From 479e9a50f38cfa8e525a697d7f5d850f851223b5 Mon Sep 17 00:00:00 2001
From: Matthew Woehlke <matthew.woehlke@kitware.com>
Date: Wed, 29 Jun 2022 14:37:16 -0400
Subject: [PATCH] Fix arrays with zero-size dimensions (#4038)
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When converting an array to an Eigen matrix, ignore the strides if any
dimension size is 0. If the array is empty, the strides aren't relevant,
and especially numpy ≥ 1.23 explicitly sets the strides to 0 in this
case. (See numpy commit dd5ab7b11520.)

Update tests to verify that this works, and continues to work.
---
 include/pybind11/eigen.h | 14 ++++++++++----
 tests/test_eigen.cpp     |  7 +++++--
 tests/test_eigen.py      |  7 +++++++
 3 files changed, 22 insertions(+), 6 deletions(-)

diff --git a/include/pybind11/eigen.h b/include/pybind11/eigen.h
index f658168de..71df3eb8b 100644
--- a/include/pybind11/eigen.h
+++ b/include/pybind11/eigen.h
@@ -111,10 +111,16 @@ struct EigenConformable {
     bool stride_compatible() const {
         // To have compatible strides, we need (on both dimensions) one of fully dynamic strides,
         // matching strides, or a dimension size of 1 (in which case the stride value is
-        // irrelevant)
-        return !negativestrides
-               && (props::inner_stride == Eigen::Dynamic || props::inner_stride == stride.inner()
-                   || (EigenRowMajor ? cols : rows) == 1)
+        // irrelevant). Alternatively, if any dimension size is 0, the strides are not relevant
+        // (and numpy ≥ 1.23 sets the strides to 0 in that case, so we need to check explicitly).
+        if (negativestrides) {
+            return false;
+        }
+        if (rows == 0 || cols == 0) {
+            return true;
+        }
+        return (props::inner_stride == Eigen::Dynamic || props::inner_stride == stride.inner()
+                || (EigenRowMajor ? cols : rows) == 1)
                && (props::outer_stride == Eigen::Dynamic || props::outer_stride == stride.outer()
                    || (EigenRowMajor ? rows : cols) == 1);
     }
diff --git a/tests/test_eigen.cpp b/tests/test_eigen.cpp
index 74e76eb7e..591dacc62 100644
--- a/tests/test_eigen.cpp
+++ b/tests/test_eigen.cpp
@@ -345,10 +345,13 @@ TEST_SUBMODULE(eigen, m) {
         },
         py::arg{}.noconvert());
 
-    // test_issue738
-    // Issue #738: 1xN or Nx1 2D matrices were neither accepted nor properly copied with an
+    // test_issue738, test_zero_length
+    // Issue #738: 1×N or N×1 2D matrices were neither accepted nor properly copied with an
     // incompatible stride value on the length-1 dimension--but that should be allowed (without
     // requiring a copy!) because the stride value can be safely ignored on a size-1 dimension.
+    // Similarly, 0×N or N×0 matrices were not accepted--again, these should be allowed since
+    // they contain no data. This particularly affects numpy ≥ 1.23, which sets the strides to
+    // 0 if any dimension size is 0.
     m.def("iss738_f1",
           &adjust_matrix<const Eigen::Ref<const Eigen::MatrixXd> &>,
           py::arg{}.noconvert());
diff --git a/tests/test_eigen.py b/tests/test_eigen.py
index 093773a51..a1c114aed 100644
--- a/tests/test_eigen.py
+++ b/tests/test_eigen.py
@@ -744,6 +744,13 @@ def test_issue738():
     )
 
 
+@pytest.mark.parametrize("func", [m.iss738_f1, m.iss738_f2])
+@pytest.mark.parametrize("sizes", [(0, 2), (2, 0)])
+def test_zero_length(func, sizes):
+    """Ignore strides on a length-0 dimension (even if they would be incompatible length > 1)"""
+    assert np.all(func(np.zeros(sizes)) == np.zeros(sizes))
+
+
 def test_issue1105():
     """Issue 1105: 1xN or Nx1 input arrays weren't accepted for eigen
     compile-time row vectors or column vector"""