From 502f7179801a9b6ae5f6d6df60e72d5064702925 Mon Sep 17 00:00:00 2001
From: Gael Guennebaud <g.gael@free.fr>
Date: Wed, 16 Jan 2019 14:33:45 +0100
Subject: [PATCH] bug #1646: disable aliasing detection for empty and 1x1
 expression

---
 Eigen/src/Core/Transpose.h |  3 ++-
 test/adjoint.cpp           | 11 +++++++++++
 2 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/Eigen/src/Core/Transpose.h b/Eigen/src/Core/Transpose.h
index d7c204579..6255a2f6d 100644
--- a/Eigen/src/Core/Transpose.h
+++ b/Eigen/src/Core/Transpose.h
@@ -392,7 +392,8 @@ struct checkTransposeAliasing_impl<Derived, OtherDerived, false>
 template<typename Dst, typename Src>
 void check_for_aliasing(const Dst &dst, const Src &src)
 {
-  internal::checkTransposeAliasing_impl<Dst, Src>::run(dst, src);
+  if(src.size()>1)
+    internal::checkTransposeAliasing_impl<Dst, Src>::run(dst, src);
 }
 
 } // end namespace internal
diff --git a/test/adjoint.cpp b/test/adjoint.cpp
index 4e1e4b5e8..7c8081ec4 100644
--- a/test/adjoint.cpp
+++ b/test/adjoint.cpp
@@ -171,6 +171,17 @@ void adjoint_extra()
   c = MatrixXd::Ones(10,10) * 1.0 + c;
   c = c + MatrixXd::Ones(10,10) .cwiseProduct( MatrixXd::Zero(10,10) );
   c = MatrixXd::Ones(10,10) * MatrixXd::Zero(10,10);
+
+  // regression for bug 1646
+  for (int j = 0; j < 10; ++j) {
+    c.col(j).head(j) = c.row(j).head(j);
+  }
+
+  a.conservativeResize(1,1);
+  a = a.transpose();
+
+  a.conservativeResize(0,0);
+  a = a.transpose();
 }
 
 EIGEN_DECLARE_TEST(adjoint)