Throw exception on returning a unique_ptr or shared_ptr nullptr (or any other...

Throw exception on returning a unique_ptr or shared_ptr nullptr (or any other holder type) from py::init, rather than crashing (#2430)

include/pybind11/detail/init.h

@@ -132,6 +132,7 @@ void construct(value_and_holder &v_h, Alias<Class> *alias_ptr, bool) {
 template <typename Class>
 void construct(value_and_holder &v_h, Holder<Class> holder, bool need_alias) {
     auto *ptr = holder_helper<Holder<Class>>::get(holder);
+    no_nullptr(ptr);
     // If we need an alias, check that the held pointer is actually an alias instance
     if (Class::has_alias && need_alias && !is_alias<Class>(ptr))
         throw type_error("pybind11::init(): construction failed: returned holder-wrapped instance "

tests/test_factory_constructors.cpp

@@ -154,6 +154,8 @@ TEST_SUBMODULE(factory_constructors, m) {
     MAKE_TAG_TYPE(TF4);
     MAKE_TAG_TYPE(TF5);
     MAKE_TAG_TYPE(null_ptr);
+    MAKE_TAG_TYPE(null_unique_ptr);
+    MAKE_TAG_TYPE(null_shared_ptr);
     MAKE_TAG_TYPE(base);
     MAKE_TAG_TYPE(invalid_base);
     MAKE_TAG_TYPE(alias);
@@ -194,6 +196,8 @@ TEST_SUBMODULE(factory_constructors, m) {
 
         // Returns nullptr:
         .def(py::init([](null_ptr_tag) { return (TestFactory3 *) nullptr; }))
+        .def(py::init([](null_unique_ptr_tag) { return std::unique_ptr<TestFactory3>(); }))
+        .def(py::init([](null_shared_ptr_tag) { return std::shared_ptr<TestFactory3>(); }))
 
         .def_readwrite("value", &TestFactory3::value)
         ;

tests/test_factory_constructors.py

@@ -41,9 +41,12 @@ def test_init_factory_basic():
     z3 = m.TestFactory3("bye")
     assert z3.value == "bye"
 
-    with pytest.raises(TypeError) as excinfo:
-        m.TestFactory3(tag.null_ptr)
-    assert str(excinfo.value) == "pybind11::init(): factory function returned nullptr"
+    for null_ptr_kind in [tag.null_ptr,
+                          tag.null_unique_ptr,
+                          tag.null_shared_ptr]:
+        with pytest.raises(TypeError) as excinfo:
+            m.TestFactory3(null_ptr_kind)
+        assert str(excinfo.value) == "pybind11::init(): factory function returned nullptr"
 
     assert [i.alive() for i in cstats] == [3, 3, 3]
     assert ConstructorStats.detail_reg_inst() == n_inst + 9