gh-143164: Fix incorrect error message for ctypes bitfield overflow (GH-143165)

Signed-off-by: Yongtao Huang <yongtaoh2022@gmail.com>

Author: hyongtao-code

Lib/test/test_ctypes/test_struct_fields.py

@@ -130,6 +130,21 @@ class S(Structure):
             self.check_struct(S)
             self.assertEqual(S.largeField.bit_size, size * 8)
 
+    def test_bitfield_overflow_error_message(self):
+        with self.assertRaisesRegex(
+            ValueError,
+            r"bit field 'x' overflows its type \(2 \+ 7 > 8\)",
+        ):
+            CField(
+                name="x",
+                type=c_byte,
+                byte_size=1,
+                byte_offset=0,
+                index=0,
+                _internal_use=True,
+                bit_size=7,
+                bit_offset=2,
+            )
 
     # __set__ and __get__ should raise a TypeError in case their self
     # argument is not a ctype instance.

Misc/NEWS.d/next/Library/2025-12-25-08-58-55.gh-issue-142164.XrFztf.rst

@@ -0,0 +1 @@
+Fix the ctypes bitfield overflow error message to report the correct offset and size calculation.

Modules/_ctypes/cfield.c

@@ -160,8 +160,8 @@ PyCField_new_impl(PyTypeObject *type, PyObject *name, PyObject *proto,
         if ((bitfield_size + bit_offset) > byte_size * 8) {
             PyErr_Format(
                 PyExc_ValueError,
-                "bit field %R overflows its type (%zd + %zd >= %zd)",
-                name, bit_offset, byte_size*8);
+                "bit field %R overflows its type (%zd + %zd > %zd)",
+                name, bit_offset, bitfield_size, byte_size * 8);
             goto error;
         }
     }