From 11eb66e1b5c33f0c532b95ccb6e687867bbb87d8 Mon Sep 17 00:00:00 2001
From: Rasmus Munk Larsen <4643818-rmlarsen1@users.noreply.gitlab.com>
Date: Thu, 26 Feb 2026 06:17:39 -0800
Subject: [PATCH] Remove pre-C++14 workarounds from unsupported/ tensor code

libeigen/eigen!2218

Co-authored-by: Rasmus Munk Larsen <rmlarsen@gmail.com>
---
 Eigen/src/Core/util/EmulateArray.h            | 23 ++--------
 .../CXX11/src/TensorSymmetry/StaticSymmetry.h |  4 +-
 .../TensorSymmetry/util/TemplateGroupTheory.h | 11 ++---
 .../Eigen/CXX11/src/util/CXX11Workarounds.h   | 46 +------------------
 unsupported/test/CMakeLists.txt               |  2 +-
 5 files changed, 12 insertions(+), 74 deletions(-)

diff --git a/Eigen/src/Core/util/EmulateArray.h b/Eigen/src/Core/util/EmulateArray.h
index 6c4c22d41..9414cb2bb 100644
--- a/Eigen/src/Core/util/EmulateArray.h
+++ b/Eigen/src/Core/util/EmulateArray.h
@@ -231,37 +231,20 @@ template <typename T, std::size_t N>
 using array = std::array<T, N>;
 
 namespace internal {
-/* std::get is only constexpr in C++14, not yet in C++11
- *     - libstdc++ from version 4.7 onwards has it nevertheless,
- *                                          so use that
- *     - libstdc++ older versions: use _M_instance directly
- *     - libc++ all versions so far: use __elems_ directly
- *     - all other libs: use std::get to be portable, but
- *                       this may not be constexpr
- */
-#if defined(__GLIBCXX__) && __GLIBCXX__ < 20120322
-#define STD_GET_ARR_HACK a._M_instance[I_]
-#elif defined(_LIBCPP_VERSION)
-#define STD_GET_ARR_HACK a.__elems_[I_]
-#else
-#define STD_GET_ARR_HACK std::template get<I_, T, N>(a)
-#endif
 
 template <std::size_t I_, class T, std::size_t N>
 constexpr T& array_get(std::array<T, N>& a) {
-  return (T&)STD_GET_ARR_HACK;
+  return std::get<I_>(a);
 }
 template <std::size_t I_, class T, std::size_t N>
 constexpr T&& array_get(std::array<T, N>&& a) {
-  return (T&&)STD_GET_ARR_HACK;
+  return std::get<I_>(std::move(a));
 }
 template <std::size_t I_, class T, std::size_t N>
 constexpr T const& array_get(std::array<T, N> const& a) {
-  return (T const&)STD_GET_ARR_HACK;
+  return std::get<I_>(a);
 }
 
-#undef STD_GET_ARR_HACK
-
 }  // end namespace internal
 }  // end namespace Eigen
 
diff --git a/unsupported/Eigen/CXX11/src/TensorSymmetry/StaticSymmetry.h b/unsupported/Eigen/CXX11/src/TensorSymmetry/StaticSymmetry.h
index 66a982bde..6ee508f2f 100644
--- a/unsupported/Eigen/CXX11/src/TensorSymmetry/StaticSymmetry.h
+++ b/unsupported/Eigen/CXX11/src/TensorSymmetry/StaticSymmetry.h
@@ -150,8 +150,8 @@ struct tensor_static_symgroup_do_apply<internal::type_list<first, next...>> {
   }
 };
 
-template <EIGEN_TPL_PP_SPEC_HACK_DEF(typename, empty)>
-struct tensor_static_symgroup_do_apply<internal::type_list<EIGEN_TPL_PP_SPEC_HACK_USE(empty)>> {
+template <>
+struct tensor_static_symgroup_do_apply<internal::type_list<>> {
   template <typename Op, typename RV, std::size_t SGNumIndices, typename Index, std::size_t NumIndices,
             typename... Args>
   static inline RV run(const std::array<Index, NumIndices>&, RV initial, Args&&...) {
diff --git a/unsupported/Eigen/CXX11/src/TensorSymmetry/util/TemplateGroupTheory.h b/unsupported/Eigen/CXX11/src/TensorSymmetry/util/TemplateGroupTheory.h
index aa16f3cd4..df8da6c38 100644
--- a/unsupported/Eigen/CXX11/src/TensorSymmetry/util/TemplateGroupTheory.h
+++ b/unsupported/Eigen/CXX11/src/TensorSymmetry/util/TemplateGroupTheory.h
@@ -132,8 +132,8 @@ struct strip_identities<Equality, id, type_list<t, ts...>> {
       Equality<id, t>::global_flags | strip_identities<Equality, id, type_list<ts...>>::global_flags;
 };
 
-template <template <typename, typename> class Equality, typename id EIGEN_TPL_PP_SPEC_HACK_DEFC(typename, ts)>
-struct strip_identities<Equality, id, type_list<EIGEN_TPL_PP_SPEC_HACK_USE(ts)>> {
+template <template <typename, typename> class Equality, typename id>
+struct strip_identities<Equality, id, type_list<>> {
   typedef type_list<> type;
   constexpr static int global_flags = 0;
 };
@@ -278,10 +278,9 @@ struct dimino_add_cosets_for_rep<Multiply, Equality, id, sub_group_elements, ele
 };
 
 template <template <typename, typename> class Multiply, template <typename, typename> class Equality, typename id,
-          typename sub_group_elements, typename elements EIGEN_TPL_PP_SPEC_HACK_DEFC(typename, empty),
-          typename rep_element, int sub_group_size>
-struct dimino_add_cosets_for_rep<Multiply, Equality, id, sub_group_elements, elements,
-                                 type_list<EIGEN_TPL_PP_SPEC_HACK_USE(empty)>, rep_element, sub_group_size> {
+          typename sub_group_elements, typename elements, typename rep_element, int sub_group_size>
+struct dimino_add_cosets_for_rep<Multiply, Equality, id, sub_group_elements, elements, type_list<>, rep_element,
+                                 sub_group_size> {
   typedef elements type;
   constexpr static int global_flags = 0;
 };
diff --git a/unsupported/Eigen/CXX11/src/util/CXX11Workarounds.h b/unsupported/Eigen/CXX11/src/util/CXX11Workarounds.h
index 632f43706..ffc198c33 100644
--- a/unsupported/Eigen/CXX11/src/util/CXX11Workarounds.h
+++ b/unsupported/Eigen/CXX11/src/util/CXX11Workarounds.h
@@ -10,29 +10,11 @@
 #ifndef EIGEN_CXX11WORKAROUNDS_H
 #define EIGEN_CXX11WORKAROUNDS_H
 
-/* COMPATIBILITY CHECKS
- * (so users of compilers that are too old get some realistic error messages)
- */
-#if defined(__INTEL_COMPILER) && (__INTEL_COMPILER < 1310)
-#error Intel Compiler only supports required C++ features since version 13.1.
-// note that most stuff in principle works with 13.0 but when combining
-// some features, at some point 13.0 will just fail with an internal assertion
-#elif defined(__GNUC__) && !defined(__clang__) && !defined(__INTEL_COMPILER) && \
-    (__GNUC__ < 4 || (__GNUC__ == 4 && __GNUC_MINOR__ < 6))
-// G++ < 4.6 by default will continue processing the source files - even if we use #error to make
-// it error out. For this reason, we use the pragma to make sure G++ aborts at the first error
-// it sees. Unfortunately, that is still not our #error directive, but at least the output is
-// short enough the user has a chance to see that the compiler version is not sufficient for
-// the funky template mojo we use.
-#pragma GCC diagnostic error "-Wfatal-errors"
-#error GNU C++ Compiler (g++) only supports required C++ features since version 4.6.
-#endif
-
 namespace Eigen {
 
 namespace internal {
 
-/* std::get is only constexpr in C++14, not yet in C++11
+/* array_get overloads for std::vector, used by tensor code.
  */
 
 template <std::size_t I_, class T>
@@ -48,32 +30,6 @@ constexpr T const& array_get(std::vector<T> const& a) {
   return a[I_];
 }
 
-/* Suppose you have a template of the form
- * template<typename T> struct X;
- * And you want to specialize it in such a way:
- *    template<typename S1, typename... SN> struct X<Foo<S1, SN...>> { ::: };
- *    template<>                            struct X<Foo<>>          { ::: };
- * This will work in Intel's compiler 13.0, but only to some extent in g++ 4.6, since
- * g++ can only match templates called with parameter packs if the number of template
- * arguments is not a fixed size (so inside the first specialization, referencing
- * X<Foo<Sn...>> will fail in g++). On the other hand, g++ will accept the following:
- *    template<typename S...> struct X<Foo<S...>> { ::: }:
- * as an additional (!) specialization, which will then only match the empty case.
- * But Intel's compiler 13.0 won't accept that, it will only accept the empty syntax,
- * so we have to create a workaround for this.
- */
-#if defined(__GNUC__) && !defined(__INTEL_COMPILER)
-#define EIGEN_TPL_PP_SPEC_HACK_DEF(mt, n) mt... n
-#define EIGEN_TPL_PP_SPEC_HACK_DEFC(mt, n) , EIGEN_TPL_PP_SPEC_HACK_DEF(mt, n)
-#define EIGEN_TPL_PP_SPEC_HACK_USE(n) n...
-#define EIGEN_TPL_PP_SPEC_HACK_USEC(n) , n...
-#else
-#define EIGEN_TPL_PP_SPEC_HACK_DEF(mt, n)
-#define EIGEN_TPL_PP_SPEC_HACK_DEFC(mt, n)
-#define EIGEN_TPL_PP_SPEC_HACK_USE(n)
-#define EIGEN_TPL_PP_SPEC_HACK_USEC(n)
-#endif
-
 }  // end namespace internal
 
 }  // end namespace Eigen
diff --git a/unsupported/test/CMakeLists.txt b/unsupported/test/CMakeLists.txt
index 4fb5125d8..4160fad49 100644
--- a/unsupported/test/CMakeLists.txt
+++ b/unsupported/test/CMakeLists.txt
@@ -231,7 +231,7 @@ ei_add_test(cxx11_tensor_thread_local "-pthread" "${CMAKE_THREAD_LIBS_INIT}")
 ei_add_test(cxx11_tensor_thread_pool "-pthread" "${CMAKE_THREAD_LIBS_INIT}")
 ei_add_test(cxx11_tensor_trace)
 ei_add_test(cxx11_tensor_volume_patch)
-#  ei_add_test(cxx11_tensor_symmetry)
+ei_add_test(cxx11_tensor_symmetry)
 if("${CMAKE_SIZEOF_VOID_P}" EQUAL "8" AND NOT CMAKE_CXX_COMPILER_ID STREQUAL "MSVC")
   # This test requires __uint128_t which is only available on 64bit systems
   ei_add_test(cxx11_tensor_uint128)