Make xfailing test for root model extra stop xfailing

docs/errors/usage_errors.md

@@ -900,7 +900,7 @@ except PydanticUserError as exc_info:
     assert exc_info.code == 'invalid_annotated_type'
 ```
 
-## `config` is unused with TypeAdapter {#type-adapter-config-unused}
+## `config` is unused with `TypeAdapter` {#type-adapter-config-unused}
 
 You will get this error if you try to pass `config` to `TypeAdapter` when the type is a type that
 has its own config that cannot be overridden (currently this is only `BaseModel`, `TypedDict` and `dataclass`):
@@ -939,4 +939,23 @@ class MyTypedDict(TypedDict):
 TypeAdapter(MyTypedDict)  # ok
 ```
 
+## Cannot specify `model_config['extra']` with `RootModel` {#root-model-extra}
+
+Because `RootModel` is not capable of storing or even accepting extra fields during initialization, we raise an error
+if you try to specify a value for the config setting `'extra'` when creating a subclass of `RootModel`:
+
+```py
+from pydantic import PydanticUserError, RootModel
+
+try:
+
+    class MyRootModel(RootModel):
+        model_config = {'extra': 'allow'}
+        root: int
+
+except PydanticUserError as exc_info:
+    assert exc_info.code == 'root-model-extra'
+```
+
+
 {% endraw %}

pydantic/errors.py

@@ -56,6 +56,7 @@
     'multiple-field-serializers',
     'invalid_annotated_type',
     'type-adapter-config-unused',
+    'root-model-extra',
 ]
 
 

pydantic/root_model.py

@@ -7,6 +7,7 @@
 
 from pydantic_core import PydanticUndefined
 
+from . import PydanticUserError
 from ._internal import _repr
 from .main import BaseModel, _object_setattr
 
@@ -43,6 +44,14 @@ class RootModel(BaseModel, typing.Generic[RootModelRootType]):
 
     root: RootModelRootType
 
+    def __init_subclass__(cls, **kwargs):
+        extra = cls.model_config.get('extra')
+        if extra is not None:
+            raise PydanticUserError(
+                "`RootModel` does not support setting `model_config['extra']`", code='root-model-extra'
+            )
+        super().__init_subclass__(**kwargs)
+
     def __init__(__pydantic_self__, root: RootModelRootType = PydanticUndefined, **data) -> None:  # type: ignore
         __tracebackhide__ = True
         if data:

tests/test_root_model.py

@@ -344,7 +344,6 @@ class B(BaseModel):
     assert B(root=42) != R(42)
 
 
-@pytest.mark.xfail(reason='TODO: raise error for `extra` with `RootModel`')
 @pytest.mark.parametrize('extra_value', ['ignore', 'allow', 'forbid'])
 def test_extra_error(extra_value):
     with pytest.raises(PydanticUserError, match='extra'):