C++14 requested but C++17 used

I know there README says g++-5.0 but it also says "The compiler has to support C++14". The latter is not even true for the 2.6 release:

In file included from /home/mblatt/src/dune/current/dune-common/dune/common/test/filledarraytest.cc:12:0:
/home/mblatt/src/dune/current/dune-common/dune/common/filledarray.hh: In instantiation of ‘constexpr std::array<T, n> Dune::filledArray(const T&) [with long unsigned int n = 2ul; T = double]’:
/home/mblatt/src/dune/current/dune-common/dune/common/test/filledarraytest.cc:18:40:   required from here
/home/mblatt/src/dune/current/dune-common/dune/common/filledarray.hh:38:3: error: body of constexpr function ‘constexpr std::array<T, n> Dune::filledArray(const T&) [with long unsigned int n = 2ul; T = double]’ not a return-statement
   }
   ^
dune/common/test/CMakeFiles/filledarraytest.dir/build.make:54: recipe for target 'dune/common/test/CMakeFiles/filledarraytest.dir/filledarraytest.cc.o' failed
make[3]: *** [dune/common/test/CMakeFiles/filledarraytest.dir/filledarraytest.cc.o] Error 1
CMakeFiles/Makefile2:2826: recipe for target 'dune/common/test/CMakeFiles/filledarraytest.dir/all' failed
make[2]: *** [dune/common/test/CMakeFiles/filledarraytest.dir/all] Error 2
CMakeFiles/Makefile2:1039: recipe for target 'CMakeFiles/build_tests.dir/rule' failed
make[1]: *** [CMakeFiles/build_tests.dir/rule] Error 2
Makefile:558: recipe for target 'build_tests' failed
make: *** [build_tests] Error 2