Add invalid schema placeholder

python/pydantic_core/core_schema.py

@@ -435,6 +435,26 @@ def model_ser_schema(cls: Type[Any], schema: CoreSchema) -> ModelSerSchema:
 ]
 
 
+class InvalidSchema(TypedDict, total=False):
+    type: Required[Literal['invalid']]
+    ref: str
+    metadata: Dict[str, Any]
+
+
+def invalid_schema(ref: str | None = None, metadata: Dict[str, Any] | None = None) -> InvalidSchema:
+    """
+    Returns an invalid schema, used to indicate that a schema is invalid.
+
+        Returns a schema that matches any value, e.g.:
+
+    Args:
+        ref: optional unique identifier of the schema, used to reference the schema in other places
+        metadata: Any other information you want to include with the schema, not used by pydantic-core
+    """
+
+    return _dict_not_none(type='invalid', ref=ref, metadata=metadata)
+
+
 class ComputedField(TypedDict, total=False):
     type: Required[Literal['computed-field']]
     property_name: Required[str]
@@ -2458,6 +2478,7 @@ class UnionSchema(TypedDict, total=False):
     type: Required[Literal['union']]
     choices: Required[List[Union[CoreSchema, Tuple[CoreSchema, str]]]]
     # default true, whether to automatically collapse unions with one element to the inner validator
+    tagged_union_tag: str
     auto_collapse: bool
     custom_error_type: str
     custom_error_message: str
@@ -3826,6 +3847,7 @@ def definition_reference_schema(
 # union which kills performance not just for pydantic, but even for code using pydantic
 if not MYPY:
     CoreSchema = Union[
+        InvalidSchema,
         AnySchema,
         NoneSchema,
         BoolSchema,
@@ -3882,6 +3904,7 @@ def definition_reference_schema(
 
 # to update this, call `pytest -k test_core_schema_type_literal` and copy the output
 CoreSchemaType = Literal[
+    'invalid',
     'any',
     'none',
     'bool',

tests/test_schema_functions.py

@@ -291,6 +291,7 @@ def args(*args, **kwargs):
     (core_schema.decimal_schema, args(), {'type': 'decimal'}),
     (core_schema.decimal_schema, args(multiple_of=5, gt=1.2), {'type': 'decimal', 'multiple_of': 5, 'gt': 1.2}),
     (core_schema.complex_schema, args(), {'type': 'complex'}),
+    (core_schema.invalid_schema, args(), {'type': 'invalid'}),
 ]
 
 
@@ -299,7 +300,7 @@ def test_schema_functions(function, args_kwargs, expected_schema):
     args, kwargs = args_kwargs
     schema = function(*args, **kwargs)
     assert schema == expected_schema
-    if schema.get('type') in {None, 'definition-ref', 'typed-dict-field', 'model-field'}:
+    if schema.get('type') in {None, 'definition-ref', 'typed-dict-field', 'model-field', 'invalid'}:
         return
 
     v = SchemaValidator(schema)