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[ty] Fix 'too many cycle iterations' for unions of literals (#20137)

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## Summary

Decrease the maximum number of literals in a union before we collapse to
the supertype. The better fix for this will be
https://github.com/astral-sh/ty/issues/957, but it is very tempting to
solve this for now by simply decreasing the limit by one, to get below
the salsa limit of 200.

closes https://github.com/astral-sh/ty/issues/660

## Test Plan

Added a regression test that would previously lead to a "too many cycle
iterations" panic.
This commit is contained in:
David Peter 2025-08-28 16:46:37 +02:00 • committed by GitHub
parent b3c4005289
commit 1842cfe333
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14
crates/ty_python_semantic/resources/mdtest/attributes.md
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@ -2309,6 +2309,20 @@ reveal_type(Toggle().x) # revealed: Literal[True]
reveal_type(Toggle().y) # revealed: Unknown | Literal[True]
```
Make sure that the growing union of literals `Literal[0, 1, 2, ...]` collapses to `int` during
fixpoint iteration. This is a regression test for <https://github.com/astral-sh/ty/issues/660>.
```py
class Counter:
def __init__(self: "Counter"):
self.count = 0
def increment(self: "Counter"):
self.count = self.count + 1
reveal_type(Counter().count) # revealed: Unknown | int
```
### Builtin types attributes
This test can probably be removed eventually, but we currently include it because we do not yet

5
crates/ty_python_semantic/src/types/builder.rs
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@ -202,7 +202,10 @@ enum ReduceResult<'db> {
// TODO increase this once we extend `UnionElement` throughout all union/intersection
// representations, so that we can make large unions of literals fast in all operations.
const MAX_UNION_LITERALS: usize = 200;
//
// For now (until we solve https://github.com/astral-sh/ty/issues/957), keep this number
// below 200, which is the salsa fixpoint iteration limit.
const MAX_UNION_LITERALS: usize = 199;
pub(crate) struct UnionBuilder<'db> {
elements: Vec<UnionElement<'db>>,