From 1aa8dd17451108871ade6b1ea86d6bcea9d3215a Mon Sep 17 00:00:00 2001
From: Roland Dreier <roland@purestorage.com>
Date: Tue, 11 Jun 2019 13:25:12 -0700
Subject: [PATCH] Fix assertion failure for unions (#1685) (#1709)

In def_readonly and def_readwrite, there is an assertion that the member comes
from the class or a base class:

    static_assert(std::is_base_of<C, type>::value, "...");

However, if C and type are the same type, is_base_of will still only be true
if they are the same _non-union_ type.  This means we can't define accessors
for the members of a union type because of this assertion.

Update the assertion to test

    std::is_same<C, type>::value || std::is_base_of<C, type>::value

which will allow union types, or members of base classes.

Also add a basic unit test for accessing unions.
---
 include/pybind11/pybind11.h |  4 ++--
 tests/CMakeLists.txt        |  1 +
 tests/test_union.cpp        | 22 ++++++++++++++++++++++
 tests/test_union.py         |  8 ++++++++
 4 files changed, 33 insertions(+), 2 deletions(-)
 create mode 100644 tests/test_union.cpp
 create mode 100644 tests/test_union.py

diff --git a/include/pybind11/pybind11.h b/include/pybind11/pybind11.h
index ca0b1a709..f1d91c788 100644
--- a/include/pybind11/pybind11.h
+++ b/include/pybind11/pybind11.h
@@ -1185,7 +1185,7 @@ public:
 
     template <typename C, typename D, typename... Extra>
     class_ &def_readwrite(const char *name, D C::*pm, const Extra&... extra) {
-        static_assert(std::is_base_of<C, type>::value, "def_readwrite() requires a class member (or base class member)");
+        static_assert(std::is_same<C, type>::value || std::is_base_of<C, type>::value, "def_readwrite() requires a class member (or base class member)");
         cpp_function fget([pm](const type &c) -> const D &{ return c.*pm; }, is_method(*this)),
                      fset([pm](type &c, const D &value) { c.*pm = value; }, is_method(*this));
         def_property(name, fget, fset, return_value_policy::reference_internal, extra...);
@@ -1194,7 +1194,7 @@ public:
 
     template <typename C, typename D, typename... Extra>
     class_ &def_readonly(const char *name, const D C::*pm, const Extra& ...extra) {
-        static_assert(std::is_base_of<C, type>::value, "def_readonly() requires a class member (or base class member)");
+        static_assert(std::is_same<C, type>::value || std::is_base_of<C, type>::value, "def_readonly() requires a class member (or base class member)");
         cpp_function fget([pm](const type &c) -> const D &{ return c.*pm; }, is_method(*this));
         def_property_readonly(name, fget, return_value_policy::reference_internal, extra...);
         return *this;
diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index a31d5b8b7..42640d0b3 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -59,6 +59,7 @@ set(PYBIND11_TEST_FILES
   test_stl.cpp
   test_stl_binders.cpp
   test_tagbased_polymorphic.cpp
+  test_union.cpp
   test_virtual_functions.cpp
 )
 
diff --git a/tests/test_union.cpp b/tests/test_union.cpp
new file mode 100644
index 000000000..7b98ea216
--- /dev/null
+++ b/tests/test_union.cpp
@@ -0,0 +1,22 @@
+/*
+    tests/test_class.cpp -- test py::class_ definitions and basic functionality
+
+    Copyright (c) 2019 Roland Dreier <roland.dreier@gmail.com>
+
+    All rights reserved. Use of this source code is governed by a
+    BSD-style license that can be found in the LICENSE file.
+*/
+
+#include "pybind11_tests.h"
+
+TEST_SUBMODULE(union_, m) {
+    union TestUnion {
+        int value_int;
+        unsigned value_uint;
+    };
+
+    py::class_<TestUnion>(m, "TestUnion")
+        .def(py::init<>())
+        .def_readonly("as_int", &TestUnion::value_int)
+        .def_readwrite("as_uint", &TestUnion::value_uint);
+}
diff --git a/tests/test_union.py b/tests/test_union.py
new file mode 100644
index 000000000..e1866e701
--- /dev/null
+++ b/tests/test_union.py
@@ -0,0 +1,8 @@
+from pybind11_tests import union_ as m
+
+
+def test_union():
+    instance = m.TestUnion()
+
+    instance.as_uint = 10
+    assert instance.as_int == 10