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Make fixed-size Matrix and Array trivially copyable after C++20

Lingzhu Xiangrequested to merge
xlz/eigen:trivially-copyable into master

Making them trivially copyable allows using std::memcpy() without undefined behaviors.

Only Matrix and Array with trivially copyable DenseStorage are marked as trivially copyable with an additional type trait.

As described in http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p0848r3.html it requires extremely verbose SFINAE to make the special member functions of fixed-size Matrix and Array trivial, unless C++20 concepts are available to simplify the selection of trivial special member functions given template parameters. Therefore only make this feature available to compilers that support C++20 P0848R3.

Fix #1855.

I have run the official and unsupported tests locally with passing results using the same commands in ci/test.gitlab-ci.yml and cmake -DCMAKE_CXX_STANDARD=20, but it's not clear to me how the CI script itself should be updated.

Please also let me know if any documentation is needed.

Edited by Lingzhu Xiang

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