From d46d726e9d06416c7d1d5a81bdeae04c629f93d8 Mon Sep 17 00:00:00 2001
From: Christoph Hertzberg <chtz@informatik.uni-bremen.de>
Date: Mon, 13 Apr 2020 16:41:20 +0200
Subject: CommaInitializer wrongfully asserted for 0-sized blocks
 commainitialier unit-test never actually called `test_block_recursion`, which
 also was not correctly implemented and would have caused too deep template
 recursion.

---
 Eigen/src/Core/CommaInitializer.h |  2 +-
 test/commainitializer.cpp         | 26 +++++++++++++++++---------
 2 files changed, 18 insertions(+), 10 deletions(-)

diff --git a/Eigen/src/Core/CommaInitializer.h b/Eigen/src/Core/CommaInitializer.h
index 53066c653..c0e29c75c 100644
--- a/Eigen/src/Core/CommaInitializer.h
+++ b/Eigen/src/Core/CommaInitializer.h
@@ -43,7 +43,7 @@ struct CommaInitializer
   inline CommaInitializer(XprType& xpr, const DenseBase<OtherDerived>& other)
     : m_xpr(xpr), m_row(0), m_col(other.cols()), m_currentBlockRows(other.rows())
   {
-    eigen_assert(m_xpr.rows() > 0 && m_xpr.cols() > 0
+    eigen_assert(m_xpr.rows() >= other.rows() && m_xpr.cols() >= other.cols()
       && "Cannot comma-initialize a 0x0 matrix (operator<<)");
     m_xpr.block(0, 0, other.rows(), other.cols()) = other;
   }
diff --git a/test/commainitializer.cpp b/test/commainitializer.cpp
index 3cb94da62..f535117d5 100644
--- a/test/commainitializer.cpp
+++ b/test/commainitializer.cpp
@@ -34,8 +34,14 @@ void test_blocks()
 
   if(N1 > 0)
   {
-    VERIFY_RAISES_ASSERT((m_fixed << mat11, mat12, mat11, mat21, mat22));
-    VERIFY_RAISES_ASSERT((m_fixed << mat11, mat12, mat21, mat21, mat22));
+    if(M1 > 0)
+    {
+      VERIFY_RAISES_ASSERT((m_fixed << mat11, mat12, mat11, mat21, mat22));
+    }
+    if(M2 > 0)
+    {
+      VERIFY_RAISES_ASSERT((m_fixed << mat11, mat12, mat21, mat21, mat22));
+    }
   }
   else
   {
@@ -49,20 +55,22 @@ void test_blocks()
 }
 
 
-template<int N>
+template<int depth, int N=0>
 struct test_block_recursion
 {
   static void run()
   {
-    test_blocks<(N>>6)&3, (N>>4)&3, (N>>2)&3, N & 3>();
-    test_block_recursion<N-1>::run();
+    test_block_recursion<depth-1, N>::run();
+    test_block_recursion<depth-1, N + (1 << (depth-1))>::run();
   }
 };
 
-template<>
-struct test_block_recursion<-1>
+template<int N>
+struct test_block_recursion<0,N>
 {
-  static void run() { }
+  static void run() {
+    test_blocks<(N>>6)&3, (N>>4)&3, (N>>2)&3, N & 3>();
+  }
 };
 
 EIGEN_DECLARE_TEST(commainitializer)
@@ -102,5 +110,5 @@ EIGEN_DECLARE_TEST(commainitializer)
 
 
   // recursively test all block-sizes from 0 to 3:
-  test_block_recursion<(1<<8) - 1>();
+  test_block_recursion<8>::run();
 }
-- 
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