Sentinel and enum type merge issues with generic

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@edgarrmondragon 已經在處理了。

開始於 2026年11月2日。

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難度
4/5
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Issue 類型
缺陷
描述清晰度
描述清楚
活躍度
活躍
技術堆疊
python
領域
compilers

研究方向

Start by running the supplied examples through mypy and compare direct assignment with assigning the result to a temporary. Check both the sentinel and enum cases against the working int example. Done means the direct assignments are accepted and the revealed types match the expected results.

由索引模型根據 Issue 內容生成。

描述

bug 
from typing_extensions import sentinel

Undefined = sentinel("Undefined")

def none_if_undefined[_T](value: _T | Undefined) -> _T | None:
    return None if value is Undefined else value

def func() -> None:
    device_id: str | Undefined | None = Undefined
    device_id = none_if_undefined(device_id)  # <-- error
    reveal_type(device_id)

test.py:10: error: Argument 1 to "none_if_undefined" has incompatible type "str | Undefined | None"; expected "str | Undefined"  [arg-type]
test.py:11: note: Revealed type is "str | Undefined | None"

However, assigning the result from none_if_undefined to a temporary variable first works as expected.

def func() -> None:
    device_id: str | Undefined | None = Undefined
    x = none_if_undefined(device_id)
    reveal_type(x)
    device_id = x
    reveal_type(device_id)

test.py:11: note: Revealed type is "str | None"
test.py:13: note: Revealed type is "str | None"

--
A similar issues exists for enums

from enum import Enum
from typing import Literal

class E(Enum):
    SPECIAL = 1

def none_if_special[_T](value: _T | Literal[E.SPECIAL]) -> _T | None:
    return None if value is E.SPECIAL else value

def func() -> None:
    device_id: str | Literal[E.SPECIAL] | None = E.SPECIAL
    device_id = none_if_special(device_id)  # <-- error
    reveal_type(device_id)

test.py:12: error: Argument 1 to "none_if_special" has incompatible type "str | Literal[E.SPECIAL] | None"; expected "str | Literal[E.SPECIAL]"  [arg-type]
test.py:13: note: Revealed type is "str | Literal[test7.E.SPECIAL] | None"

--
For completeness the related example with just int - that works fine -- no errors.

def none_if_int[_T](value: _T | int) -> _T | None:
    return None if isinstance(value, int) else value

def func() -> None:
    device_id: str | int | None = 42
    device_id = none_if_int(device_id)
    reveal_type(device_id)
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從這裡開始

  1. 先讀完整個 Issue，再讀專案的貢獻指南。
  2. 在 Issue 下留言說明你要接手 —— 這能避免兩個人做同樣的事。
  3. Fork 儲存庫，在一個分支上完成修改。
  4. 送出 Pull Request，並在描述裡引用這個 Issue 編號。

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