Fix EIGEN_NO_AUTOMATIC_RESIZING not resizing empty destinations

libeigen/eigen!2219

Co-authored-by: Rasmus Munk Larsen <rmlarsen@gmail.com>

Eigen/src/Core/AssignEvaluator.h

@@ -810,6 +810,9 @@ EIGEN_DEVICE_FUNC EIGEN_STRONG_INLINE constexpr void resize_if_allowed(DstXprTyp
         (dst.size() == 0 || (DstXprType::IsVectorAtCompileTime ? (dst.size() == src.size())
                                                                : (dst.rows() == dstRows && dst.cols() == dstCols))) &&
         "Size mismatch. Automatic resizing is disabled because EIGEN_NO_AUTOMATIC_RESIZING is defined");
+    if (dst.size() == 0) {
+      dst.resize(dstRows, dstCols);
+    }
 #else
     dst.resize(dstRows, dstCols);
     eigen_assert(dst.rows() == dstRows && dst.cols() == dstCols);

Eigen/src/Core/PlainObjectBase.h

@@ -685,6 +685,7 @@ class PlainObjectBase : public internal::dense_xpr_base<Derived>::type {
     eigen_assert((this->size() == 0 || (IsVectorAtCompileTime ? (this->size() == other.size())
                                                               : (rows() == other.rows() && cols() == other.cols()))) &&
                  "Size mismatch. Automatic resizing is disabled because EIGEN_NO_AUTOMATIC_RESIZING is defined");
+    if (this->size() == 0) resizeLike(other);
     EIGEN_ONLY_USED_FOR_DEBUG(other);
 #else
     resizeLike(other);

test/CMakeLists.txt

@@ -217,6 +217,7 @@ ei_add_test(indexed_view)
 ei_add_test(reshape)
 ei_add_test(swap)
 ei_add_test(resize)
+ei_add_test(no_automatic_resizing)
 ei_add_test(conservative_resize)
 ei_add_test(product_small)
 ei_add_test(product_large)

test/evaluators.cpp

@@ -33,6 +33,7 @@ EIGEN_STRONG_INLINE DstXprType& copy_using_evaluator(const PlainObjectBase<DstXp
   eigen_assert((dst.size() == 0 || (IsVectorAtCompileTime ? (dst.size() == src.size())
                                                           : (dst.rows() == src.rows() && dst.cols() == src.cols()))) &&
                "Size mismatch. Automatic resizing is disabled because EIGEN_NO_AUTOMATIC_RESIZING is defined");
+  if (dst.size() == 0) dst.const_cast_derived().resizeLike(src.derived());
 #else
   dst.const_cast_derived().resizeLike(src.derived());
 #endif

test/no_automatic_resizing.cpp

@@ -0,0 +1,106 @@
+// This file is part of Eigen, a lightweight C++ template library
+// for linear algebra.
+//
+// Copyright (C) 2026 Rasmus Munk Larsen <rmlarsen@gmail.com>
+//
+// This Source Code Form is subject to the terms of the Mozilla
+// Public License v. 2.0. If a copy of the MPL was not distributed
+// with this file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+// Test that EIGEN_NO_AUTOMATIC_RESIZING still allows assignment to
+// default-constructed (empty) matrices and arrays.
+
+#define EIGEN_NO_AUTOMATIC_RESIZING
+#include "main.h"
+
+template <typename Scalar>
+void testNoAutomaticResizing() {
+  using Matrix = Eigen::Matrix<Scalar, Eigen::Dynamic, Eigen::Dynamic>;
+  using Vector = Eigen::Matrix<Scalar, Eigen::Dynamic, 1>;
+  using RowVector = Eigen::Matrix<Scalar, 1, Eigen::Dynamic>;
+  using Array = Eigen::Array<Scalar, Eigen::Dynamic, Eigen::Dynamic>;
+  using ArrayVector = Eigen::Array<Scalar, Eigen::Dynamic, 1>;
+
+  const Index rows = internal::random<Index>(1, 50);
+  const Index cols = internal::random<Index>(1, 50);
+
+  // Assignment of Zero expression to default-constructed matrix.
+  {
+    Matrix M;
+    M = Matrix::Zero(rows, cols);
+    VERIFY_IS_EQUAL(M.rows(), rows);
+    VERIFY_IS_EQUAL(M.cols(), cols);
+    VERIFY_IS_EQUAL(M.norm(), Scalar(0));
+  }
+
+  // Assignment of Zero expression to default-constructed array.
+  {
+    Array A;
+    A = Array::Zero(rows, cols);
+    VERIFY_IS_EQUAL(A.rows(), rows);
+    VERIFY_IS_EQUAL(A.cols(), cols);
+  }
+
+  // Assignment of Ones expression to default-constructed matrix.
+  {
+    Matrix M;
+    M = Matrix::Ones(rows, cols);
+    VERIFY_IS_EQUAL(M.rows(), rows);
+    VERIFY_IS_EQUAL(M.cols(), cols);
+  }
+
+  // Assignment of Random expression to default-constructed matrix.
+  {
+    Matrix M;
+    M = Matrix::Random(rows, cols);
+    VERIFY_IS_EQUAL(M.rows(), rows);
+    VERIFY_IS_EQUAL(M.cols(), cols);
+  }
+
+  // Assignment from another matrix to default-constructed matrix.
+  {
+    Matrix src = Matrix::Random(rows, cols);
+    Matrix dst;
+    dst = src;
+    VERIFY_IS_EQUAL(dst.rows(), rows);
+    VERIFY_IS_EQUAL(dst.cols(), cols);
+    VERIFY_IS_APPROX(dst, src);
+  }
+
+  // Vector assignment to default-constructed vector.
+  {
+    Vector v;
+    v = Vector::Zero(rows);
+    VERIFY_IS_EQUAL(v.size(), rows);
+  }
+
+  // RowVector assignment to default-constructed row vector.
+  {
+    RowVector v;
+    v = RowVector::Zero(cols);
+    VERIFY_IS_EQUAL(v.size(), cols);
+  }
+
+  // Array vector assignment to default-constructed array vector.
+  {
+    ArrayVector v;
+    v = ArrayVector::Zero(rows);
+    VERIFY_IS_EQUAL(v.size(), rows);
+  }
+
+  // Column access after Zero initialization (reproducer for reported bug).
+  {
+    Array A;
+    A = Array::Zero(rows, cols);
+    for (Index j = 0; j < cols; ++j) {
+      auto c = A.col(j);
+      VERIFY_IS_EQUAL(c.rows(), rows);
+    }
+  }
+}
+
+EIGEN_DECLARE_TEST(no_automatic_resizing) {
+  CALL_SUBTEST_1(testNoAutomaticResizing<float>());
+  CALL_SUBTEST_2(testNoAutomaticResizing<double>());
+  CALL_SUBTEST_3(testNoAutomaticResizing<std::complex<double>>());
+}