From 36ca36d0def87e3cf10fb0b3d75c6be87b3d3698 Mon Sep 17 00:00:00 2001
From: Pavel Guzenfeld <pavelguzenfeld@gmail.com>
Date: Thu, 19 Mar 2026 03:24:09 +0000
Subject: [PATCH] Guard redundant constexpr static member redeclarations for
 C++17+

libeigen/eigen!2299

Closes #3061

Co-authored-by: Pavel Guzenfeld <67074795+PavelGuzenfeld@users.noreply.github.com>
---
 Eigen/src/Core/arch/Default/BFloat16.h | 3 +++
 Eigen/src/Core/arch/Default/Half.h     | 3 +++
 2 files changed, 6 insertions(+)

diff --git a/Eigen/src/Core/arch/Default/BFloat16.h b/Eigen/src/Core/arch/Default/BFloat16.h
index 8f1f09208..78d2a9592 100644
--- a/Eigen/src/Core/arch/Default/BFloat16.h
+++ b/Eigen/src/Core/arch/Default/BFloat16.h
@@ -210,6 +210,8 @@ struct numeric_limits_bfloat16_impl {
   static EIGEN_CONSTEXPR Eigen::bfloat16 denorm_min() { return Eigen::bfloat16_impl::raw_uint16_to_bfloat16(0x0001); }
 };
 
+// Redundant out-of-class definitions are required pre-C++17 but deprecated since.
+#if EIGEN_COMP_CXXVER < 17
 template <typename T>
 EIGEN_CONSTEXPR const bool numeric_limits_bfloat16_impl<T>::is_specialized;
 template <typename T>
@@ -259,6 +261,7 @@ template <typename T>
 EIGEN_CONSTEXPR const bool numeric_limits_bfloat16_impl<T>::traps;
 template <typename T>
 EIGEN_CONSTEXPR const bool numeric_limits_bfloat16_impl<T>::tinyness_before;
+#endif
 }  // end namespace bfloat16_impl
 }  // end namespace Eigen
 
diff --git a/Eigen/src/Core/arch/Default/Half.h b/Eigen/src/Core/arch/Default/Half.h
index 671a9dcc1..56aeb3fa8 100644
--- a/Eigen/src/Core/arch/Default/Half.h
+++ b/Eigen/src/Core/arch/Default/Half.h
@@ -301,6 +301,8 @@ struct numeric_limits_half_impl {
   static _EIGEN_MAYBE_CONSTEXPR Eigen::half denorm_min() { return Eigen::half_impl::raw_uint16_to_half(0x0001); }
 };
 
+// Redundant out-of-class definitions are required pre-C++17 but deprecated since.
+#if EIGEN_COMP_CXXVER < 17
 template <typename T>
 constexpr const bool numeric_limits_half_impl<T>::is_specialized;
 template <typename T>
@@ -350,6 +352,7 @@ template <typename T>
 constexpr const bool numeric_limits_half_impl<T>::traps;
 template <typename T>
 constexpr const bool numeric_limits_half_impl<T>::tinyness_before;
+#endif
 }  // end namespace half_impl
 }  // end namespace Eigen