Fix iteration over union (when self type is involved) (#17976)

Fixes #17945

Author: hauntsaninja

mypy/checkexpr.py

@@ -3805,6 +3805,7 @@ def check_method_call_by_name(
             is_operator=True,
             msg=self.msg,
             original_type=original_type,
+            self_type=base_type,
             chk=self.chk,
             in_literal_context=self.is_literal_context(),
         )

test-data/unit/check-selftype.test

@@ -2160,3 +2160,19 @@ class MyProtocol(Protocol):
 
 def test() -> None: ...
 value: MyProtocol = test
+
+[case testSelfTypeUnionIter]
+from typing import Self, Iterator, Generic, TypeVar, Union
+
+T = TypeVar("T")
+
+class range(Generic[T]):
+    def __iter__(self) -> Self: ...
+    def __next__(self) -> T: ...
+
+class count:
+    def __iter__(self) -> Iterator[int]: ...
+
+def foo(x: Union[range[int], count]) -> None:
+    for item in x:
+        reveal_type(item)  # N: Revealed type is "builtins.int"