AnyStr does not accept str|bytes in single-argument functions of StdLib #20340

MyPy cannot unfold AnyStr with a single-argument StdLib function (re.escape). The typeshed defines the function as:

def escape(pattern: AnyStr) -> AnyStr: ...

AnyStr = TypeVar("AnyStr", str, bytes)

There is nothing wrong with the definition, all is good in typeshed. My suspicion is that it is something wrong with type inference.

I see that AnyStr is deprecated, but in Python 3.18, so roughly in 4 years — a long time to wait. Some clarity on how this is expected to be used, would be nice.

My code supports both strings and bytes at runtime, they both behave similarly, never mix-up together. Now, I have to duplicate the code fragments first for if isinstance(val, str):…, then else:… for bytes — while there is no difference in the code pieces.

To Reproduce

import re
val: str | bytes = 'hello'
re.escape(val)

Expected Behavior

It accepts either str ot bytes, and returns the respective type back.

Actual Behavior

_repro_pattern.py:3: error: Value of type variable "AnyStr" of "escape" cannot be "str | bytes"  [type-var]

Your Environment

MyPy 1.19.0.
Python 3.14.0 (the same in others).

mypy --strict _repro_pattern.py