Add support for C++23 multidimensional subscript operator

I'm not sure where to put tests for this, assuming they're needed. They also wouldn't run in CI anyway since CI only exercises the C++17 codepaths.

See merge request libeigen/eigen!2053

Eigen/src/Core/DenseCoeffsBase.h

@@ -113,6 +113,16 @@ class DenseCoeffsBase<Derived, ReadOnlyAccessors> : public EigenBase<Derived> {
     return coeff(row, col);
   }
 
+#if __cpp_multidimensional_subscript >= 202110L
+  /** \returns the coefficient at given the given row and column.
+   *
+   * \sa operator[](Index,Index), operator[](Index)
+   */
+  EIGEN_DEVICE_FUNC EIGEN_STRONG_INLINE constexpr CoeffReturnType operator[](Index row, Index col) const {
+    return operator()(row, col);
+  }
+#endif
+
   /** Short version: don't use this function, use
    * \link operator[](Index) const \endlink instead.
    *
@@ -316,12 +326,21 @@ class DenseCoeffsBase<Derived, WriteAccessors> : public DenseCoeffsBase<Derived,
    *
    * \sa operator[](Index)
    */
-
   EIGEN_DEVICE_FUNC EIGEN_STRONG_INLINE constexpr Scalar& operator()(Index row, Index col) {
     eigen_assert(row >= 0 && row < rows() && col >= 0 && col < cols());
     return coeffRef(row, col);
   }
 
+#if __cpp_multidimensional_subscript >= 202110L
+  /** \returns a reference to the coefficient at given the given row and column.
+   *
+   * \sa operator[](Index)
+   */
+  EIGEN_DEVICE_FUNC EIGEN_STRONG_INLINE constexpr Scalar& operator[](Index row, Index col) {
+    return operator()(row, col);
+  }
+#endif
+
   /** Short version: don't use this function, use
    * \link operator[](Index) \endlink instead.
    *

Eigen/src/Core/TriangularMatrix.h

@@ -89,6 +89,11 @@ class TriangularBase : public EigenBase<Derived> {
     return coeffRef(row, col);
   }
 
+#if __cpp_multidimensional_subscript >= 202110L
+  EIGEN_DEVICE_FUNC inline Scalar operator[](Index row, Index col) const { return operator()(row, col); }
+  EIGEN_DEVICE_FUNC inline Scalar& operator[](Index row, Index col) { return operator()(row, col); }
+#endif
+
 #ifndef EIGEN_PARSED_BY_DOXYGEN
   EIGEN_DEVICE_FUNC inline const Derived& derived() const { return *static_cast<const Derived*>(this); }
   EIGEN_DEVICE_FUNC inline Derived& derived() { return *static_cast<Derived*>(this); }

Eigen/src/Geometry/Transform.h

@@ -365,6 +365,15 @@ class Transform {
    * \sa MatrixBase::operator(Index,Index) */
   EIGEN_DEVICE_FUNC inline Scalar& operator()(Index row, Index col) { return m_matrix(row, col); }
 
+#if __cpp_multidimensional_subscript >= 202110L
+  /** shortcut for m_matrix(row,col);
+   * \sa MatrixBase::operator(Index,Index) const */
+  EIGEN_DEVICE_FUNC inline Scalar operator[](Index row, Index col) const { return m_matrix[row, col]; }
+  /** shortcut for m_matrix(row,col);
+   * \sa MatrixBase::operator(Index,Index) */
+  EIGEN_DEVICE_FUNC inline Scalar& operator[](Index row, Index col) { return m_matrix[row, col]; }
+#endif
+
   /** \returns a read-only expression of the transformation matrix */
   EIGEN_DEVICE_FUNC inline const MatrixType& matrix() const { return m_matrix; }
   /** \returns a writable expression of the transformation matrix */

unsupported/Eigen/CXX11/src/Tensor/TensorContractionMapper.h

@@ -117,6 +117,10 @@ class SimpleTensorContractionMapper {
     return m_tensor.coeff(computeIndex(row, col));
   }
 
+#if __cpp_multidimensional_subscript >= 202110L
+  EIGEN_DEVICE_FUNC EIGEN_STRONG_INLINE Scalar operator[](Index row, Index col) const { return operator()(row, col); }
+#endif
+
   EIGEN_DEVICE_FUNC EIGEN_STRONG_INLINE Index computeIndex(Index row, Index col) const {
     const bool left = (side == Lhs);
     EIGEN_UNUSED_VARIABLE(left);  // annoying bug in g++8.1: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85963

unsupported/Eigen/src/Polynomials/Companion.h

@@ -63,6 +63,10 @@ class companion {
     }
   }
 
+#if __cpp_multidimensional_subscript >= 202110L
+  EIGEN_STRONG_INLINE const Scalar_ operator[](Index row, Index col) const { return operator()(row, col); }
+#endif
+
  public:
   template <typename VectorType>
   void setPolynomial(const VectorType& poly) {

unsupported/Eigen/src/SparseExtra/RandomSetter.h

@@ -280,6 +280,11 @@ class RandomSetter {
     return m_hashmaps[outerMajor][key].value;
   }
 
+#if __cpp_multidimensional_subscript >= 202110L
+  /** \returns a reference to the coefficient at given coordinates \a row, \a col */
+  Scalar& operator[](Index row, Index col) { return operator()(row, col); }
+#endif
+
   /** \returns the number of non zero coefficients
    *
    * \note According to the underlying map/hash_map implementation,