# Cycles

## Function signature

Deferred annotations can result in cycles in resolving a function signature:

```py
from __future__ import annotations

# error: [invalid-type-form]
def f(x: f):
    pass

reveal_type(f)  # revealed: def f(x: Unknown) -> Unknown
```

## Unpacking

See: <https://github.com/astral-sh/ty/issues/364>

```py
class Point:
    def __init__(self, x: int = 0, y: int = 0) -> None:
        self.x = x
        self.y = y

    def replace_with(self, other: "Point") -> None:
        self.x, self.y = other.x, other.y

p = Point()
reveal_type(p.x)  # revealed: Unknown | int
reveal_type(p.y)  # revealed: Unknown | int
```

## Self-referential bare type alias

```toml
[environment]
python-version = "3.12"  # typing.TypeAliasType
```

```py
from typing import Union, TypeAliasType, Sequence, Mapping

A = list["A | None"]

def f(x: A):
    # TODO: should be `list[A | None]`?
    reveal_type(x)  # revealed: list[Divergent]
    # TODO: should be `A | None`?
    reveal_type(x[0])  # revealed: Divergent

JSONPrimitive = Union[str, int, float, bool, None]
JSONValue = TypeAliasType("JSONValue", 'Union[JSONPrimitive, Sequence["JSONValue"], Mapping[str, "JSONValue"]]')

def _(x: JSONValue):
    reveal_type(x)  # revealed: Sequence[JSONValue] | int | float | None | Mapping[str, JSONValue]
```

## Self-referential legacy type variables

```py
from typing import Generic, TypeVar

B = TypeVar("B", bound="Base")

class Base(Generic[B]):
    pass
```

## Parameter default values

This is a regression test for <https://github.com/astral-sh/ty/issues/1402>. When a parameter has a
default value that references the callable itself, we currently prevent infinite recursion by simply
falling back to `Unknown` for the type of the default value, which does not have any practical
impact except for the displayed type. We could also consider inferring `Divergent` when we encounter
too many layers of nesting (instead of just one), but that would require a type traversal which
could have performance implications. So for now, we mainly make sure not to panic or stack overflow
for these seeminly rare cases.

### Functions

```py
class C:
    def f(self: "C"):
        def inner_a(positional=self.a):
            return
        self.a = inner_a
        # revealed: def inner_a(positional=...) -> Unknown
        reveal_type(inner_a)

        def inner_b(*, kw_only=self.b):
            return
        self.b = inner_b
        # revealed: def inner_b(*, kw_only=...) -> Unknown
        reveal_type(inner_b)

        def inner_c(positional_only=self.c, /):
            return
        self.c = inner_c
        # revealed: def inner_c(positional_only=..., /) -> Unknown
        reveal_type(inner_c)

        def inner_d(*, kw_only=self.d):
            return
        self.d = inner_d
        # revealed: def inner_d(*, kw_only=...) -> Unknown
        reveal_type(inner_d)
```

We do, however, still check assignability of the default value to the parameter type:

```py
class D:
    def f(self: "D"):
        # error: [invalid-parameter-default] "Default value of type `Unknown | (def inner_a(a: int = ...) -> Unknown)` is not assignable to annotated parameter type `int`"
        def inner_a(a: int = self.a): ...
        self.a = inner_a
```

### Lambdas

```py
class C:
    def f(self: "C"):
        self.a = lambda positional=self.a: positional
        self.b = lambda *, kw_only=self.b: kw_only
        self.c = lambda positional_only=self.c, /: positional_only
        self.d = lambda *, kw_only=self.d: kw_only

        # revealed: (positional=...) -> Unknown | ((positional=...) -> Divergent) | ((positional=...) -> Unknown) | ((positional=...) -> Divergent)
        reveal_type(self.a)

        # revealed: (*, kw_only=...) -> Unknown | ((*, kw_only=...) -> Divergent) | ((*, kw_only=...) -> Unknown) | ((*, kw_only=...) -> Divergent)
        reveal_type(self.b)

        # revealed: (positional_only=..., /) -> Unknown | ((positional_only=..., /) -> Divergent) | ((positional_only=..., /) -> Unknown) | ((positional_only=..., /) -> Divergent)
        reveal_type(self.c)

        # revealed: (*, kw_only=...) -> Unknown | ((*, kw_only=...) -> Divergent) | ((*, kw_only=...) -> Unknown) | ((*, kw_only=...) -> Divergent)
        reveal_type(self.d)
```

## Self-referential implicit attributes

```py
class Cyclic:
    def __init__(self, data: str | dict):
        self.data = data

    def update(self):
        if isinstance(self.data, str):
            self.data = {"url": self.data}

# revealed: Unknown | str | dict[Unknown, Unknown] | dict[str, str]
reveal_type(Cyclic("").data)
```

## Decorator defined on a base class with constrained typevars, accessed from a subclass with decorated generic parameters

This example was minimized from
[a real issue in `robotframework`](https://github.com/astral-sh/ty/issues/2637#issuecomment-3807037935).
It created
[a complicated cycle with multiple cycle heads](https://gist.github.com/oconnor663/c996ed2cc97d172dd4b9a8d8207dc7ac),
which also involved
[a tricky Salsa behavior that comes up when a query oscillates between being a cycle head and not being one](https://gist.github.com/oconnor663/c2a7662e3d88048b691754da957121d1).

`entry.py`:

```py
from derived import Derived

Derived.decorate
# revealed: bound method <class 'Derived'>.decorate[T](item_class: type[T]) -> type[T]
reveal_type(Derived.decorate)
```

`derived.py`:

```py
from ty_extensions import reveal_mro
import bases

class Derived(bases.GenericBase["Foo", "Bar"]): ...

@Derived.decorate
class Foo(bases.Foo): ...

# revealed: <class 'Foo'>
reveal_type(Foo)
# revealed: (<class 'derived.Foo'>, <class 'bases.Foo'>, <class 'object'>)
reveal_mro(Foo)

@Derived.decorate
class Bar(bases.Bar): ...

# revealed: <class 'Bar'>
reveal_type(Bar)
# revealed: (<class 'derived.Bar'>, <class 'bases.Bar'>, <class 'object'>)
reveal_mro(Bar)
```

`bases.py`:

```py
from typing import Generic, TypeVar, Type
from ty_extensions import reveal_mro

T = TypeVar("T")
B1 = TypeVar("B1", bound="Foo")
B2 = TypeVar("B2", bound="Bar")

class GenericBase(Generic[B1, B2]):
    @classmethod
    def decorate(cls, item_class: Type[T]) -> Type[T]:
        return item_class

# revealed: <class 'GenericBase'>
reveal_type(GenericBase)
# revealed: (<class 'GenericBase[Unknown, Unknown]'>, typing.Generic, <class 'object'>)
reveal_mro(GenericBase)
# revealed: (<class 'GenericBase[Foo, Bar]'>, typing.Generic, <class 'object'>)
reveal_mro(GenericBase["Foo", "Bar"])

class Foo: ...
class Bar: ...
```
