second example made clear.