[Dune] PQkLocalFiniteElement in dune-localfunction

[Simon Praetorius]

Hello,

I'm currently working on calculating derivatives of solution-vectors 
build on functionspacebases from dune-function, i.e. PQkNodalBasis with 
order k > 0. When evaluating the derivative I get an error about non 
matching types, since for some reasons the PQkLocalFiniteElementCache 
generates a LocalBasisType for P0LocalFiniteElements only (independent 
of k, always the order 0 is chosen). Is this a bug or a feature or 
simply not yet implemented for general k?

Best regards,
Simon

Details:
------------------
The evaluation of a partial derivative is called by

pqknodalbasis.localView().tree().finiteElement().localBasis().template 
evaluate<1>(direction, position, derivativeVector);

with
decltype(direction) = std::array<int, 1>
decltype(position) = FieldVector<double,dim>
decltype(derivativeVector) = std::vector< DerivativeType >

using DerivativeType = 
PQkNodalBasis<...>::LocalView::Tree::FiniteElement::Traits::LocalBasisType::Traits::RangeType

The problem is in the LocalBasisType, i.e.

using LocalBasisType = 
decltype(pqknodalbasis.localView().tree().finiteElement().localBasis())::(...)
  = LocalBasisVirtualInterface< LocalBasisTraits<DF,n,D,RF,m,R,J, *** 0 
*** > >

Thus, the maximal order of implemented partial derivatives is always set 
to 0.

See also PQkLocalFiniteElementCache>D,R,dim,k>::FiniteElementType in 
pqkfactory.hh:115

ErrorMessage:
-------------------------
File://dune/localfunctions/common/virtualinterface.hh: In instantiation 
of 'void Dune::LocalBasisVirtualInterface<T>::evaluate(const 
std::array<int, k>&, const typename 
Dune::LocalBasisVirtualInterface<T>::Traits::DomainType&, 
std::vector<typename T::RangeType>&) const [with int k = 1; T = 
Dune::LocalBasisTraits<double, 2, Dune::FieldVector<double, 2>, double, 
1, Dune::FieldVector<double, 1>, Dune::FieldMatrix<double, 1, 2>, 0>; 
typename Dune::LocalBasisVirtualInterface<T>::Traits::DomainType = 
Dune::FieldVector<double, 2>; typename T::RangeType = 
Dune::FieldVector<double, 1>]':
(...)
File://dune/localfunctions/common/virtualinterface.hh:211:44: error: 
invalid initialization of reference of type 'const OrderKBaseInterface& 
{aka const 
Dune::LocalBasisVirtualInterfaceBase<Dune::LocalBasisTraits<double, 2, 
Dune::FieldVector<double, 2>, double, 1, Dune::FieldVector<double, 1>, 
Dune::FieldMatrix<double, 1, 2>, 1> >&}' from expression of type 'const 
Dune::LocalBasisVirtualInterface<Dune::LocalBasisTraits<double, 2, 
Dune::FieldVector<double, 2>, double, 1, Dune::FieldVector<double, 1>, 
Dune::FieldMatrix<double, 1, 2>, 0> >'
        const OrderKBaseInterface& asBase = *this;