mypy types set of `StrEnum` as set of `str`

[injust]

Bug Report

mypy types a set of StrEnum as set[str].

To Reproduce

from enum import Enum, StrEnum

class Tag(Enum):
    pass

class StrTag(StrEnum):
    pass

reveal_type(set(Tag))  # note: Revealed type is "builtins.set[__main__.Tag]"
reveal_type(set(StrTag))  # note: Revealed type is "builtins.set[builtins.str]"

https://mypy-play.net/?mypy=latest&python=3.13&gist=237de7c0610830acd5a5104d18541e3e

Expected Behavior

mypy should type set(StrTag) as set[StrTag], not set[str].

Actual Behavior

main.py:10: note: Revealed type is "builtins.set[builtins.str]"

Your Environment

• Mypy version used: 1.14.0
• Python version used: 3.13

[sobolevn]

This happens because str is also Iterable and str is closer in the MRO than Enum. Adding def __iter__(self) -> Self: ... to StrEnum solves the issue. But, I am not sure that this is the correct / full fix.

[injust]

I think python/cpython#128659 is the root cause.

[sobolevn]

I don't think so: mypy does not use runtime analysis. So, these are two different cases.

[hauntsaninja]

This was fixed by #18587