Allow raw unions without base classes in is_accessible_base_of (#2320)

Co-authored-by: Henry Schreiner <HenrySchreinerIII@gmail.com>

docs/changelog.rst

@@ -98,6 +98,11 @@ See :ref:`upgrade-guide-2.6` for help upgrading to the new version.
   ``get_type_overload`` is deprecated.
   `#2325 <https://github.com/pybind/pybind11/pull/2325>`_
 
+* `py::class_<union_type>` is now supported. Note that writing to one data
+  member of the union and reading another (type punning) is UB in C++. Thus
+  pybind11-bound enums should never be used for such conversion.
+  `#2320 <https://github.com/pybind/pybind11/pull/2320>`_.
+
 Smaller or developer focused features:
 
 * Moved ``mkdoc.py`` to a new repo, `pybind11-mkdoc`_.
@@ -164,7 +169,6 @@ Smaller or developer focused features:
 .. _pre-commit: https://pre-commit.com
 
 
-
 v2.5.0 (Mar 31, 2020)
 -----------------------------------------------------
 

include/pybind11/detail/common.h

@@ -631,8 +631,9 @@ template <typename Base, typename Derived> using is_strict_base_of = bool_consta
 
 /// Like is_base_of, but also requires that the base type is accessible (i.e. that a Derived pointer
 /// can be converted to a Base pointer)
+/// For unions, `is_base_of<T, T>::value` is False, so we need to check `is_same` as well.
 template <typename Base, typename Derived> using is_accessible_base_of = bool_constant<
-    std::is_base_of<Base, Derived>::value && std::is_convertible<Derived *, Base *>::value>;
+    (std::is_same<Base, Derived>::value || std::is_base_of<Base, Derived>::value) && std::is_convertible<Derived *, Base *>::value>;
 
 template <template<typename...> class Base>
 struct is_template_base_of_impl {

tests/pybind11_tests.h

@@ -50,6 +50,12 @@ public:
     IncType &operator=(IncType &&) = delete;
 };
 
+/// A simple union for basic testing
+union IntFloat {
+    int i;
+    float f;
+};
+
 /// Custom cast-only type that casts to a string "rvalue" or "lvalue" depending on the cast context.
 /// Used to test recursive casters (e.g. std::tuple, stl containers).
 struct RValueCaster {};

tests/test_opaque_types.cpp

@@ -64,4 +64,10 @@ TEST_SUBMODULE(opaque_types, m) {
         result->push_back("some value");
         return std::unique_ptr<StringList>(result);
     });
+
+    // test unions
+    py::class_<IntFloat>(m, "IntFloat")
+        .def(py::init<>())
+        .def_readwrite("i", &IntFloat::i)
+        .def_readwrite("f", &IntFloat::f);
 }

tests/test_opaque_types.py

@@ -45,3 +45,11 @@ def test_pointers(msg):
     ptr = m.return_unique_ptr()
     assert "StringList" in repr(ptr)
     assert m.print_opaque_list(ptr) == "Opaque list: [some value]"
+
+
+def test_unions():
+    int_float_union = m.IntFloat()
+    int_float_union.i = 42
+    assert int_float_union.i == 42
+    int_float_union.f = 3.0
+    assert int_float_union.f == 3.0