From 0b541f64c472976b2fee1ec9919bc7b02a798242 Mon Sep 17 00:00:00 2001
From: Sheidan <37596668+Sh3idan@users.noreply.github.com>
Date: Wed, 17 Jan 2024 19:20:39 +0100
Subject: [PATCH] gh-105102: Fix nested unions in structures when the system
 byteorder is the opposite (GH-105106)

---
 Lib/ctypes/_endian.py                          |  4 ++--
 Lib/test/test_ctypes/test_byteswap.py          | 18 ++++++++++++++++++
 ...3-05-30-18-30-11.gh-issue-105102.SnpK04.rst |  2 ++
 3 files changed, 22 insertions(+), 2 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Library/2023-05-30-18-30-11.gh-issue-105102.SnpK04.rst

diff --git a/Lib/ctypes/_endian.py b/Lib/ctypes/_endian.py
index 3febb3118b8..6382dd22b8a 100644
--- a/Lib/ctypes/_endian.py
+++ b/Lib/ctypes/_endian.py
@@ -15,8 +15,8 @@ def _other_endian(typ):
     # if typ is array
     if isinstance(typ, _array_type):
         return _other_endian(typ._type_) * typ._length_
-    # if typ is structure
-    if issubclass(typ, Structure):
+    # if typ is structure or union
+    if issubclass(typ, (Structure, Union)):
         return typ
     raise TypeError("This type does not support other endian: %s" % typ)
 
diff --git a/Lib/test/test_ctypes/test_byteswap.py b/Lib/test/test_ctypes/test_byteswap.py
index b97b57646ec..78eff0392c4 100644
--- a/Lib/test/test_ctypes/test_byteswap.py
+++ b/Lib/test/test_ctypes/test_byteswap.py
@@ -363,6 +363,24 @@ class Test(unittest.TestCase):
                 self.assertEqual(s.point.x, 1)
                 self.assertEqual(s.point.y, 2)
 
+    def test_build_struct_union_opposite_system_byteorder(self):
+        # gh-105102
+        if sys.byteorder == "little":
+            _Structure = BigEndianStructure
+            _Union = BigEndianUnion
+        else:
+            _Structure = LittleEndianStructure
+            _Union = LittleEndianUnion
+
+        class S1(_Structure):
+            _fields_ = [("a", c_byte), ("b", c_byte)]
+
+        class U1(_Union):
+            _fields_ = [("s1", S1), ("ab", c_short)]
+
+        class S2(_Structure):
+            _fields_ = [("u1", U1), ("c", c_byte)]
+
 
 if __name__ == "__main__":
     unittest.main()
diff --git a/Misc/NEWS.d/next/Library/2023-05-30-18-30-11.gh-issue-105102.SnpK04.rst b/Misc/NEWS.d/next/Library/2023-05-30-18-30-11.gh-issue-105102.SnpK04.rst
new file mode 100644
index 00000000000..7ca21afefa3
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2023-05-30-18-30-11.gh-issue-105102.SnpK04.rst
@@ -0,0 +1,2 @@
+Allow :class:`ctypes.Union` to be nested in :class:`ctypes.Structure` when
+the system endianness is the opposite of the classes.