make mypy happy

fastapi/_compat/v2.py

@@ -1,6 +1,6 @@
 import re
 import warnings
-from collections.abc import Sequence
+from collections.abc import Callable, Sequence
 from copy import copy
 from dataclasses import dataclass, is_dataclass
 from enum import Enum
@@ -39,11 +39,16 @@ from pydantic_core.core_schema import (
 RequiredParam = PydanticUndefined
 Undefined = PydanticUndefined
 
-# eval_type_lenient has been deprecated since Pydantic v2.10.0b1 (PR #10530)
-evaluate_forwardref = getattr(_pydantic_typing_extra, "try_eval_type", None)
-if evaluate_forwardref is None:
-    evaluate_forwardref = _pydantic_typing_extra.eval_type_lenient
-assert evaluate_forwardref is not None
+
+def define_forwardref() -> Callable[..., Any]:
+    # eval_type_lenient has been deprecated since Pydantic v2.10.0b1 (PR #10530)
+    eval_type = getattr(_pydantic_typing_extra, "try_eval_type", None)
+    if eval_type is not None:
+        return cast(Callable[..., Any], eval_type)
+    return _pydantic_typing_extra.eval_type_lenient
+
+
+evaluate_forwardref = define_forwardref()
 
 
 class GenerateJsonSchema(_GenerateJsonSchema):