BUG: ensure passing np.dtype to itself doesn't crash

Author: ngoldbaum

numpy/core/src/multiarray/descriptor.c

@@ -1478,6 +1478,11 @@ PyArray_DTypeOrDescrConverterRequired(PyObject *obj, npy_dtype_info *dt_info)
     dt_info->descr = NULL;
 
     if (PyObject_TypeCheck(obj, &PyArrayDTypeMeta_Type)) {
+        if (obj == (PyObject *)&PyArrayDescr_Type) {
+            PyErr_SetString(PyExc_TypeError,
+                            "Cannot convert np.dtype into a dtype.");
+            return NPY_FAIL;
+        }
         Py_INCREF(obj);
         dt_info->dtype = (PyArray_DTypeMeta *)obj;
         dt_info->descr = NULL;

numpy/core/tests/test_dtype.py

@@ -1898,3 +1898,9 @@ def test_result_type_integers_and_unitless_timedelta64():
     td = np.timedelta64(4)
     result = np.result_type(0, td)
     assert_dtype_equal(result, td.dtype)
+
+
+def test_creating_dtype_with_dtype_class_errors():
+    # Regression test for #25031, calling `np.dtype` with itself segfaulted.
+    with pytest.raises(TypeError, match="Cannot convert np.dtype into a"):
+        np.array(np.ones(10), dtype=np.dtype)