Add unit tests for bug #981: valid and invalid usage of ternary operator

failtest/ternary_2.cpp

@@ -0,0 +1,13 @@
+#include "../Eigen/Core"
+
+using namespace Eigen;
+
+int main(int argc,char **)
+{
+  VectorXf a(10), b(10);
+#ifdef EIGEN_SHOULD_FAIL_TO_BUILD
+  b = argc>1 ? 2*a : a+a;
+#else
+  b = argc>1 ? VectorXf(2*a) : VectorXf(a+a);
+#endif
+}