From 8d0e84788189748d9e906cc7f807507a3cb4b2ef Mon Sep 17 00:00:00 2001
From: Zachary Ferguson <zy.fergus@gmail.com>
Date: Fri, 31 Jul 2026 10:12:30 -0700
Subject: [PATCH] Support Eigen 5: reimplement removed internal::make_coherent

Eigen 5.0 removed Eigen::internal::make_coherent from
unsupported/Eigen/src/AutoDiff/AutoDiffScalar.h, which
AutomaticDifferentiation.hh references in the
pow(AutoDiffScalar, AutoDiffScalar) overload. Because the call is
qualified, name lookup happens at template definition time, so any TU
including this header fails to compile against Eigen >= 5 even if pow is
never instantiated (this also breaks all of MeshFEMSparse, whose
SparseMatrices.hh includes this header).

Reimplement it with the Eigen 3.4 semantics behind a version guard.
Note Eigen 5 moved to semantic versioning: EIGEN_WORLD_VERSION remains 3
forever and the new major version lives in EIGEN_MAJOR_VERSION.
---
 .../MeshFEMCore/AutomaticDifferentiation.hh   | 27 +++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/src/lib/MeshFEMCore/AutomaticDifferentiation.hh b/src/lib/MeshFEMCore/AutomaticDifferentiation.hh
index bfea387..fc3ecd5 100644
--- a/src/lib/MeshFEMCore/AutomaticDifferentiation.hh
+++ b/src/lib/MeshFEMCore/AutomaticDifferentiation.hh
@@ -4,6 +4,33 @@
 #include <MeshFEMCore/Types.hh>
 #include <unsupported/Eigen/AutoDiff>
 
+// Eigen 5 removed Eigen::internal::make_coherent from AutoDiffScalar.h;
+// reimplement it with the Eigen 3.4 semantics: if exactly one of the two
+// derivative vectors is empty, resize it to match the other and zero it.
+// (Note: Eigen 5 moved to semantic versioning — EIGEN_WORLD_VERSION remains 3
+// forever and the major version lives in EIGEN_MAJOR_VERSION.)
+#if EIGEN_MAJOR_VERSION >= 5
+namespace Eigen {
+namespace internal {
+    template<typename DerTypeA, typename DerTypeB>
+    inline void make_coherent(const DerTypeA &a, const DerTypeB &b) {
+        // Eigen 3.4's implementation const-casts too (the derivatives are
+        // semantically mutable scratch space of the AutoDiffScalar pair).
+        DerTypeA &a_ref = const_cast<DerTypeA &>(a);
+        DerTypeB &b_ref = const_cast<DerTypeB &>(b);
+        if (a_ref.size() == 0 && b_ref.size() != 0) {
+            a_ref.resize(b_ref.size());
+            a_ref.setZero();
+        }
+        else if (b_ref.size() == 0 && a_ref.size() != 0) {
+            b_ref.resize(a_ref.size());
+            b_ref.setZero();
+        }
+    }
+} // namespace internal
+} // namespace Eigen
+#endif
+
 namespace MeshFEM {
 
 using ADReal = Eigen::AutoDiffScalar<Eigen::Matrix<Real, 1, 1>>;