From 4ae564735564d8afcac260915d77f14e78da7904 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Antonio=20S=C3=A1nchez?= <cantonios@google.com>
Date: Mon, 25 Aug 2025 18:17:18 +0000
Subject: [PATCH] Fix direct index aliased assignment.

---
 Eigen/src/Core/IndexedView.h |  6 ++++++
 test/indexed_view.cpp        | 10 ++++++++++
 2 files changed, 16 insertions(+)

diff --git a/Eigen/src/Core/IndexedView.h b/Eigen/src/Core/IndexedView.h
index 358239ca8..10562c194 100644
--- a/Eigen/src/Core/IndexedView.h
+++ b/Eigen/src/Core/IndexedView.h
@@ -308,6 +308,12 @@ struct unary_evaluator<IndexedView<ArgType, RowIndices, ColIndices>, IndexBased>
   const XprType& m_xpr;
 };
 
+// Catch assignments to an IndexedView.
+template <typename ArgType, typename RowIndices, typename ColIndices>
+struct evaluator_assume_aliasing<IndexedView<ArgType, RowIndices, ColIndices>> {
+  static const bool value = true;
+};
+
 }  // end namespace internal
 
 }  // end namespace Eigen
diff --git a/test/indexed_view.cpp b/test/indexed_view.cpp
index 064cc4ac7..9f0df851d 100644
--- a/test/indexed_view.cpp
+++ b/test/indexed_view.cpp
@@ -840,11 +840,21 @@ void check_tutorial_examples() {
   }
 }
 
+void check_aliasing() {
+  Eigen::Vector<float, 5> z = {0.0f, 1.1f, 2.2f, 3.3f, 4.4f};
+  std::vector<int> left_indices = {0, 1, 3, 4};
+  std::vector<int> right_indices = {1, 3, 4, 0};
+  z(left_indices) = z(right_indices);
+  Eigen::Vector<float, 5> expected = {1.1f, 3.3f, 2.2f, 4.4f, 0.0f};
+  VERIFY_IS_EQUAL(z, expected);
+}
+
 EIGEN_DECLARE_TEST(indexed_view) {
   for (int i = 0; i < g_repeat; i++) {
     CALL_SUBTEST_1(check_indexed_view());
   }
   CALL_SUBTEST_1(check_tutorial_examples());
+  CALL_SUBTEST_1(check_aliasing());
 
   // static checks of some internals:
   STATIC_CHECK((internal::is_valid_index_type<int>::value));