From 5bc944a3efaf19ecff71b11c52fb531b6821ecc6 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Damiano=20Franz=C3=B2?= <franzo.damiano@gmail.com>
Date: Wed, 8 Oct 2025 00:41:26 +0200
Subject: [PATCH] Fix jacobi svd for TriangularBase

---
 Eigen/src/SVD/JacobiSVD.h | 19 +++++++++++++++++++
 test/jacobisvd.cpp        | 18 ++++++++++++++++++
 2 files changed, 37 insertions(+)

diff --git a/Eigen/src/SVD/JacobiSVD.h b/Eigen/src/SVD/JacobiSVD.h
index da2f29593..dfcb6df5e 100644
--- a/Eigen/src/SVD/JacobiSVD.h
+++ b/Eigen/src/SVD/JacobiSVD.h
@@ -571,6 +571,11 @@ class JacobiSVD : public SVDBase<JacobiSVD<MatrixType_, Options_> > {
     compute_impl(matrix, internal::get_computation_options(Options));
   }
 
+  template <typename Derived>
+  explicit JacobiSVD(const TriangularBase<Derived>& matrix) {
+    compute_impl(matrix, internal::get_computation_options(Options));
+  }
+
   /** \brief Constructor performing the decomposition of given matrix using specified options
    *         for computing unitaries.
    *
@@ -601,6 +606,11 @@ class JacobiSVD : public SVDBase<JacobiSVD<MatrixType_, Options_> > {
     return compute_impl(matrix, m_computationOptions);
   }
 
+  template <typename Derived>
+  JacobiSVD& compute(const TriangularBase<Derived>& matrix) {
+    return compute_impl(matrix, m_computationOptions);
+  }
+
   /** \brief Method performing the decomposition of given matrix, as specified by
    *         the `computationOptions` parameter.
    *
@@ -638,6 +648,8 @@ class JacobiSVD : public SVDBase<JacobiSVD<MatrixType_, Options_> > {
   }
 
  private:
+  template <typename Derived>
+  JacobiSVD& compute_impl(const TriangularBase<Derived>& matrix, unsigned int computationOptions);
   template <typename Derived>
   JacobiSVD& compute_impl(const MatrixBase<Derived>& matrix, unsigned int computationOptions);
 
@@ -676,6 +688,13 @@ class JacobiSVD : public SVDBase<JacobiSVD<MatrixType_, Options_> > {
   WorkMatrixType m_workMatrix;
 };
 
+template <typename MatrixType, int Options>
+template <typename Derived>
+JacobiSVD<MatrixType, Options>& JacobiSVD<MatrixType, Options>::compute_impl(const TriangularBase<Derived>& matrix,
+                                                                             unsigned int computationOptions) {
+  return compute_impl(matrix.toDenseMatrix(), computationOptions);
+}
+
 template <typename MatrixType, int Options>
 template <typename Derived>
 JacobiSVD<MatrixType, Options>& JacobiSVD<MatrixType, Options>::compute_impl(const MatrixBase<Derived>& matrix,
diff --git a/test/jacobisvd.cpp b/test/jacobisvd.cpp
index cf5cc5fdb..7a6d4906f 100644
--- a/test/jacobisvd.cpp
+++ b/test/jacobisvd.cpp
@@ -94,6 +94,19 @@ void jacobisvd_verify_inputs(const MatrixType& input = MatrixType()) {
           (int)ColPivHouseholderQRPreconditioner));
 }
 
+template <typename MatrixType>
+void svd_triangular_matrix(const MatrixType& input = MatrixType()) {
+  MatrixType matrix(input.rows(), input.cols());
+  svd_fill_random(matrix);
+  // Make sure that we only consider the 'Lower' part of the matrix.
+  MatrixType matrix_self_adj = matrix.template selfadjointView<Lower>().toDenseMatrix();
+
+  JacobiSVD<MatrixType, ComputeFullV> svd_triangular(matrix.template selfadjointView<Lower>());
+  JacobiSVD<MatrixType, ComputeFullV> svd_full(matrix_self_adj);
+
+  VERIFY_IS_APPROX(svd_triangular.singularValues(), svd_full.singularValues());
+}
+
 namespace Foo {
 // older compiler require a default constructor for Bar
 // cf: https://stackoverflow.com/questions/7411515/
@@ -211,5 +224,10 @@ EIGEN_DECLARE_TEST(jacobisvd) {
 
   CALL_SUBTEST_55(svd_underoverflow<void>());
 
+  // Check that the TriangularBase constructor works
+  CALL_SUBTEST_56((svd_triangular_matrix<Matrix3d>()));
+  CALL_SUBTEST_57((svd_triangular_matrix<Matrix4f>()));
+  CALL_SUBTEST_58((svd_triangular_matrix<Matrix<double, 10, 10>>()));
+
   msvc_workaround();
 }
-- 
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