size of array is not an integral constant-expression

#include <vector>
using namespace std;

template <typename Iter>
int
foo(Iter first, Iter last, int nn)
{
 const size_t n = last - first;
 double buf[n];
 return 0;
}

int
main(int argc, char **argv)
{
 vector<double> x;
 foo(x.begin(), x.end(), argc);
 return 0;
}

foo.cc:17:   instantiated from here
foo.cc:9: error: size of array is not an integral constant-expression

g++ 4.2.1

Is this error specific to g++ 4.x?  g++ 3.6.4 and g++ 2.9.5 have no
problems with it, but that doesn't mean they are right.  Is there some
reason to expect this to fail.

There are a few interesting workarounds that point to this being
unexpected behavior... I'll post those next.