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[ty] Use 'unknown' specialization for upper bound on Self #20325

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Merged
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Sep 10, 2025
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[ty] Use 'unknown' specialization for upper bound on Self #20325

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19 changes: 18 additions & 1 deletion crates/ty_python_semantic/resources/mdtest/annotations/self.md
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Expand Up @@ -2,7 +2,7 @@

```toml
[environment]
python-version = "3.11"
python-version = "3.13"
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Default values for type parameters have only been introduced recently (https://peps.python.org/pep-0696/)

```

`Self` is treated as if it were a `TypeVar` bound to the class it's being used on.
Expand Down Expand Up @@ -147,6 +147,23 @@ class Shape:
return self
```

## `Self` for classes with a default value for their generic parameter

This is a regression test for <https://github.com/astral-sh/ty/issues/1156>.

```py
from typing import Self

class Container[T = bytes]:
def __init__(self: Self, data: T | None = None) -> None:
self.data = data

reveal_type(Container()) # revealed: Container[bytes]
reveal_type(Container(1)) # revealed: Container[int]
reveal_type(Container("a")) # revealed: Container[str]
reveal_type(Container(b"a")) # revealed: Container[bytes]
```

## Invalid Usage

`Self` cannot be used in the signature of a function or variable.
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4 changes: 1 addition & 3 deletions crates/ty_python_semantic/src/types.rs
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Expand Up @@ -5708,9 +5708,7 @@ impl<'db> Type<'db> {
],
});
};
let instance = Type::ClassLiteral(class).to_instance(db).expect(
"nearest_enclosing_class must return type that can be instantiated",
);
Comment on lines -5711 to -5713
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This would previously use the "default" specialization, and so we would end up with an upper bound of Container[bytes] for the example in the mdtest. With the change here, we instead use Container[Unknown] as the upper bound.

let instance = Type::instance(db, class.unknown_specialization(db));
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I wondered if class.top_materialization() would work here, but it looks like under the hood that uses default_specialization(), so it suffers from the same issue as our current logic on main. (It probably shouldn't use default_specialization()?)

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It probably shouldn't use default_specialization()

I think you're right — it should translate typevars into the bound/constraints in covariant position, and to Never (the implicit lower bound of every typevar) in contravariant position. (With the caveat that we can't really express the constraints of a constrained typevar as a single type — we'd need a "one-of" connective instead of union, and an "instance of this type but not any subtypes".)

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I opened astral-sh/ty#1164

let class_definition = class.definition(db);
let typevar = TypeVarInstance::new(
db,
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