Hi,
I have had versions 3.2.4 - 3.2.8 of the Eigen library successfully compiling with an example program on a Linux system (Xubuntu, "Wily Werewolf") using the Arduino/Teensyduino IDE (v1.6.. When I try to swap in version 3.3.1 beta 1, however, a number of errors are generated, e.g. :

In file included from /home/matt/Downloads/arduino-1.6.8/hardware/teensy/avr/cores/teensy3/WProgram.h:15:0,
from /home/matt/Downloads/arduino-1.6.8/hardware/teensy/avr/cores/teensy3/Arduino.h:1,
from /tmp/build05ea79966117b24e0fb82be583ac0586.tmp/sketch/Eigen_test.ino.cpp:1:
/home/matt/Downloads/arduino-1.6.8/hardware/teensy/avr/cores/teensy3/wiring.h:80:19: error: expected unqualified-id before '{' token
#define round(x) ({ \
^
/home/matt/Arduino/libraries/Eigen331b1/Eigen/src/Core/MathFunctions.h:936:45: note: in expansion of macro 'round'
inline EIGEN_MATHFUNC_RETVAL(round, Scalar) round(const Scalar& x)

(There are others like that, though excluded here).

It seems to me that perhaps the 3.3.1 version of the library might require some specific compiler directives that were not needed before. Can anyone tell me if that is correct and if so, what those directives should be? Or, if I am completely off base and someone can point that out (along with a solution), I'd be appreciative.
Regards,
Matt

Either #include Eigen's headers first, or #undef the offending macros like "round" before including Eigen's header (or any c++ header):

#ifdef round
#undef round
#endif

Thank you - that fixed it.
Regards,
Matt