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47eac21072
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.