From 6854da2ea0116f7f042c9b1a9d6fe3c21718c86a Mon Sep 17 00:00:00 2001
From: Charles Schlosser <cs.schlosser@gmail.com>
Date: Mon, 7 Jul 2025 17:32:54 +0000
Subject: [PATCH] Fix 1x1 selfadjoint matrix-vector product bug

---
 Eigen/src/Core/products/SelfadjointMatrixVector.h | 5 +++++
 test/product_selfadjoint.cpp                      | 4 ++++
 2 files changed, 9 insertions(+)

diff --git a/Eigen/src/Core/products/SelfadjointMatrixVector.h b/Eigen/src/Core/products/SelfadjointMatrixVector.h
index 10f60266d..f7387601f 100644
--- a/Eigen/src/Core/products/SelfadjointMatrixVector.h
+++ b/Eigen/src/Core/products/SelfadjointMatrixVector.h
@@ -173,6 +173,11 @@ 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);
 
diff --git a/test/product_selfadjoint.cpp b/test/product_selfadjoint.cpp
index 8d1bda582..b61f30090 100644
--- a/test/product_selfadjoint.cpp
+++ b/test/product_selfadjoint.cpp
@@ -57,6 +57,10 @@ void product_selfadjoint(const MatrixType& m) {
         v1.tail(rows - 1) * v2.head(cols - 1).adjoint() + v2.head(cols - 1) * v1.tail(rows - 1).adjoint();
     VERIFY_IS_APPROX(m2, m3.template triangularView<Lower>().toDenseMatrix());
   }
+
+  // matrix-vector
+  m2 = m1.template triangularView<Lower>();
+  VERIFY_IS_APPROX(m1 * m4, m2.template selfadjointView<Lower>() * m4);
 }
 
 EIGEN_DECLARE_TEST(product_selfadjoint) {