From d2d2e8d8ae3d925849df2606637c69c0214c6b68 Mon Sep 17 00:00:00 2001
From: Andrew Johnson <andrew.johnson@arjohnsonau.com>
Date: Tue, 30 Jun 2026 19:54:00 +0800
Subject: [PATCH 1/2] Enable backwards-compatibility Eigen < 5

---
 stan/math/fwd/fun/Eigen_NumTraits.hpp         |  2 ++
 stan/math/prim/meta/lazy_select_evaluator.hpp | 10 +++++-----
 stan/math/rev/core/Eigen_NumTraits.hpp        |  2 ++
 3 files changed, 9 insertions(+), 5 deletions(-)

diff --git a/stan/math/fwd/fun/Eigen_NumTraits.hpp b/stan/math/fwd/fun/Eigen_NumTraits.hpp
index 20827456d09..9e627e46d1e 100644
--- a/stan/math/fwd/fun/Eigen_NumTraits.hpp
+++ b/stan/math/fwd/fun/Eigen_NumTraits.hpp
@@ -218,6 +218,7 @@ struct ScalarBinaryOpTraits<std::complex<stan::math::fvar<T>>,
 
 namespace internal {
 
+#if EIGEN_VERSION_AT_LEAST(5, 0, 0)
 /**
  * Partial specialization of Eigen's ternary evaluator for `.select()`
  * expressions on matrices of `fvar<T>`, restoring the lazy (Eigen 3.x)
@@ -247,6 +248,7 @@ struct ternary_evaluator<
       Arg1, Arg2, Arg3>;
   using Base::Base;
 };
+#endif
 
 }  // namespace internal
 
diff --git a/stan/math/prim/meta/lazy_select_evaluator.hpp b/stan/math/prim/meta/lazy_select_evaluator.hpp
index 0cc579598f7..d3958169d83 100644
--- a/stan/math/prim/meta/lazy_select_evaluator.hpp
+++ b/stan/math/prim/meta/lazy_select_evaluator.hpp
@@ -71,11 +71,11 @@ struct lazy_select_evaluator
                    & static_cast<int>(Arg3Flags)
                    & (StorageOrdersAgree ? Eigen::LinearAccessBit : 0))),
     Flags = (Flags0 & ~Eigen::RowMajorBit) | (Arg1Flags & Eigen::RowMajorBit),
-    Alignment = Eigen::internal::plain_enum_min(
-        Eigen::internal::plain_enum_min(
-            Eigen::internal::evaluator<Arg1>::Alignment,
-            Eigen::internal::evaluator<Arg2>::Alignment),
-        Eigen::internal::evaluator<Arg3>::Alignment)
+    Alignment = std::min({
+      static_cast<int>(Eigen::internal::evaluator<Arg1>::Alignment),
+      static_cast<int>(Eigen::internal::evaluator<Arg2>::Alignment),
+      static_cast<int>(Eigen::internal::evaluator<Arg3>::Alignment)
+    })
   };
 
   EIGEN_DEVICE_FUNC explicit lazy_select_evaluator(const XprType& xpr)
diff --git a/stan/math/rev/core/Eigen_NumTraits.hpp b/stan/math/rev/core/Eigen_NumTraits.hpp
index 7719e9a4aa3..ed5eebad4af 100644
--- a/stan/math/rev/core/Eigen_NumTraits.hpp
+++ b/stan/math/rev/core/Eigen_NumTraits.hpp
@@ -269,6 +269,7 @@ struct ScalarBinaryOpTraits<std::complex<stan::math::var>,
 
 namespace internal {
 
+#if EIGEN_VERSION_AT_LEAST(5, 0, 0)
 /**
  * Partial specialization of Eigen's ternary evaluator for `.select()`
  * expressions on matrices of `var`, restoring the lazy (Eigen 3.x)
@@ -299,6 +300,7 @@ struct ternary_evaluator<
       Arg1, Arg2, Arg3>;
   using Base::Base;
 };
+#endif
 
 /**
  * Enable linear access of inputs when using read_vi_val_adj.

From ad5ec5c5bfacfe4108c9f7089f1a6893f5f21563 Mon Sep 17 00:00:00 2001
From: Stan Jenkins <mc.stanislaw@gmail.com>
Date: Tue, 30 Jun 2026 07:59:20 -0400
Subject: [PATCH 2/2] [Jenkins] auto-formatting by clang-format version
 10.0.0-4ubuntu1

---
 stan/math/prim/meta/lazy_select_evaluator.hpp | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/stan/math/prim/meta/lazy_select_evaluator.hpp b/stan/math/prim/meta/lazy_select_evaluator.hpp
index d3958169d83..0543167b4ed 100644
--- a/stan/math/prim/meta/lazy_select_evaluator.hpp
+++ b/stan/math/prim/meta/lazy_select_evaluator.hpp
@@ -71,11 +71,10 @@ struct lazy_select_evaluator
                    & static_cast<int>(Arg3Flags)
                    & (StorageOrdersAgree ? Eigen::LinearAccessBit : 0))),
     Flags = (Flags0 & ~Eigen::RowMajorBit) | (Arg1Flags & Eigen::RowMajorBit),
-    Alignment = std::min({
-      static_cast<int>(Eigen::internal::evaluator<Arg1>::Alignment),
-      static_cast<int>(Eigen::internal::evaluator<Arg2>::Alignment),
-      static_cast<int>(Eigen::internal::evaluator<Arg3>::Alignment)
-    })
+    Alignment
+    = std::min({static_cast<int>(Eigen::internal::evaluator<Arg1>::Alignment),
+                static_cast<int>(Eigen::internal::evaluator<Arg2>::Alignment),
+                static_cast<int>(Eigen::internal::evaluator<Arg3>::Alignment)})
   };
 
   EIGEN_DEVICE_FUNC explicit lazy_select_evaluator(const XprType& xpr)