[dune-functions] More build failures

[Carsten Gräser]

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.

Best,
Carsten


BTW:
As it turns out calling a constexpr member function
gives only a constexpr if the type is literal. Hence
calling size() for a FieldVector is not constexpr
because FieldVector is not literal. Maybe we can change this
by declaring the default constructor constexpr.

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