From ac3ef16f30b7708a3c6c39b964e1f294dd16b8a3 Mon Sep 17 00:00:00 2001
From: Tyler Veness <calcmogul@gmail.com>
Date: Thu, 23 Oct 2025 07:03:37 +0000
Subject: [PATCH] Fix SparseVector::insertBack() with custom scalar types
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It fixes this compiler error:
```
/home/tav/git/Sleipnir/build/_deps/eigen3-src/Eigen/src/SparseCore/SparseVector.h:143:19: error: cannot convert ‘int’ to ‘const Eigen::internal::CompressedStorage<boost::decimal::decimal64_t, int>::Scalar&’ {aka ‘const boost::decimal::decimal64_t&’}
  143 |     m_data.append(0, i);
      |                   ^
      |                   |
      |                   int
```

This change matches what SparseMatrix does:
https://gitlab.com/libeigen/eigen/-/blob/master/Eigen/src/SparseCore/SparseMatrix.h#L430-L438

See merge request libeigen/eigen!2040
---
 Eigen/src/SparseCore/SparseVector.h | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/Eigen/src/SparseCore/SparseVector.h b/Eigen/src/SparseCore/SparseVector.h
index d19a00dd9..8b34a9ac9 100644
--- a/Eigen/src/SparseCore/SparseVector.h
+++ b/Eigen/src/SparseCore/SparseVector.h
@@ -140,7 +140,7 @@ class SparseVector : public SparseCompressedBase<SparseVector<Scalar_, Options_,
     return insertBack(inner);
   }
   inline Scalar& insertBack(Index i) {
-    m_data.append(0, i);
+    m_data.append(Scalar(0), i);
     return m_data.value(m_data.size() - 1);
   }
 
@@ -150,7 +150,7 @@ class SparseVector : public SparseCompressedBase<SparseVector<Scalar_, Options_,
     return insertBackUnordered(inner);
   }
   inline Scalar& insertBackUnordered(Index i) {
-    m_data.append(0, i);
+    m_data.append(Scalar(0), i);
     return m_data.value(m_data.size() - 1);
   }
 
@@ -367,7 +367,7 @@ class SparseVector : public SparseCompressedBase<SparseVector<Scalar_, Options_,
 
   /** \internal \deprecated use insertBack(Index) */
   EIGEN_DEPRECATED_WITH_REASON("Use .insertBack() instead.") Scalar& fill(Index i) {
-    m_data.append(0, i);
+    m_data.append(Scalar(0), i);
     return m_data.value(m_data.size() - 1);
   }