Compile- and run-time assertions for the construction of Ref<const>.

failtest/ref_6.cpp

@@ -0,0 +1,15 @@
+#include "../Eigen/Core"
+
+using namespace Eigen;
+
+void call_ref(Ref<const VectorXf, 0, InnerStride<2>>) {}
+
+int main() {
+  VectorXf a(10);
+  Map<const VectorXf, 0, InnerStride<2>> m(a.data(), 5);
+#ifdef EIGEN_SHOULD_FAIL_TO_BUILD
+  call_ref(a);
+#else
+  call_ref(m);
+#endif
+}