Raise error when Pydantic dataclasses are used on models

pydantic/dataclasses.py

@@ -10,8 +10,10 @@
 
 from ._internal import _config, _decorators, _typing_extra
 from ._internal import _dataclasses as _pydantic_dataclasses
+from ._internal._utils import is_model_class
 from ._migration import getattr_migration
 from .config import ConfigDict
+from .errors import PydanticUserError
 from .fields import Field, FieldInfo
 
 if TYPE_CHECKING:
@@ -190,6 +192,12 @@ def create_dataclass(cls: type[Any]) -> type[PydanticDataclass]:
  Returns:
  A Pydantic dataclass.
  """
+ if is_model_class(cls):
+ raise PydanticUserError(
+ f'Cannot create a Pydantic dataclass from {cls.__name__} as it is already a Pydantic model',
+ code=None,
+ )
+
  original_cls = cls
 
  config_dict = config

tests/test_dataclasses.py

@@ -40,6 +40,16 @@
 from pydantic.json_schema import model_json_schema
 
 
+def test_cannot_create_dataclass_from_basemodel_subclass():
+ msg = 'Cannot create a Pydantic dataclass from SubModel as it is already a Pydantic model'
+
+ with pytest.raises(PydanticUserError, match=msg):
+
+ @pydantic.dataclasses.dataclass
+ class SubModel(BaseModel):
+ pass
+
+
 def test_simple():
  @pydantic.dataclasses.dataclass
  class MyDataclass: