Comment 7 for bug 923397

extern const int v1[];
const int __attribute__((aligned(16))) v1[] = { 0 };
extern const int __attribute__((aligned(16))) v2[];
const int v2[] = { 0 };
extern const int __attribute__((aligned(16))) v3[];
const int __attribute__((aligned(16))) v3[] = { 0 };
const int __attribute__((aligned(16))) v4[] = { 0 };
int test[(__alignof__ (v4) != __alignof__ (v1)/* { dg-bogus "is negative" } */
 || __alignof__ (v4) != __alignof__ (v2)
 || __alignof__ (v4) != __alignof__ (v3)) ? -1 : 0];

fails with gcc 4.7, worked with 4.6, but it looks like the actual bug is of an earlier date, when DECL_USER_ALIGN bit has been moved from tree_decl_common etc.
down into tree_base.