Fix GCC 7.5 issue with distributed::Matrix

[MarcelKoch]

This PR uses the xstd::void_t again in is_matrix_type_builder. I think there is a bug in GCC 7.5 bug when std::void_t is used for is_matrix_type_builder. I wasn't able to reproduce this issue to actually find out if it's a bug or not, but since this is not an issue in newer gcc version, I don't think it's worth to put more effort into it.

[yhmtsai]

could you paste the error message in the pr?
It might be helpful for searching when we face the same issue in other places.

[MarcelKoch]

I think this is the important part:

In file included from /home/marcel/projects/ginkgo/include/ginkgo/ginkgo.hpp:64:0,
                 from /home/marcel/projects/ginkgo/benchmark/solver/distributed/solver.cpp:10:
/home/marcel/projects/ginkgo/include/ginkgo/core/distributed/matrix.hpp: In instantiation of ‘struct gko::detail::is_matrix_type_builder<std::unique_ptr<gko::LinOp, std::default_delete<gko::LinOp> >, double, int, void>’:
/home/marcel/projects/ginkgo/include/ginkgo/core/distributed/matrix.hpp:481:73:   required by substitution of ‘template<class LocalMatrixType, class NonLocalMatrixType, class> static std::unique_ptr<gko::experimental::distributed::Matrix<double, int, long int>, std::default_delete<gko::experimental::distributed::Matrix<double, int, long int> > > gko::experimental::distributed::Matrix<double, int, long int>::create<LocalMatrixType, NonLocalMatrixType, <template-parameter-1-3> >(std::shared_ptr<const gko::Executor>, gko::experimental::mpi::communicator, LocalMatrixType, NonLocalMatrixType) [with LocalMatrixType = std::unique_ptr<gko::LinOp, std::default_delete<gko::LinOp> >; NonLocalMatrixType = std::unique_ptr<gko::LinOp, std::default_delete<gko::LinOp> >; <template-parameter-1-3> = <missing>]’
/home/marcel/projects/ginkgo/benchmark/utils/generator.hpp:245:49:   required from here
/home/marcel/projects/ginkgo/include/ginkgo/core/distributed/matrix.hpp:59:51: error: ‘class std::unique_ptr<gko::LinOp, std::default_delete<gko::LinOp> >’ has no member named ‘create’
         decltype(std::declval<Builder>().template create<ValueType, IndexType>(
                  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/marcel/projects/ginkgo/include/ginkgo/core/distributed/matrix.hpp:59:51: error: ‘class std::unique_ptr<gko::LinOp, std::default_delete<gko::LinOp> >’ has no member named ‘create’

Somehow the missing create function is not triggering SFINAE in this context.

[greole]

LGTM!

[greole]

Maybe add some documentation on what the intended use is.

[MarcelKoch]

There is no use for users for this. On the developer side it's used directly below, so I don't think it requires documentation.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.96%. Comparing base (f6eb48a) to head (ca3f152).
Report is 3 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1696      +/-   ##
===========================================
- Coverage    90.24%   89.96%   -0.28%     
===========================================
  Files          763      763              
  Lines        62841    62877      +36     
===========================================
- Hits         56713    56570     -143     
- Misses        6128     6307     +179     

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