Fix unsupported types bug with plain validator (#8710)

Co-authored-by: David Montague <35119617+dmontagu@users.noreply.github.com>

pydantic/functional_validators.py

@@ -153,8 +153,19 @@ class Model(BaseModel):
     func: core_schema.NoInfoValidatorFunction | core_schema.WithInfoValidatorFunction
 
     def __get_pydantic_core_schema__(self, source_type: Any, handler: _GetCoreSchemaHandler) -> core_schema.CoreSchema:
-        schema = handler(source_type)
-        serialization = core_schema.wrap_serializer_function_ser_schema(function=lambda v, h: h(v), schema=schema)
+        # Note that for some valid uses of PlainValidator, it is not possible to generate a core schema for the
+        # source_type, so calling `handler(source_type)` will error, which prevents us from generating a proper
+        # serialization schema. To work around this for use cases that will not involve serialization, we simply
+        # catch any PydanticSchemaGenerationError that may be raised while attempting to build the serialization schema
+        # and abort any attempts to handle special serialization.
+        from pydantic import PydanticSchemaGenerationError
+
+        try:
+            schema = handler(source_type)
+            serialization = core_schema.wrap_serializer_function_ser_schema(function=lambda v, h: h(v), schema=schema)
+        except PydanticSchemaGenerationError:
+            serialization = None
+
         info_arg = _inspect_validator(self.func, 'plain')
         if info_arg:
             func = cast(core_schema.WithInfoValidatorFunction, self.func)

tests/test_validators.py

@@ -2823,3 +2823,19 @@ class Blah(BaseModel):
     data = blah.model_dump()
     assert isinstance(data['foo'], ser_type)
     assert isinstance(data['bar'], ser_type)
+
+
+def test_plain_validator_with_unsupported_type() -> None:
+    class UnsupportedClass:
+        pass
+
+    PreviouslySupportedType = Annotated[
+        UnsupportedClass,
+        PlainValidator(lambda _: UnsupportedClass()),
+    ]
+
+    type_adapter = TypeAdapter(PreviouslySupportedType)
+
+    model = type_adapter.validate_python('abcdefg')
+    assert isinstance(model, UnsupportedClass)
+    assert isinstance(type_adapter.dump_python(model), UnsupportedClass)