Fix mishandling of typing.Self in attrs generated inits

mypy/plugins/attrs.py

@@ -56,6 +56,7 @@
     deserialize_and_fixup_type,
 )
 from mypy.server.trigger import make_wildcard_trigger
+from mypy.state import state
 from mypy.typeops import get_type_vars, make_simplified_union, map_type_from_supertype
 from mypy.types import (
     AnyType,
@@ -123,10 +124,19 @@ def __init__(
         self.context = context
         self.init_type = init_type
 
+    def expand_type(self, typ: Type | None, ctx: mypy.plugin.ClassDefContext) -> Type | None:
+        if typ is not None and self.info.self_type is not None:
+            # In general, it is not safe to call `expand_type()` during semantic analyzis,
+            # 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).
+            return expand_type(typ, {self.info.self_type.id: fill_typevars(ctx.cls.info)})
+        return typ
+
     def argument(self, ctx: mypy.plugin.ClassDefContext) -> Argument:
         """Return this attribute as an argument to __init__."""
         assert self.init
-        init_type: Type | None = None
+        init_type: Type | None
         if self.converter:
             if self.converter.init_type:
                 init_type = self.converter.init_type
@@ -171,6 +181,9 @@ def argument(self, ctx: mypy.plugin.ClassDefContext) -> Argument:
         else:
             arg_kind = ARG_OPT if self.has_default else ARG_POS
 
+        with state.strict_optional_set(ctx.api.options.strict_optional):
+            init_type = self.expand_type(init_type, ctx)
+
         # Attrs removes leading underscores when creating the __init__ arguments.
         return Argument(Var(self.name.lstrip("_"), init_type), init_type, None, arg_kind)
 

mypy/plugins/dataclasses.py

@@ -2,7 +2,7 @@
 
 from __future__ import annotations
 
-from typing import Iterator, Optional
+from typing import Iterator
 from typing_extensions import Final
 
 from mypy import errorcodes, message_registry
@@ -120,7 +120,7 @@ def to_argument(self, current_info: TypeInfo) -> Argument:
             kind=arg_kind,
         )
 
-    def expand_type(self, current_info: TypeInfo) -> Optional[Type]:
+    def expand_type(self, current_info: TypeInfo) -> Type | None:
         if self.type is not None and self.info.self_type is not None:
             # In general, it is not safe to call `expand_type()` during semantic analyzis,
             # however this plugin is called very late, so all types should be fully ready.

test-data/unit/check-plugin-attrs.test

@@ -1978,6 +1978,31 @@ D(1, "").b = "2" # E: Cannot assign to final attribute "b"
 
 [builtins fixtures/property.pyi]
 
+[case testSelfInClassInit]
+# flags: --strict-optional
+from attrs import define
+from typing import Union, Self
+
+@define
+class C:
+    a: Union[Self, None] = None
+
+reveal_type(C) # N: Revealed type is "def (a: Union[__main__.C, None] =) -> __main__.C"
+C(C())
+C(None)
+reveal_type(C(C()).a)  # N: Revealed type is "Union[__main__.C, None]"
+
+@define
+class S(C):
+    ...
+
+reveal_type(S) # N: Revealed type is "def (a: Union[__main__.S, None] =) -> __main__.S"
+S(S())
+S(None)
+reveal_type(S(S()).a)  # N: Revealed type is "Union[__main__.S, None]"
+
+[builtins fixtures/property.pyi]
+
 [case testEvolve]
 import attr