_Alignas for members using clang & C11

I'm having trouble with -Wpadded using C11 and structs.

I already read the alignment of structure elements with _Alignas , and I looked in the clang docs and saw that it is now supported.

In addition, I am using a very new version of clang, which I recently created from the torso.

$ clang --version
clang version 3.3 (trunk 175473)
Target: x86_64-unknown-linux-gnu
Thread model: posix

The problem I am facing is this:

#include <stdlib.h>
#include <stdalign.h>

struct foo{
   void* a;
   int b;
};

int main() {
   struct foo instance;

   instance.a = NULL;
   instance.b = 2;

   return 0;
}

What throws me this warning:

$ clang -Weverything -std=c11 t.c 
t.c:4:8: warning: padding size of 'struct foo' with 4 bytes to alignment boundary [-Wpadded]
struct foo{
       ^
1 warning generated.

Isn't that what for _Alignas? I tried putting it before the int declaration, for example:

struct foo{
   void* a;
   _Alignas(void*) int b;
};

But the same warning remains. I also tried putting _Alignas in different places, but to no avail. What am I missing here?

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