Add support for typing.Self (fix #5992)

pydantic/_internal/_generate_schema.py

@@ -859,8 +859,6 @@ def match_type(self, obj: Any) -> core_schema.CoreSchema:  # noqa: C901
 
         if _typing_extra.is_dataclass(obj):
             return self._dataclass_schema(obj, None)
-        # if _typing_extra.is_self(obj):
-        #     pass
         res = self._get_prepare_pydantic_annotations_for_known_type(obj, ())
         if res is not None:
             source_type, annotations = res

tests/test_types_self.py

@@ -37,4 +37,39 @@ class SelfRef(BaseModel):
         ValidationError,
         match=r'ref\.ref\s+Input should be a valid dictionary or instance of SelfRef \[type=model_type,',
     ):
-        SelfRef(data=1, ref={'data': 2, 'ref': 3}).model_dump() == {'data': 1, 'ref': {'data': 2, 'ref': None}}
+        SelfRef(data=1, ref={'data': 2, 'ref': 3}).model_dump()
+
+
+def test_recursive_model_with_subclass(Self):
+    """
+    Self refs should be valid in covariant direction
+    """
+
+    class SelfRef(BaseModel):
+        data: int
+        ref: Self | None = None
+
+    class SubSelfRef(SelfRef):
+        pass
+
+    assert SubSelfRef(data=1, ref=SubSelfRef(data=2)).model_dump() == {'data': 1, 'ref': {'data': 2, 'ref': None}}
+    assert SelfRef(data=1, ref=SubSelfRef(data=2)).model_dump() == {'data': 1, 'ref': {'data': 2, 'ref': None}}
+
+
+def test_recursive_model_with_subclass_invalid(Self):
+    """
+    Self refs are invalid in contravariant direction
+    """
+
+    class SelfRef(BaseModel):
+        data: int
+        ref: Self | None = None
+
+    class SubSelfRef(SelfRef):
+        pass
+
+    with pytest.raises(
+        ValidationError,
+        match=r'ref\s+Input should be a valid dictionary or instance of SubSelfRef \[type=model_type,',
+    ):
+        SubSelfRef(data=1, ref=SelfRef(data=2)).model_dump()