From b5d218d8574c665d7d4ac3ac21f7bf15b8dabbfa Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?J=C3=B6rg=20Buchwald?= <joerg.buchwald@ufz.de>
Date: Thu, 13 Jan 2022 01:24:20 +0100
Subject: [PATCH] fix compilation issue with gcc < 10 and -std=c++2a

(cherry picked from commit d1bf05639476adbb222e8d09da2faf6ef1ed66f2)
---
 Eigen/src/Core/util/Meta.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Eigen/src/Core/util/Meta.h b/Eigen/src/Core/util/Meta.h
index 0e21fe37f..b67f50d0e 100755
--- a/Eigen/src/Core/util/Meta.h
+++ b/Eigen/src/Core/util/Meta.h
@@ -477,7 +477,7 @@ template<typename T, std::size_t N> struct array_size<std::array<T,N> > {
   *
   * For C++20, this function just forwards to `std::ssize`, or any ADL discoverable `ssize` function.
   */
-#if EIGEN_COMP_CXXVER < 20
+#if EIGEN_COMP_CXXVER < 20  || EIGEN_GNUC_AT_MOST(9,4)
 template <typename T>
 EIGEN_CONSTEXPR auto index_list_size(const T& x) {
   using R = std::common_type_t<std::ptrdiff_t, std::make_signed_t<decltype(x.size())>>;