From 74020a084bc3cdd98822f38ea808226df7aa292a Mon Sep 17 00:00:00 2001
From: Alexander Alekhin <alexander.a.alekhin@gmail.com>
Date: Sun, 31 May 2020 15:22:03 +0000
Subject: [PATCH] core: fix builds with eigen helper header

---
 modules/core/include/opencv2/core/eigen.hpp | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/modules/core/include/opencv2/core/eigen.hpp b/modules/core/include/opencv2/core/eigen.hpp
index 8c250efe4c..8afc06caa7 100644
--- a/modules/core/include/opencv2/core/eigen.hpp
+++ b/modules/core/include/opencv2/core/eigen.hpp
@@ -45,9 +45,14 @@
 #ifndef OPENCV_CORE_EIGEN_HPP
 #define OPENCV_CORE_EIGEN_HPP
 
+#ifndef EIGEN_WORLD_VERSION
+#error "Wrong usage of OpenCV's Eigen utility header. Include Eigen's headers first. See https://github.com/opencv/opencv/issues/17366"
+#endif
+
 #include "opencv2/core.hpp"
 
-#if EIGEN_WORLD_VERSION == 3 && EIGEN_MAJOR_VERSION >= 3
+#if EIGEN_WORLD_VERSION == 3 && EIGEN_MAJOR_VERSION >= 3 \
+    && defined(CV_CXX11) && defined(CV_CXX_STD_ARRAY)
 #include <unsupported/Eigen/CXX11/Tensor>
 #define OPENCV_EIGEN_TENSOR_SUPPORT
 #endif // EIGEN_WORLD_VERSION == 3 && EIGEN_MAJOR_VERSION >= 3
@@ -157,7 +162,7 @@ Eigen::TensorMap<Eigen::Tensor<float, 3, Eigen::RowMajor>> a_tensormap = cv2eige
 \endcode
 */
 template <typename _Tp> static inline
-Eigen::TensorMap<Eigen::Tensor<_Tp, 3, Eigen::RowMajor>> cv2eigen_tensormap(const cv::InputArray &src)
+Eigen::TensorMap<Eigen::Tensor<_Tp, 3, Eigen::RowMajor>> cv2eigen_tensormap(InputArray src)
 {
     Mat mat = src.getMat();
     CV_CheckTypeEQ(mat.type(), CV_MAKETYPE(traits::Type<_Tp>::value, mat.channels()), "");