From b2814d53a707f699e5c56f565847e7020654efc2 Mon Sep 17 00:00:00 2001
From: Antonio Sanchez <cantonios@google.com>
Date: Tue, 16 Jan 2024 08:50:26 -0800
Subject: [PATCH] Fix stableNorm when input is zero-sized.

---
 Eigen/src/Core/StableNorm.h | 4 ++++
 test/stable_norm.cpp        | 7 +++++++
 2 files changed, 11 insertions(+)

diff --git a/Eigen/src/Core/StableNorm.h b/Eigen/src/Core/StableNorm.h
index 1ec71403c..be80b7fac 100644
--- a/Eigen/src/Core/StableNorm.h
+++ b/Eigen/src/Core/StableNorm.h
@@ -55,6 +55,10 @@ typename MatrixType::RealScalar stable_norm_impl(const MatrixType& mat) {
   RealScalar invScale(1);
   RealScalar ssq(0);  // sum of squares
 
+  if (mat.size() == 0) {
+    return RealScalar(0);
+  }
+
   stable_norm_kernel(mat, ssq, scale, invScale);
   return scale * sqrt(ssq);
 }
diff --git a/test/stable_norm.cpp b/test/stable_norm.cpp
index c16e44d8d..e4c13f761 100644
--- a/test/stable_norm.cpp
+++ b/test/stable_norm.cpp
@@ -209,6 +209,11 @@ void stable_norm(const MatrixType& m) {
   }
 }
 
+void test_empty() {
+  Eigen::VectorXf empty(0);
+  VERIFY_IS_EQUAL(empty.stableNorm(), 0.0f);
+}
+
 template <typename Scalar>
 void test_hypot() {
   typedef typename NumTraits<Scalar>::Real RealScalar;
@@ -235,6 +240,8 @@ void test_hypot() {
 }
 
 EIGEN_DECLARE_TEST(stable_norm) {
+  CALL_SUBTEST_1(test_empty());
+
   for (int i = 0; i < g_repeat; i++) {
     CALL_SUBTEST_3(test_hypot<double>());
     CALL_SUBTEST_4(test_hypot<float>());