From 9210e71fb378a0f1542272506dc2759b6c147237 Mon Sep 17 00:00:00 2001
From: Erik Schultheis <erik.schultheis@aalto.fi>
Date: Wed, 5 Jan 2022 00:46:09 +0000
Subject: [PATCH] ensure that eigen::internal::size is not found by ADL, rename
 to ssize and...

---
 Eigen/src/Core/IndexedView.h           |  4 ++--
 Eigen/src/Core/util/Meta.h             | 25 +++++++++++++++++++------
 Eigen/src/plugins/IndexedViewMethods.h |  6 +++---
 3 files changed, 24 insertions(+), 11 deletions(-)

diff --git a/Eigen/src/Core/IndexedView.h b/Eigen/src/Core/IndexedView.h
index d27ec8037..e7ca88beb 100644
--- a/Eigen/src/Core/IndexedView.h
+++ b/Eigen/src/Core/IndexedView.h
@@ -124,10 +124,10 @@ public:
   {}
 
   /** \returns number of rows */
-  Index rows() const { return internal::size(m_rowIndices); }
+  Index rows() const { return internal::index_list_size(m_rowIndices); }
 
   /** \returns number of columns */
-  Index cols() const { return internal::size(m_colIndices); }
+  Index cols() const { return internal::index_list_size(m_colIndices); }
 
   /** \returns the nested expression */
   const typename internal::remove_all<XprType>::type&
diff --git a/Eigen/src/Core/util/Meta.h b/Eigen/src/Core/util/Meta.h
index b5db0d951..c0232c212 100755
--- a/Eigen/src/Core/util/Meta.h
+++ b/Eigen/src/Core/util/Meta.h
@@ -212,20 +212,33 @@ template<typename T, std::size_t N> struct array_size<std::array<T,N> > {
   enum { value = N };
 };
 
+
 /** \internal
-  * Analogue of the std::size free function.
-  * It returns the size of the container or view \a x of type \c T
+  * Analogue of the std::ssize free function.
+  * It returns the signed size of the container or view \a x of type \c T
   *
   * It currently supports:
   *  - any types T defining a member T::size() const
   *  - plain C arrays as T[N]
   *
+  * For C++20, this function just forwards to `std::ssize`, or any ADL discoverable `ssize` function.
   */
-template<typename T>
-EIGEN_CONSTEXPR Index size(const T& x) { return x.size(); }
+#if EIGEN_COMP_CXXVER < 20
+template <typename T>
+EIGEN_CONSTEXPR auto index_list_size(const T& x) {
+  using R = std::common_type_t<std::ptrdiff_t, std::make_signed_t<decltype(x.size())>>;
+  return static_cast<R>(x.size());
+}
 
-template<typename T,std::size_t N>
-EIGEN_CONSTEXPR Index size(const T (&) [N]) { return N; }
+template<typename T, std::ptrdiff_t N>
+EIGEN_CONSTEXPR std::ptrdiff_t index_list_size(const T (&)[N]) { return N; }
+#else
+template <typename T>
+EIGEN_CONSTEXPR auto index_list_size(T&& x) {
+  using std::ssize;
+  return ssize(std::forward<T>(x));
+}
+#endif // EIGEN_COMP_CXXVER
 
 /** \internal
   * Convenient struct to get the result type of a nullary, unary, binary, or
diff --git a/Eigen/src/plugins/IndexedViewMethods.h b/Eigen/src/plugins/IndexedViewMethods.h
index 8acd87e3d..8b97c98cc 100644
--- a/Eigen/src/plugins/IndexedViewMethods.h
+++ b/Eigen/src/plugins/IndexedViewMethods.h
@@ -90,8 +90,8 @@ operator()(const RowIndices& rowIndices, const ColIndices& colIndices) EIGEN_IND
   return BlockType(derived(),
                    internal::first(actualRowIndices),
                    internal::first(actualColIndices),
-                   internal::size(actualRowIndices),
-                   internal::size(actualColIndices));
+                   internal::index_list_size(actualRowIndices),
+                   internal::index_list_size(actualColIndices));
 }
 
 // The following overload returns a Scalar
@@ -165,7 +165,7 @@ operator()(const Indices& indices) EIGEN_INDEXED_VIEW_METHOD_CONST
   EIGEN_STATIC_ASSERT_VECTOR_ONLY(Derived)
   typename IvcType<Indices>::type actualIndices = ivcSize(indices);
   return VectorBlock<EIGEN_INDEXED_VIEW_METHOD_CONST Derived,internal::array_size<Indices>::value>
-            (derived(), internal::first(actualIndices), internal::size(actualIndices));
+            (derived(), internal::first(actualIndices), internal::index_list_size(actualIndices));
 }
 
 template<typename IndexType>