numpy: fix refcount leak to dtype singleton (pybind#1860)

PyArray_DescrFromType returns a new reference, not borrowed one

Author: pv

include/pybind11/numpy.h

@@ -1044,7 +1044,7 @@ struct npy_format_descriptor<T, enable_if_t<satisfies_any_of<T, std::is_arithmet
 
     static pybind11::dtype dtype() {
         if (auto ptr = npy_api::get().PyArray_DescrFromType_(value))
-            return reinterpret_borrow<pybind11::dtype>(ptr);
+            return reinterpret_steal<pybind11::dtype>(ptr);
         pybind11_fail("Unsupported buffer format!");
     }
 };

tests/test_numpy_array.py

@@ -434,3 +434,14 @@ def test_array_create_and_resize(msg):
 def test_index_using_ellipsis():
     a = m.index_using_ellipsis(np.zeros((5, 6, 7)))
     assert a.shape == (6,)
+
+
+@pytest.unsupported_on_pypy
+def test_dtype_refcount_leak():
+    from sys import getrefcount
+    dtype = np.dtype(np.float_)
+    a = np.array([1], dtype=dtype)
+    before = getrefcount(dtype)
+    m.ndim(a)
+    after = getrefcount(dtype)
+    assert after == before