From 17200570239f23b2f0d3b434bc0269c46c409791 Mon Sep 17 00:00:00 2001
From: Steve Bronder <stevo15025@gmail.com>
Date: Fri, 21 May 2021 16:25:32 +0000
Subject: Adds macro for checking if C++14 variable templates are supported

---
 Eigen/src/Core/util/IntegralConstant.h |  2 +-
 Eigen/src/Core/util/Macros.h           | 18 +++++++++++++-----
 2 files changed, 14 insertions(+), 6 deletions(-)

diff --git a/Eigen/src/Core/util/IntegralConstant.h b/Eigen/src/Core/util/IntegralConstant.h
index ef3fdfb94..d457e02ee 100644
--- a/Eigen/src/Core/util/IntegralConstant.h
+++ b/Eigen/src/Core/util/IntegralConstant.h
@@ -184,7 +184,7 @@ template<int N, int DynamicKey> struct cleanup_index_type<std::integral_constant
 
 #ifndef EIGEN_PARSED_BY_DOXYGEN
 
-#if EIGEN_HAS_CXX14
+#if EIGEN_HAS_CXX14_VARIABLE_TEMPLATES
 template<int N>
 static const internal::FixedInt<N> fix{};
 #else
diff --git a/Eigen/src/Core/util/Macros.h b/Eigen/src/Core/util/Macros.h
index d2e844928..fca94a989 100644
--- a/Eigen/src/Core/util/Macros.h
+++ b/Eigen/src/Core/util/Macros.h
@@ -162,8 +162,8 @@
 
 /// \internal EIGEN_COMP_IBM set to xlc version if the compiler is IBM XL C++
 // XLC   version
-// 3.1   0x0301	
-// 4.5   0x0405	
+// 3.1   0x0301
+// 4.5   0x0405
 // 5.0   0x0500
 // 12.1  0x0C01
 #if defined(__IBMCPP__) || defined(__xlc__) || defined(__ibmxl__)
@@ -637,6 +637,14 @@
   #define EIGEN_COMP_CXXVER 03
 #endif
 
+#ifndef EIGEN_HAS_CXX14_VARIABLE_TEMPLATES
+  #if defined(__cpp_variable_templates) && __cpp_variable_templates >= 201304 && EIGEN_MAX_CPP_VER>=14
+    #define EIGEN_HAS_CXX14_VARIABLE_TEMPLATES 1
+  #else
+    #define EIGEN_HAS_CXX14_VARIABLE_TEMPLATES 0
+  #endif
+#endif
+
 
 // The macros EIGEN_HAS_CXX?? defines a rough estimate of available c++ features
 // but in practice we should not rely on them but rather on the availabilty of
@@ -833,7 +841,7 @@
   #endif
 #endif
 
-// NOTE: the required Apple's clang version is very conservative 
+// NOTE: the required Apple's clang version is very conservative
 //       and it could be that XCode 9 works just fine.
 // NOTE: the MSVC version is based on https://en.cppreference.com/w/cpp/compiler_support
 //       and not tested.
@@ -962,7 +970,7 @@
   #endif
   #define EIGEN_DEVICE_FUNC __attribute__((flatten)) __attribute__((always_inline))
 // All functions callable from CUDA/HIP code must be qualified with __device__
-#elif defined(EIGEN_GPUCC) 
+#elif defined(EIGEN_GPUCC)
     #define EIGEN_DEVICE_FUNC __host__ __device__
 #else
   #define EIGEN_DEVICE_FUNC
@@ -989,7 +997,7 @@
   #else
     #define eigen_plain_assert(x)
   #endif
-#else 
+#else
   #if EIGEN_SAFE_TO_USE_STANDARD_ASSERT_MACRO
     namespace Eigen {
     namespace internal {
-- 
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