Eigen: treat 1x1 matrices as matrices and not scalars

- also adds a test for return value passing, which was (ultimately) not
  found to be the reason of the reported issue

include/nanobind/eigen/dense.h

@@ -26,7 +26,7 @@ template <typename T> using DMap = Eigen::Map<T, 0, DStride>;
 NAMESPACE_BEGIN(detail)
 
 template <typename T>
-constexpr int NumDimensions = int(T::MaxSizeAtCompileTime) == 1 ? 0 : bool(T::IsVectorAtCompileTime) ? 1 : 2;
+constexpr int NumDimensions = bool(T::IsVectorAtCompileTime) ? 1 : 2;
 
 template <typename T>
 using array_for_eigen_t = ndarray<

src/nb_ndarray.cpp

@@ -504,7 +504,6 @@ ndarray_handle *ndarray_create(void *value, size_t ndim, const size_t *shape_in,
             --i;
         }
     }
-
     ndarray->dltensor.data = (void *) value_rounded;
     ndarray->dltensor.device.device_type = device_type;
     ndarray->dltensor.device.device_id = device_id;

tests/test_eigen.cpp

@@ -117,6 +117,16 @@ NB_MODULE(test_eigen_ext, m) {
         return m;
     });
 
+    /// issue #166
+    using Matrix1d = Eigen::Matrix<double,1,1>;
+    try {
+        m.def(
+            "default_arg", [](Matrix1d a, Matrix1d b) { return a + b; },
+            "a"_a = Matrix1d::Zero(), "b"_a = Matrix1d::Zero());
+    } catch (...) {
+        // Ignore (NumPy not installed, etc.)
+    }
+
     struct Buffer {
         uint32_t x[30] { };
 

tests/test_eigen.py

@@ -243,3 +243,8 @@ def test_sparse_failures():
     # undo sabotage of the module
     sys.path = sys_path
     scipy.sparse.csr_matrix = csr_matrix
+
+@needs_numpy_and_eigen
+def test_eigen_scalar_default():
+    x = t.default_arg()
+    assert x==0