From d8c8ee6fb2cfe1af53b020ff10a70fbd9b47ccad Mon Sep 17 00:00:00 2001
From: Tyler Veness <calcmogul@gmail.com>
Date: Thu, 5 Mar 2026 17:43:21 -0800
Subject: [PATCH] Fix crash on construction of SparseMatrix with zero-length
 diagonal

libeigen/eigen!2249
---
 Eigen/src/SparseCore/SparseMatrix.h | 2 +-
 test/sparse_basic.cpp               | 4 ++++
 2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/Eigen/src/SparseCore/SparseMatrix.h b/Eigen/src/SparseCore/SparseMatrix.h
index 782e59511..36646c8ab 100644
--- a/Eigen/src/SparseCore/SparseMatrix.h
+++ b/Eigen/src/SparseCore/SparseMatrix.h
@@ -1008,7 +1008,7 @@ class SparseMatrix : public SparseCompressedBase<SparseMatrix<Scalar_, Options_,
 
     const bool overwrite = internal::is_same<Func, internal::assign_op<Scalar, Scalar>>::value;
     if (overwrite) {
-      if ((m_outerSize != n) || (m_innerSize != n)) resize(n, n);
+      if ((m_outerSize != n) || (m_innerSize != n) || (n == 0)) resize(n, n);
     }
 
     if (m_data.size() == 0 || overwrite) {
diff --git a/test/sparse_basic.cpp b/test/sparse_basic.cpp
index b239eab58..f1fd6b771 100644
--- a/test/sparse_basic.cpp
+++ b/test/sparse_basic.cpp
@@ -871,6 +871,10 @@ void sparse_basic(const SparseMatrixType& ref) {
     m2 -= d.asDiagonal();
     refMat2 -= d.asDiagonal();
     VERIFY_IS_APPROX(m2, refMat2);
+
+    // zero-length diagonal
+    d = DenseVector(0);
+    SparseMatrixType m4(d.asDiagonal());
   }
 
   // test conservative resize