Fix AttributeError when using generic SerializableType subclass

When using PEP 695 syntax to create a generic class, some
private attributes are set up by the runtime in __init_subclass__().
Those attributes are needed when specializing the generic class.

SerializableType did not call super().__init_subclass__() and
thus those attributes were missing, leading to an AttributeError.

mashumaro/types.py

@@ -29,6 +29,7 @@ def __init_subclass__(
         ] = Sentinel.MISSING,
         **kwargs: Any,
     ):
+        super().__init_subclass__(**kwargs)
         if use_annotations is not Sentinel.MISSING:
             cls.__use_annotations__ = use_annotations
 
@@ -61,6 +62,7 @@ def __init_subclass__(
         ] = Sentinel.MISSING,
         **kwargs: Any,
     ):
+        super().__init_subclass__(**kwargs)
         if use_annotations is not Sentinel.MISSING:
             cls.__use_annotations__ = use_annotations
 

tests/test_generics_pep_695.py

@@ -4,6 +4,7 @@
 
 from mashumaro import DataClassDictMixin
 from mashumaro.mixins.json import DataClassJSONMixin
+from mashumaro.types import SerializableType
 from tests.entities import MyGenericDataClass, SerializableTypeGenericList
 
 
@@ -18,6 +19,18 @@ class Bar(Foo):
     pass
 
 
+@dataclass
+class GenericSerializableType[T](SerializableType):
+    value: object
+
+    def _serialize(self):
+        return self.value
+
+    @classmethod
+    def _deserialize(cls, value):
+        return cls(value)
+
+
 def test_one_generic():
     @dataclass
     class A[T]:
@@ -238,3 +251,11 @@ def test_self_referenced_generic_no_max_recursion_error():
     assert Bar.from_dict({"x": 42, "y": {"x": 33, "y": None}}) == obj
     assert obj.to_json() == '{"x": 42, "y": {"x": 33, "y": null}}'
     assert Bar.from_json('{"x": 42, "y": {"x": 33, "y": null}}') == obj
+
+
+# Regression test for https://github.com/Fatal1ty/mashumaro/issues/274.
+def test_generic_serializable_type_getitem():
+    @dataclass
+    class DataClass(DataClassDictMixin):
+        # Simply specializing the type would lead to an AttributeError.
+        x: GenericSerializableType[int]