[eigen] Failing stable_norm_1 test

[Jitse Niesen <jitse@xxxxxxxxxxxxxxxxx>]

Hello,

I just spent some time on the stable_norm_1 test, which fails on both my 
computer and Gael's computer when using gcc 4.3. It looks similar to the 
issue which you solved by the empty struct magic (well done!).

The details are as follows. I constructed the following (fairly) minimal 
test example:

#include <iostream>
#include <Eigen/Dense>

int main()
{
  Eigen::Matrix<float,1,1> v;
  v(0,0) = 0.1;
  std::cerr << "blue norm = " << v.colwise().blueNorm() << "\n";
  std::cerr << "status = " << v.colwise().hypotNorm().isApprox(v.colwise().norm(), 1e-4) << "\n";
}

When compiling with g++ (version 4.3.3) -O2 -g and running under valgrind, 
this produces:

Use of uninitialised value of size 4
    at 0x8048F28: bool Eigen::DenseBase<Eigen::PartialReduxExpr<Eigen::Matrix<float, 1, 1, 0, 1, 1>, Eigen::ei_member_hypotNorm<float>, 0> >::isApprox<Eigen::PartialReduxExpr<Eigen::Matrix<float, 1, 1, 0, 1, 1>, Eigen::ei_member_norm<float>, 0> >(Eigen::DenseBase<Eigen::PartialReduxExpr<Eigen::Matrix<float, 1, 1, 0, 1, 1>, Eigen::ei_member_norm<float>, 0> > const&, float) const (in /scratch/1/users/jitse/work/c/eigen/eigenfoo)
    by 0x8048AF5: main (in /scratch/1/users/jitse/work/c/eigen/eigenfoo)

and the same error with the same stack trace a bit later, at 0x8048F2A.

The result of the program is also wrong: it should print status = 1, but 
it says status = 0.

The error goes away when compiling with -fno-strict-aliasing.
I tried compiling with -Wstrict-aliasing=2 but that does not produce any 
warnings.

Over to the gurus!

Cheers,
Jitse