[dune-functions] More build failures

[Christian Engwer]

On Fri, Sep 18, 2015 at 01:59:58PM +0200, Carsten Gräser wrote:
> Hi,
> 
> [...]
> > Ups, you are right... but the error is still the same
> >       in-class initializer for static data member is not a constant expression
> > 	      static const int value = ((const typename std::decay<T>::type*)(nullptr))->size();
> > 		                           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> it's really hard to call a constexpr method if you don't
> have an object. After digging some more time it seems that
> the only possible way is to call a constructor inside of
> decltype. Unfortunately the constructor arguments are not
> known. Maybe it's OK to restrict this to default-constructible
> types. I'll have look.

After discussing with Jö, we agreed that it is necessary to have
default constructible types. casting from a null ptr and then calling
a member is undefined behaviour. On the other hand we can safely
assume that all types we want to pass are either std::array or
somethingin our scopte. Thus we can always make sure that they are
default constructible.

With this assumption the code got a lot simpler and I pushed a fix
which works with clang and g++.

Ciao
Christian