A lot of the relatively complicated feature detection code could be simplified by using built-in feature-tests of C++, e.g., // Does the compiler support variadic templates? #ifndef EIGEN_HAS_VARIADIC_TEMPLATES #if EIGEN_MAX_CPP_VER>=11 && defined(__cpp_variadic_templates) #define EIGEN_HAS_VARIADIC_TEMPLATES __cpp_variadic_templates #else #define EIGEN_HAS_VARIADIC_TEMPLATES 0 #endif #endif This needs to be tested with older compiler however, and we either need to still add some extra tests, or just un-support compilers which do not support these feature-tests. https://en.cppreference.com/w/cpp/feature_test
I would not bother until we're done with 3.4 and start 3.5 with an increased minimal c++ version (c++11 or c++14). Such an increase in the minimal c++11 version will allow us to cleanup most of those checks anyway, and then, yes, we should use __cpp_* for c++17/20 features.
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