Proposed resolution (approved by CWG 2023-04-28):

Insert a new paragraph after 13.7.4 [temp.variadic] paragraph 9 as follows:

The instantiation of a sizeof... expression (7.6.2.5 [expr.sizeof]) produces an integral constant with value N.

The instantiation of an alignment-specifier with an ellipsis produces E₁ E₂ ... E_N.

[Accepted as a DR at the June, 2023 meeting.]

Subclause 13.7.4 [temp.variadic] paragraph 11 specifies:

The instantiation of any other pack expansion produces a list of elements E1, E2, ... , EN.

Consider this example:

  template<class ...T>
  struct Align{
   alignas(T...) unsigned char buffer[128];
  };
  Align<int, short> a;

The pack expansion of the alignment-specifier yields alignas(int), alignas(short), an ill-formed list per the grammar in 9.12.1 [dcl.attr.grammar].