Fix arrays with zero-size dimensions (#4038)

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

@@ -111,9 +111,15 @@ 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()
+        // 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);

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());

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"""