Disallow registering dtypes multiple times

include/pybind11/numpy.h

@@ -668,6 +668,9 @@ struct npy_format_descriptor<T, enable_if_t<is_pod_struct<T>::value>> {
     }
 
     static void register_dtype(std::initializer_list<field_descriptor> fields) {
+        if (dtype_ptr)
+            pybind11_fail("NumPy: dtype is already registered");
+
         list names, formats, offsets;
         for (auto field : fields) {
             if (!field.descr)

tests/test_numpy_dtypes.cpp

@@ -335,6 +335,7 @@ test_initializer numpy_dtypes([](py::module &m) {
     m.def("f_simple", [](SimpleStruct s) { return s.y * 10; });
     m.def("f_packed", [](PackedStruct s) { return s.y * 10; });
     m.def("f_nested", [](NestedStruct s) { return s.a.y * 10; });
+    m.def("register_dtype", []() { PYBIND11_NUMPY_DTYPE(SimpleStruct, x, y, z); });
 });
 
 #undef PYBIND11_PACKED

tests/test_numpy_dtypes.py

@@ -196,3 +196,12 @@ def test_scalar_conversion():
                 with pytest.raises(TypeError) as excinfo:
                     func(arr[0])
                 assert 'incompatible function arguments' in str(excinfo.value)
+
+
+@pytest.requires_numpy
+def test_register_dtype():
+    from pybind11_tests import register_dtype
+
+    with pytest.raises(RuntimeError) as excinfo:
+        register_dtype()
+    assert 'dtype is already registered' in str(excinfo.value)