In the C++0x draft, [dcl.align] says:

"An alignment-specifier may be applied to a variable or to a class
data member, but it shall not be applied to a bit-field, a function
parameter, the formal parameter of a catch clause (15.3), or a
variable declared with the register storage class specifier.  An
alignment-specifier may also be applied to the declaration of a class
or enumeration type."

This does not allow its use on a typedef or next to a typename template parameter.  It might make sense for gcc to support that as an extension, but gcc's current behavior is not what people expect that extension to do:

$ cat test.cc
#include <iostream>

#define ALIGNED(x) __attribute__((aligned(x)))

struct Char15 {
  char x[15];
}  ALIGNED(8);

template<typename T>
void print_type_alignment(const T&) {
  struct { char c; T t; } s;
  std::cout << sizeof(T) << ' ' << ((char*)&s.t - (char*)&s.c) << '\n';
}

int main() {
  typedef char unaligned[15];
  typedef char aligned[15] ALIGNED(8);
  unaligned x[10];
  aligned y[10];
  Char15 c15[10];
  std::cout << sizeof(unaligned) << ' ' << sizeof(x) << '\n';
  std::cout << sizeof(aligned) << ' ' << sizeof(y) << '\n';
  std::cout << sizeof(Char15) << ' ' << sizeof(c15) << '\n';
  
  aligned z ALIGNED(8);
  print_type_alignment<unaligned ALIGNED(8)>(z);
}

$ g++-mp-4.6 -std=gnu++0x test.cc && ./a.out
15 150
15 152
16 160
15 1


Note that the alignment on the typedef applies to the final variable, not the defined type, which means that interior members of arrays of the defined type have an unexpected alignment. This has been reported several times before (PR43798, PR47557, PR12742, PR42098), but the core problem seems to be that alignments on typedefs aren't supported.

__attribute__((aligned)) on template arguments seems to have no effect at all.





$ g++-mp-4.6 -v
Using built-in specs.
COLLECT_GCC=g++-mp-4.6
COLLECT_LTO_WRAPPER=/opt/local/libexec/gcc/x86_64-apple-darwin10/4.6.0/lto-wrapper
Target: x86_64-apple-darwin10
Configured with: ../gcc-4.6-20110305/configure --prefix=/opt/local --build=x86_64-apple-darwin10 --enable-languages=c,c++,objc,obj-c++ --libdir=/opt/local/lib/gcc46 --includedir=/opt/local/include/gcc46 --infodir=/opt/local/share/info --mandir=/opt/local/share/man --datarootdir=/opt/local/share/gcc-4.6 --with-local-prefix=/opt/local --with-system-zlib --disable-nls --program-suffix=-mp-4.6 --with-gxx-include-dir=/opt/local/include/gcc46/c++/ --with-gmp=/opt/local --with-mpfr=/opt/local --with-mpc=/opt/local --enable-stage1-checking --disable-multilib --enable-fully-dynamic-string
Thread model: posix
gcc version 4.6.0 20110305 (experimental) (GCC)