using the "align (16) struct __uint128_t" struct from __builtins.di:

_Static_assert( _Alignof(__uint128_t) == 16, "" ); // fail

it gives the size of a pointer (4 or 8), same as if align() wasn't used


it has the correct alignment in D:

import __builtins;
pragma(msg, __uint128_t.alignof); // 16


_Alignas() in C doesn't have the problem:

struct S { _Alignas(16) unsigned long long a; unsigned long long b; };
_Static_assert( _Alignof(struct S) == 16, "this works" );
_Static_assert( sizeof(struct S) == 16, "this works" );