List[Foo] + List[Bar] now returns List[Foo | Bar] (#8293)

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

test_cases/stdlib/builtins/test_list.py

@@ -0,0 +1,19 @@
+from typing import List, Union
+from typing_extensions import assert_type
+
+
+# list.__add__ example from #8292
+class Foo:
+    def asd(self) -> int:
+        return 1
+
+
+class Bar:
+    def asd(self) -> int:
+        return 2
+
+
+combined = [Foo()] + [Bar()]
+assert_type(combined, List[Union[Foo, Bar]])
+for item in combined:
+    assert_type(item.asd(), int)