Fix reference source typing

Author: koxudaxi

src/datamodel_code_generator/reference.py

@@ -43,7 +43,6 @@
 
     import inflect
 
-    from datamodel_code_generator.model.base import DataModel, DataModelFieldBase
     from datamodel_code_generator.types import DataType
 
 
@@ -54,7 +53,7 @@ def _is_data_type(value: object) -> TypeIs[DataType]:
     return isinstance(value, DataType_)
 
 
-def _is_data_model(value: object) -> TypeIs[DataModel]:
+def _is_data_model(value: object) -> bool:
     """Check if value is a DataModel instance."""
     from datamodel_code_generator.model.base import DataModel as DataModel_  # noqa: PLC0415
 
@@ -79,25 +78,25 @@ def reference(self) -> Reference | None:
 
 if TYPE_CHECKING:
 
-    class _ReferenceSource(Protocol):
+    class _ReferenceSource(ReferenceChild, Protocol):
         """Protocol for objects that can be assigned to Reference.source."""
 
-        fields: list[DataModelFieldBase]
+        fields: Sequence[Any]
 
         @property
         def is_alias(self) -> bool:
             """Return whether this source renders as an alias."""
-            ...
+            raise NotImplementedError
 
         @property
         def module_name(self) -> str | None:
             """Return this source module name."""
-            ...
+            raise NotImplementedError
 
         @property
         def nullable(self) -> bool:
             """Return whether this source is nullable."""
-            ...
+            raise NotImplementedError
 
 else:
     _ReferenceSource = ReferenceChild
@@ -184,7 +183,7 @@ def replace_children_references(self, new_reference: Reference) -> None:
             if _is_data_type(child):
                 child.replace_reference(new_reference)
 
-    def iter_data_model_children(self) -> Iterator[DataModel]:
+    def iter_data_model_children(self) -> Iterator[Any]:
         """Yield all DataModel children."""
         for child in self.children:
             if _is_data_model(child):