gh-145166: Fix crash in tzinfo.fromutc() when subclass __new__ returns non-datetime

Author: HELLPUSYY666

Lib/_pydatetime.py

@@ -1348,6 +1348,11 @@ def fromutc(self, dt):
         delta = dtoff - dtdst
         if delta:
             dt += delta
+            if not isinstance(dt, datetime):
+                raise TypeError(
+                    f"datetime arithmetic on a subclass returned non-datetime "
+                    f"(type {type(dt).__name__})"
+                )
             dtdst = dt.dst()
             if dtdst is None:
                 raise ValueError("fromutc(): dt.dst gave inconsistent "

Lib/test/datetimetester.py

@@ -5133,6 +5133,27 @@ def test_pickling(self):
             self.assertEqual(derived.tzname(), 'cookie')
         self.assertEqual(orig.__reduce__(), orig.__reduce_ex__(2))
 
+    def test_fromutc_subclass_new_returns_non_datetime(self):
+        call_count = 0
+
+        class EvilDatetime(self.theclass):
+            def __new__(cls, *args, **kwargs):
+                nonlocal call_count
+                call_count += 1
+                if call_count > 1:
+                    return bytearray(b'\x00' * 200)
+                return super().__new__(cls, *args, **kwargs)
+
+        class SimpleTZ(tzinfo):
+            def utcoffset(self, dt): return timedelta(hours=2)
+            def dst(self, dt):       return timedelta(hours=1)
+            def tzname(self, dt):    return "Test"
+
+        tz = SimpleTZ()
+        dt = EvilDatetime(2000, 1, 1, 12, 0, 0, tzinfo=tz)
+        with self.assertRaises(TypeError):
+            tz.fromutc(dt)
+
     def test_compat_unpickle(self):
         tests = [
             b'cdatetime\ndatetime\n'

Misc/NEWS.d/next/Library/2026-02-24-19-14-43.gh-issue-145166.bG_Rp2.rst

@@ -0,0 +1,14 @@
+Fix crash in :meth:`datetime.tzinfo.fromutc` when a
+:class:`~datetime.datetime` subclass ``__new__`` returns a non-datetime
+object. A :exc:`TypeError` is now raised instead.
+
+# # Uncomment one of these "section:" lines to specify which section # this
+entry should go in in Misc/NEWS.d. # #.. section: Security #.. section: Core
+and Builtins #.. section: Library #.. section: Documentation #.. section:
+Tests #.. section: Build #.. section: Windows #.. section: macOS #..
+section: IDLE #.. section: Tools/Demos #.. section: C API
+
+# Write your Misc/NEWS.d entry below.  It should be a simple ReST paragraph.
+# Don't start with "- Issue #<n>: " or "- gh-issue-<n>: " or that sort of
+stuff.
+###########################################################################

Modules/_datetimemodule.c

@@ -4169,6 +4169,14 @@ tzinfo_fromutc(PyObject *self, PyObject *dt)
     if (result == NULL)
         goto Fail;
 
+    if (!PyDateTime_Check(result)) {
+        PyErr_Format(PyExc_TypeError,
+                    "datetime arithmetic on a subclass returned non-datetime "
+                    "(type %.200s)",
+                    Py_TYPE(result)->tp_name);
+        Py_CLEAR(result);
+        goto Fail;
+    }
     Py_DECREF(dst);
     dst = call_dst(GET_DT_TZINFO(dt), result);
     if (dst == NULL)
@@ -4180,6 +4188,15 @@ tzinfo_fromutc(PyObject *self, PyObject *dt)
                                                  (PyDateTime_Delta *)dst, 1));
         if (result == NULL)
             goto Fail;
+
+        if (!PyDateTime_Check(result)) {
+            PyErr_Format(PyExc_TypeError,
+                        "datetime arithmetic on a subclass returned "
+                        "non-datetime (type %.200s)",
+                        Py_TYPE(result)->tp_name);
+            Py_CLEAR(result);
+            goto Fail;
+        }
     }
     Py_DECREF(delta);
     Py_DECREF(dst);