[eigen] help using foo(const MatrixBase<T>&) with (u * v)

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We're trying to follow the advice on this page:


and running into trouble passing the _expression_ template (u * v)
to a function declared as suggested above.

It seems to work fine for transpose, for Map, for MatrixXd, but
here's a minimal example of the problem we're running into with

#include <Eigen/Dense>

template <typename Derived>
double foo(const Eigen::MatrixBase<Derived>& a, int m, int n) {
  return a(m,n);

int main() {
  Eigen::MatrixXd u(2,2);
  u << 1, 2, 3, 4;

  Eigen::MatrixXd v(2,3);
  v << 1, 2, 3, 4, 5, 6;

  foo(u*v, 0, 0);

  return 0;

~/temp2$ clang++ -I ~/stan/lib/eigen_3.2.2 eigen-test.cpp

~/temp2$ ./a.out
Assertion failed: (this->rows() == 1 && this->cols() == 1), function coeff, file /Users/carp/stan/lib/eigen_3.2.2/Eigen/src/Core/ProductBase.h, line 141.
Abort trap: 6

~/temp2$ clang++ --version
Apple LLVM version 6.0 (clang-600.0.56) (based on LLVM 3.5svn)
Target: x86_64-apple-darwin14.1.0
Thread model: posix


Here's the relevant part of Eigen/src/Core/ProductBase.h:

    typename Base::CoeffReturnType coeff(Index row, Index col) const
      return lhs().row(row).cwiseProduct(rhs().col(col).transpose()).sum();
      eigen_assert(this->rows() == 1 && this->cols() == 1);   // *** LINE 141 ***
      Matrix<Scalar,1,1> result = *this;
      return result.coeff(row,col);


If I change the #ifdef to an #ifndef, everything works as expected.
Are we somehow supposed to be setting the EIGEN2_SUPPORT property
ourselves?  If I do that in the compilation, it works as expected, but
I get a deprecation warning:

~/temp2$ clang++ -I ~/stan/lib/eigen_3.2.2 -DEIGEN2_SUPPORT eigen-test.cpp
In file included from eigen-test.cpp:1:
In file included from /Users/carp/stan/lib/eigen_3.2.2/Eigen/Dense:1:
In file included from /Users/carp/stan/lib/eigen_3.2.2/Eigen/Core:373:
/Users/carp/stan/lib/eigen_3.2.2/Eigen/Eigen2Support:20:2: warning: "Eigen2 support is
      deprecated in Eigen 3.2.x and it will be removed in Eigen 3.3. (Define
      EIGEN_NO_EIGEN2_DEPRECATED_WARNING to disable this warning)" [-W#warnings]
#warning "Eigen2 support is deprecated in Eigen 3.2.x and it will be removed in Eigen 3...
1 warning generated.
~/temp2$ ./a.out


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