This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 112e. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.
According to 9.12.2 [dcl.align] paragraph 6,
If the defining declaration of an entity has an alignment-specifier, any non-defining declaration of that entity shall either specify equivalent alignment or have no alignment-specifier. Conversely, if any declaration of an entity has an alignment-specifier, every defining declaration of that entity shall specify an equivalent alignment. No diagnostic is required if declarations of an entity have different alignment-specifiers in different translation units.
Because this is phrased in terms of the definition of an entity, an example like the following is presumably well-formed (even though there can be no definition of n):
alignas(8) extern int n; alignas(16) extern int n;
Is this intentional?