[datetime] Fix datetime.combine inference when tzinfo is omitted (#16811)

Author: carljm

stdlib/@tests/test_cases/check_datetime.py

@@ -58,9 +58,14 @@
 
 # Combine
 
-assert_type(datetime.combine(date(2000, 1, 1), time(12, 0)), datetime[None])
-assert_type(datetime.combine(date(2000, 1, 1), time(12, 0), tzinfo=None), datetime[None])
-assert_type(datetime.combine(date(2000, 1, 1), time(12, 0), tzinfo=UTC), datetime[tzinfo])
+d = date(2000, 1, 1)
+naive_time = time(12, 0)
+aware_time = time(12, 0, tzinfo=UTC)
+
+assert_type(datetime.combine(d, naive_time), Union[datetime[tzinfo], datetime[None]])
+assert_type(datetime.combine(d, aware_time), Union[datetime[tzinfo], datetime[None]])
+assert_type(datetime.combine(d, aware_time, tzinfo=None), datetime[None])
+assert_type(datetime.combine(d, naive_time, tzinfo=UTC), datetime[tzinfo])
 
 # Replace
 

stdlib/datetime.pyi

@@ -379,7 +379,10 @@ class datetime(date, Generic[_TzInfoT]):
 
     @overload
     @classmethod
-    def combine(cls, date: _Date, time: _Time, tzinfo: None = None) -> datetime[None]: ...
+    def combine(cls, date: _Date, time: _Time) -> datetime[_TzInfo] | datetime[None]: ...
+    @overload
+    @classmethod
+    def combine(cls, date: _Date, time: _Time, tzinfo: None) -> datetime[None]: ...
     @overload
     @classmethod
     def combine(cls, date: _Date, time: _Time, tzinfo: _TzInfo) -> datetime[_TzInfo]: ...