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[ty] avoid unions of generic aliases of the same class in fixpoint #21909

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Merged
carljm merged 2 commits into from
Dec 11, 2025
Merged

[ty] avoid unions of generic aliases of the same class in fixpoint #21909

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[ty] avoid unions of class literals / generic aliases in fixpoint
carljm Dec 10, 2025
ec113ef
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carljm Dec 11, 2025
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5 changes: 0 additions & 5 deletions crates/ty_python_semantic/resources/mdtest/generics/legacy/classes.md
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Original file line number Diff line number Diff line change
Expand Up @@ -860,9 +860,6 @@ reveal_type(Sub) # revealed: <class 'Sub'>
U = TypeVar("U")

class Base2(Generic[T, U]): ...

# TODO: no error
# error: [unsupported-base] "Unsupported class base with type `<class 'Base2[Sub2, U@Sub2]'> | <class 'Base2[Sub2[Unknown], U@Sub2]'>`"
class Sub2(Base2["Sub2", U]): ...
```

Expand All @@ -888,8 +885,6 @@ from typing_extensions import Generic, TypeVar

T = TypeVar("T")

# TODO: no error "Unsupported class base with type `<class 'list[Derived[T@Derived]]'> | <class 'list[@Todo]'>`"
# error: [unsupported-base]
class Derived(list[Derived[T]], Generic[T]): ...
```

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15 changes: 13 additions & 2 deletions crates/ty_python_semantic/src/types.rs
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Expand Up @@ -912,8 +912,19 @@ impl<'db> Type<'db> {
previous: Self,
cycle: &salsa::Cycle,
) -> Self {
UnionType::from_elements_cycle_recovery(db, [self, previous])
.recursive_type_normalized(db, cycle)
// Avoid unioning two generic aliases of the same class together; this union will never
// simplify and is likely to cause downstream problems. This introduces the theoretical
// possibility of cycle oscillation involving such types (because we are not strictly
// widening the type on each iteration), but so far we have not seen an example of that.
match (previous, self) {
(Type::GenericAlias(prev_alias), Type::GenericAlias(curr_alias))
if prev_alias.origin(db) == curr_alias.origin(db) =>
{
self
}
_ => UnionType::from_elements_cycle_recovery(db, [self, previous]),
}
.recursive_type_normalized(db, cycle)
}

fn is_none(&self, db: &'db dyn Db) -> bool {
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