tuple element types wrongly mixed as union

Bug Report

When a variable conditionally gets one or the other of a set of fixed-length tuples, instead of being inferred as a union of said tuple types, mypy sees an arbibrary-length tuple whose elements are typed with a union of all element types involved. This results false-positive errors being reported.

To Reproduce

https://mypy-play.net/?mypy=latest&python=3.12&gist=4780da0a891d33c46ee7ac0b55a05f24

Expected Behavior

Similar to what pyright reveals, a union of two tuple types with defined length and element types, and no error when assigning :

  /home/user/omaha/omaha/uitest/mypy-test.py:9:17 - information: Type of "item" is "tuple[Literal[1], Literal['a'], list[bool]] | tuple[Literal[1], Literal['a']]"

Actual Behavior

The type is instead inferred to be a tuple of arbitrary length, with all elements allowed to have any of the types seen inside the tuple:

main.py:9: note: Revealed type is "builtins.tuple[builtins.int | builtins.str | builtins.list[builtins.bool], ...]"
main.py:10: error: List item 0 has incompatible type "tuple[int | str | list[bool], ...]"; expected "tuple[int, str] | tuple[int, str, Sequence[bool]]"  [list-item]

Note that when using just item = (1, "a"), removing the conditional part, the tuple is properly identified with fixed types, and the assignment is validated by mypy:

main.py:9: note: Revealed type is "tuple[builtins.int, builtins.str]"

Your Environment

• Mypy version used: 1.18.2
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.12, 3.14.0rc2

まず、リンクされている mypy-play の再現例を mypy 1.18.2 で実行し、表示された型と代入の診断を、期待される pyright の結果と比較します。条件付き代入に関係するタプル推論の挙動を追跡します。固定長タプルの選択肢がユニオンのまま保持され、示された代入で誤検知が発生しなくなれば完了です。