Issue an error in case of direct inclusion of internal headers.
diff --git a/Eigen/src/Core/arch/GPU/PacketMath.h b/Eigen/src/Core/arch/GPU/PacketMath.h
index 25c45fd..4c68536 100644
--- a/Eigen/src/Core/arch/GPU/PacketMath.h
+++ b/Eigen/src/Core/arch/GPU/PacketMath.h
@@ -10,6 +10,8 @@
 #ifndef EIGEN_PACKET_MATH_GPU_H
 #define EIGEN_PACKET_MATH_GPU_H
 
+#include "../../InternalHeaderCheck.h"
+
 namespace Eigen {
 
 namespace internal {