This updates our function specialization inference to infer type
mappings from parameters that are generic aliases, e.g.:
```py
def f[T](x: list[T]) -> T: ...
reveal_type(f(["a", "b"])) # revealed: str
```
Though note that we're still inferring the type of list literals as
`list[Unknown]`, so for now we actually need something like the
following in our tests:
```py
def _(x: list[str]):
reveal_type(f(x)) # revealed: str
```
We were not inducting into instance types and subclass-of types when
looking for legacy typevars, nor when apply specializations.
This addresses
https://github.com/astral-sh/ruff/pull/17832#discussion_r2081502056
```py
from __future__ import annotations
from typing import TypeVar, Any, reveal_type
S = TypeVar("S")
class Foo[T]:
def method(self, other: Foo[S]) -> Foo[T | S]: ... # type: ignore[invalid-return-type]
def f(x: Foo[Any], y: Foo[Any]):
reveal_type(x.method(y)) # revealed: `Foo[Any | S]`, but should be `Foo[Any]`
```
We were not detecting that `S` made `method` generic, since we were not
finding it when searching the function signature for legacy typevars.
Function literals have an optional specialization, which is applied to
the parameter/return type annotations lazily when the function's
signature is requested. We were previously only applying this
specialization to the final overload of an overloaded function.
This manifested most visibly for `list.__add__`, which has an overloaded
definition in the typeshed:
b398b83631/crates/ty_vendored/vendor/typeshed/stdlib/builtins.pyi (L1069-L1072)
Closes https://github.com/astral-sh/ty/issues/314
It's possible for a typevar to list another typevar as its default
value:
```py
class C[T, U = T]: ...
```
When specializing this class, if a type isn't provided for `U`, we would
previously use the default as-is, leaving an unspecialized `T` typevar
in the specialization. Instead, we want to use what `T` is mapped to as
the type of `U`.
```py
reveal_type(C()) # revealed: C[Unknown, Unknown]
reveal_type(C[int]()) # revealed: C[int, int]
reveal_type(C[int, str]()) # revealed: C[int, str]
```
This is especially important for the `slice` built-in type.
#17897 added variance handling for legacy typevars — but they were only
being considered when checking generic aliases of the same class:
```py
class A: ...
class B(A): ...
class C[T]: ...
static_assert(is_subtype_of(C[B], C[A]))
```
and not for generic subclasses:
```py
class D[U](C[U]): ...
static_assert(is_subtype_of(D[B], C[A]))
```
Now we check those too!
Closes https://github.com/astral-sh/ty/issues/101
We now track the variance of each typevar, and obey the `covariant` and
`contravariant` parameters to the legacy `TypeVar` constructor. We still
don't yet infer variance for PEP-695 typevars or for the
`infer_variance` legacy constructor parameter.
---------
Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>
Co-authored-by: Carl Meyer <carl@astral.sh>
@AlexWaygood discovered that even though we've been propagating
specializations to _parent_ base classes correctly, we haven't been
passing them on to _grandparent_ base classes:
https://github.com/astral-sh/ruff/pull/17832#issuecomment-2854360969
```py
class Bar[T]:
x: T
class Baz[T](Bar[T]): ...
class Spam[T](Baz[T]): ...
reveal_type(Spam[int]().x) # revealed: `T`, but should be `int`
```
This PR updates the MRO machinery to apply the current specialization
when starting to iterate the MRO of each base class.
If a typevar is declared as having a default, we shouldn't require a
type to be specified for that typevar when explicitly specializing a
generic class:
```py
class WithDefault[T, U = int]: ...
reveal_type(WithDefault[str]()) # revealed: WithDefault[str, int]
```
---------
Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>
Fixes
https://github.com/astral-sh/ruff/pull/17832#issuecomment-2851224968. We
had a comment that we did not need to apply specializations to generic
aliases, or to the bound `self` of a bound method, because they were
already specialized. But they might be specialized with a type variable,
which _does_ need to be specialized, in the case of a "multi-step"
specialization, such as:
```py
class LinkedList[T]: ...
class C[U]:
def method(self) -> LinkedList[U]:
return LinkedList[U]()
```
---------
Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>
