Do not warn about shadowed fields if they are not redefined in a child class

pydantic/_internal/_fields.py

@@ -177,21 +177,26 @@ def collect_model_fields(  # noqa: C901
             )
 
         # when building a generic model with `MyModel[int]`, the generic_origin check makes sure we don't get
-        # "... shadows an attribute" errors
+        # "... shadows an attribute" warnings
         generic_origin = getattr(cls, '__pydantic_generic_metadata__', {}).get('origin')
         for base in bases:
             dataclass_fields = {
                 field.name for field in (dataclasses.fields(base) if dataclasses.is_dataclass(base) else ())
             }
             if hasattr(base, ann_name):
                 if base is generic_origin:
-                    # Don't error when "shadowing" of attributes in parametrized generics
+                    # Don't warn when "shadowing" of attributes in parametrized generics
                     continue
 
                 if ann_name in dataclass_fields:
-                    # Don't error when inheriting stdlib dataclasses whose fields are "shadowed" by defaults being set
+                    # Don't warn when inheriting stdlib dataclasses whose fields are "shadowed" by defaults being set
                     # on the class instance.
                     continue
+
+                if ann_name not in annotations:
+                    # Don't warn when a field exists in a parent class but has not been defined in the current class
+                    continue
+
                 warnings.warn(
                     f'Field name "{ann_name}" in "{cls.__qualname__}" shadows an attribute in parent '
                     f'"{base.__qualname__}"',

tests/test_main.py

@@ -1,6 +1,7 @@
 import json
 import platform
 import re
+import warnings
 from collections import defaultdict
 from copy import deepcopy
 from dataclasses import dataclass
@@ -3151,6 +3152,33 @@ class Three(One):
     assert getattr(Three, 'foo', None) == ' edited! edited!'
 
 
+def test_shadow_attribute_warn_for_redefined_fields() -> None:
+    """https://github.com/pydantic/pydantic/issues/9107"""
+
+    # A simple class which defines a field
+    class Parent:
+        foo: bool = False
+
+    # When inheriting from the parent class, as long as the field is not defined at all, there should be no warning
+    # about shadowed fields.
+    with warnings.catch_warnings():
+        warnings.simplefilter('error')
+
+        class ChildWithoutRedefinedField(BaseModel, Parent):
+            pass
+
+    # But when inheriting from the parent class and a parent field is redefined, a warning should be raised about
+    # shadowed fields irrespective of whether it is defined with a type that is still compatible or narrower, or
+    # with a different default that is still compatible with the type definition.
+    with pytest.warns(
+        UserWarning,
+        match=r'"foo" in ".*ChildWithRedefinedField" shadows an attribute in parent ".*Parent"',
+    ):
+
+        class ChildWithRedefinedField(BaseModel, Parent):
+            foo: bool = True
+
+
 def test_eval_type_backport():
     class Model(BaseModel):
         foo: 'list[int | str]'