[Fix error] Syntax Error Bug encountered with pyupgrade and multi-line Union

[tlambert03]

Just encountered the request for bug report when running ruff:

error: Fix introduced a syntax error. Reverting all changes.

This indicates a bug in Ruff. If you could open an issue at:

I was able to narrow it down to a pretty small reproducer.

$ tree .
.
├── main.py
└── pyproject.toml

# pyproject.toml
[project]
name = "ruffbug"
version = "0.1.0"
requires-python = ">=3.10"

[tool.ruff]
line-length = 88
target-version = "py310"
fix = true

[tool.ruff.lint]
select = ["UP"]

# main.py
from __future__ import annotations

from typing import TYPE_CHECKING

if TYPE_CHECKING:
    from typing import Literal, TypeAlias, Union

    LongLiterals: TypeAlias = Union[
        Literal["LongLiteralNumberOne"]
        | Literal["LongLiteralNumberTwo"]
        | Literal["LongLiteralNumberThree"]
    ]

$ uv run ruff check main.py --fix

error: Fix introduced a syntax error. Reverting all changes.

This indicates a bug in Ruff. If you could open an issue at:

    https://github.com/astral-sh/ruff/issues/new?title=%5BFix%20error%5D

...quoting the contents of `main.py`, the rule codes UP007, along with the `pyproject.toml` settings and executed command, we'd be very appreciative!

UP007 Use `X | Y` for type annotations
  --> main.py:8:31
   |
 6 |       from typing import Literal, TypeAlias, Union
 7 |
 8 |       LongLiterals: TypeAlias = Union[
   |  _______________________________^
 9 | |         Literal["LongLiteralNumberOne"]
10 | |         | Literal["LongLiteralNumberTwo"]
11 | |         | Literal["LongLiteralNumberThree"]
12 | |     ]
   | |_____^
   |
help: Convert to `X | Y`

Found 1 error.
[*] 1 fixable with the `--fix` option.

importantly: If I get rid of that third Literal["LongLiteralNumberThree"] (such that formatting would cause the union to fit on a single line), then a round of ruff format and ruff check work fine. It seems to only have difficulty with the multi-line union

[ntBre]

Thanks for the report! I think this is a minimal reproducer too:

from typing import Union

x: Union[
    int
    | str
]

https://play.ruff.rs/d4f8d2b2-0943-496f-92d3-4c3a674dacc2

As you noted, it's important for the union to span multiple lines.

[tlambert03]

zoom zoom. thanks both!