Why is this code accepted?

[nbecker at fred dot net]

This code compiles without complaint both on gcc-2.96 and on a recent gcc-3 snapshot,
but doesn't link.  I suspect the initialization of the static "a" is
invalid (I don't have a good reference handy), but isn't gcc behaviour
wrong?  If the initialization is invalid, why doesn't the compiler
complain (it doesn't even with -Wall)?  If not, why doesn't it link?
--------------------

cd /home/nbecker/bitexact/bitexact/
/opt/gcc/bin/g++ -o bug bug.cc
/tmp/ccZtF0aa.o: In function `X::X()   ':
/tmp/ccZtF0aa.o(.gnu.linkonce.t._ZN1XC1Ev+0xa): undefined reference to `X::a    '
/tmp/ccZtF0aa.o(.gnu.linkonce.t._ZN1XC1Ev+0xf): undefined reference to `X::a    '
collect2: ld returned 1 exit status

--------------------
#include <vector>
using namespace std;

class Y {
  vector<double> c;
public:
  Y (const double* a, const double* b) :
    c (a, b)
  {}
};

class X {
  static const double a[] = {1, 2};
  Y y;
public:
  X () :
    y (&a[0], &a[2])
  {}

};

main () {
  vector<X> x (2);
}
-------------------