[eigen] XLC Port Part III: XLC failing to instantiate a partial template specialization unless it is defined before use in a forward declaration

[Aron Ahmadia <aja2111@xxxxxxxxxxxx>]

A little more digging has turned up this nifty fact.  If I
'forward-define' the partial specialization for ei_traits in
ForwardDeclarations.h (moving it from Matrix.h):

template<typename _Scalar, int _Rows, int _Cols, int _Options, int
_MaxRows, int _MaxCols>
struct ei_traits<Matrix<_Scalar, _Rows, _Cols, _Options, _MaxRows, _MaxCols> >
{
  typedef _Scalar Scalar;
  typedef Dense StorageType;
  typedef DenseStorageMatrix DenseStorageType;
  enum {
    RowsAtCompileTime = _Rows,
    ColsAtCompileTime = _Cols,
    MaxRowsAtCompileTime = _MaxRows,
    MaxColsAtCompileTime = _MaxCols,
    Flags = ei_compute_matrix_flags<_Scalar, _Rows, _Cols, _Options,
_MaxRows, _MaxCols>::ret,
    CoeffReadCost = NumTraits<Scalar>::ReadCost
  };
};

Then

template<typename MatrixType, int BlockRows=Dynamic, int BlockCols=Dynamic,
         int _DirectAccessStatus =
(ei_traits<MatrixType>::Flags&DirectAccessBit) ? HasDirectAccess :
NoDirectAccess> class Block;

compiles (at least to instantiate a Matrix3f, make check is still
failing miserably).  This sort of problem is a real head-scratcher,
since XLC seems to handle a simple version of this correctly:

#include<iostream>
#include<vector>

template<typename T> struct A;

template<typename Scalar, typename Allocator, int _bits =
(A<std::vector<Scalar, Allocator> >::bits > 10) ? 1 : 0> struct B {
  static const int bits = _bits;
};

template<typename Scalar, typename Allocator>
struct A < std::vector <Scalar, Allocator > > {
  static const int bits = 3;
  std::vector <Scalar, Allocator> y;
};

int main()
{
  B < int, std::allocator<int> > b1;
  B < int, std::allocator<int>, 2 > b2;
  std::cout << "b1 " << b1.bits << std::endl;
  std::cout << "b2 " << b2.bits << std::endl;

  return 0;
}

This news is still positive, since we can at least identify what
exactly is causing a problem (XLC is failing to instantiate a partial
template specialization unless the specialization has already been
defined)  I tried forward-declaring the partial template
specialization, a legal maneuver in XLC, but this didn't fix things.
It's possible that "MatrixType" is not well-defined to XLC and I am
missing other forward declarations of partial specializations that
would fix things.

Alexandre, can you follow up with the IBM Compiler Team and see if
there are any outstanding issues in the XLC V9.0 that shipped with our
BlueGene/P last year that would cause this sort of trouble?

Thanks,
Aron