From bd0cd1d67b7d93605da49924588ce4bd6a816124 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Antonio=20S=C3=A1nchez?= <cantonios@google.com>
Date: Tue, 8 Jul 2025 21:48:59 +0000
Subject: [PATCH] Fix self-adjoint products when multiplying by a compile-time
 vector.

---
 Eigen/src/Core/ProductEvaluators.h                |  4 ++--
 Eigen/src/Core/products/SelfadjointMatrixVector.h | 10 +++++-----
 2 files changed, 7 insertions(+), 7 deletions(-)

diff --git a/Eigen/src/Core/ProductEvaluators.h b/Eigen/src/Core/ProductEvaluators.h
index ce8d954bf..a23004458 100644
--- a/Eigen/src/Core/ProductEvaluators.h
+++ b/Eigen/src/Core/ProductEvaluators.h
@@ -846,7 +846,7 @@ struct generic_product_impl<Lhs, Rhs, SelfAdjointShape, DenseShape, ProductTag>
 
   template <typename Dest>
   static EIGEN_DEVICE_FUNC void scaleAndAddTo(Dest& dst, const Lhs& lhs, const Rhs& rhs, const Scalar& alpha) {
-    selfadjoint_product_impl<typename Lhs::MatrixType, Lhs::Mode, false, Rhs, 0, Rhs::IsVectorAtCompileTime>::run(
+    selfadjoint_product_impl<typename Lhs::MatrixType, Lhs::Mode, false, Rhs, 0, Rhs::ColsAtCompileTime == 1>::run(
         dst, lhs.nestedExpression(), rhs, alpha);
   }
 };
@@ -858,7 +858,7 @@ struct generic_product_impl<Lhs, Rhs, DenseShape, SelfAdjointShape, ProductTag>
 
   template <typename Dest>
   static void scaleAndAddTo(Dest& dst, const Lhs& lhs, const Rhs& rhs, const Scalar& alpha) {
-    selfadjoint_product_impl<Lhs, 0, Lhs::IsVectorAtCompileTime, typename Rhs::MatrixType, Rhs::Mode, false>::run(
+    selfadjoint_product_impl<Lhs, 0, Lhs::RowsAtCompileTime == 1, typename Rhs::MatrixType, Rhs::Mode, false>::run(
         dst, lhs, rhs.nestedExpression(), alpha);
   }
 };
diff --git a/Eigen/src/Core/products/SelfadjointMatrixVector.h b/Eigen/src/Core/products/SelfadjointMatrixVector.h
index f7387601f..580f6a850 100644
--- a/Eigen/src/Core/products/SelfadjointMatrixVector.h
+++ b/Eigen/src/Core/products/SelfadjointMatrixVector.h
@@ -164,6 +164,11 @@ struct selfadjoint_product_impl<Lhs, LhsMode, false, Rhs, 0, true> {
 
   enum { LhsUpLo = LhsMode & (Upper | Lower) };
 
+  // Verify that the Rhs is a vector in the correct orientation.
+  // Otherwise, we break the assumption that we are multiplying
+  // MxN * Nx1.
+  static_assert(Rhs::ColsAtCompileTime == 1, "The RHS must be a column vector.");
+
   template <typename Dest>
   static EIGEN_DEVICE_FUNC void run(Dest& dest, const Lhs& a_lhs, const Rhs& a_rhs, const Scalar& alpha) {
     typedef typename Dest::Scalar ResScalar;
@@ -173,11 +178,6 @@ struct selfadjoint_product_impl<Lhs, LhsMode, false, Rhs, 0, true> {
 
     eigen_assert(dest.rows() == a_lhs.rows() && dest.cols() == a_rhs.cols());
 
-    if (a_lhs.rows() == 1) {
-      dest = (alpha * a_lhs.coeff(0, 0)) * a_rhs;
-      return;
-    }
-
     add_const_on_value_type_t<ActualLhsType> lhs = LhsBlasTraits::extract(a_lhs);
     add_const_on_value_type_t<ActualRhsType> rhs = RhsBlasTraits::extract(a_rhs);