[dune-functions] More build failures

[Steffen Müthing]

> Am 18.09.2015 um 21:12 schrieb Carsten Gräser <graeser at mi.fu-Berlin.de>:
> 
> Am 18.09.2015 um 15:19 schrieb 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++.
> 
> I also discussed this with Steffen. The main problem is, that
> calling a constexpr function is only a constexpr if 'this' is
> also a constexpr which is never true for the casted nullptr.
> Default constructability is only needed because one has to settle
> on one constructor to call and the default constructor is the
> only viable solution.
> 
> But this will also only solve the problem if the default
> constructor is constexpr. Hence your commit only partly
> fixes the problem because the HierarchicVectorWrapper
> is not used in the posisson example (which is not necessary
> but should also work). I just push a follow up fix allowing
> to also use FlatMultiIndex in through the wrapper.

Another point Carsten and I discussed was the annoying fact that
the size method of std::array<> isn’t constexpr, so you can’t use this
for arrays…

Steffen

> 
> Best,
> Carsten
> 
> 
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