Fix handling of optionals in mypy plugin

pydantic/mypy.py

@@ -4,6 +4,7 @@
 
 import sys
 from configparser import ConfigParser
+from contextlib import ExitStack
 from typing import Any, Callable, Iterator
 
 from mypy.errorcodes import ErrorCode
@@ -366,12 +367,10 @@ def expand_type(self, current_info: TypeInfo, api: SemanticAnalyzerPluginInterfa
             # however this plugin is called very late, so all types should be fully ready.
             # Also, it is tricky to avoid eager expansion of Self types here (e.g. because
             # we serialize attributes).
-            expanded_type = expand_type(self.type, {self.info.self_type.id: fill_typevars(current_info)})
-            if isinstance(self.type, UnionType) and not isinstance(expanded_type, UnionType):
-                if not api.final_iteration:
-                    raise _DeferAnalysis()
-            return expanded_type
-        return self.type
+            with ExitStack() as stack:
+                if MYPY_VERSION_TUPLE >= (1, 5):
+                    stack.enter_context(state.strict_optional_set(api.options.strict_optional))
+                return expand_type(self.type, {self.info.self_type.id: fill_typevars(current_info)})
 
     def to_var(self, current_info: TypeInfo, api: SemanticAnalyzerPluginInterface, use_alias: bool) -> Var:
         """Based on mypy.plugins.dataclasses.DataclassAttribute.to_var."""