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[ty] Simplify unions of enum literals and subtypes thereof (#20324)

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## Summary

When adding an enum literal `E = Literal[Color.RED]` to a union which
already contained a subtype of that enum literal(!), we were previously
not simplifying the union correctly. My assumption is that our property
tests didn't catch that earlier, because the only possible non-trivial
subytpe of an enum literal that I can think of is `Any & E`. And in
order for that to be detected by the property tests, it would have to
randomly generate `Any & E | E` and then also compare that with `E` on
the other side (in an equivalence test, or the subtyping-antisymmetry
test).

closes https://github.com/astral-sh/ty/issues/1155

## Test Plan

* Added a regression test.
* I also ran the property tests for a while, but probably not for two
months worth of daily CI runs.
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David Peter 2025-09-10 15:54:06 +02:00 • committed by GitHub
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10
crates/ty_python_semantic/resources/mdtest/union_types.md
View file

@ -118,7 +118,8 @@ def _(
```py
from enum import Enum
from typing import Literal
from typing import Literal, Any
from ty_extensions import Intersection
class Color(Enum):
RED = "red"
@ -139,6 +140,13 @@ def _(
reveal_type(u4) # revealed: Literal[Color.RED, Color.GREEN]
reveal_type(u5) # revealed: Color
reveal_type(u6) # revealed: Color
def _(
u1: Intersection[Literal[Color.RED], Any] | Literal[Color.RED],
u2: Literal[Color.RED] | Intersection[Literal[Color.RED], Any],
):
reveal_type(u1) # revealed: Literal[Color.RED]
reveal_type(u2) # revealed: Literal[Color.RED]
```
## Do not erase `Unknown`