See http://stackoverflow.com/questions/44033691/is-it-possible-to-use-eigen-3-3-3-on-windows-with-arm-neon-defined When Eigen is used on Windows with __ARM_NEON__ defined, the following compilation error is produced: eigen\src/Core/arch/NEON/PacketMath.h(109): error C2766: explicit specialization; 'Eigen::internal::unpacket_traits<Eigen::internal::Packet4f>' has already been defined Packet4f and Packet4i are defined (in PacketMath.h) as: typedef float32x4_t Packet4f; typedef int32x4_t Packet4i; And float32x4_t and int32x4_t are defined (in MSVC's arm_neon.h) as: typedef __n128 float32x4_t; typedef __n128 int32x4_t; Presumably the preprocessor checks need to be updated to handle this case specifically for MSVC.
here is an attempt to fix the issue: https://bitbucket.org/eigen/eigen/commits/c576f16f3a596 anybody should be able to check the success of it there: https://godbolt.org/g/qPhNgh once the trunk version of Eigen will be updated (nightly).
This sounds like a problem that could be solved cleaner with Meta-Packets (Bug 692)
another smaller fix: https://bitbucket.org/eigen/eigen/commits/2af650906b57/
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