[ty] Support __init_subclass__ (#20190)

## Summary

`__init_subclass__` is implicitly a classmethod.

closes https://github.com/astral-sh/ty/issues/1106

## Test Plan

Regression test

crates/ty_ide/src/completion.rs

@@ -1411,7 +1411,7 @@ quux.<CURSOR>
         __getstate__ :: bound method Quux.__getstate__() -> object
         __hash__ :: bound method Quux.__hash__() -> int
         __init__ :: bound method Quux.__init__() -> Unknown
-        __init_subclass__ :: bound method Quux.__init_subclass__() -> None
+        __init_subclass__ :: bound method type[Quux].__init_subclass__() -> None
         __module__ :: str
         __ne__ :: bound method Quux.__ne__(value: object, /) -> bool
         __new__ :: bound method Quux.__new__() -> Quux
@@ -1456,7 +1456,7 @@ quux.b<CURSOR>
         __getstate__ :: bound method Quux.__getstate__() -> object
         __hash__ :: bound method Quux.__hash__() -> int
         __init__ :: bound method Quux.__init__() -> Unknown
-        __init_subclass__ :: bound method Quux.__init_subclass__() -> None
+        __init_subclass__ :: bound method type[Quux].__init_subclass__() -> None
         __module__ :: str
         __ne__ :: bound method Quux.__ne__(value: object, /) -> bool
         __new__ :: bound method Quux.__new__() -> Quux
@@ -1506,7 +1506,7 @@ C.<CURSOR>
         __getstate__ :: def __getstate__(self) -> object
         __hash__ :: def __hash__(self) -> int
         __init__ :: def __init__(self) -> None
-        __init_subclass__ :: def __init_subclass__(cls) -> None
+        __init_subclass__ :: bound method <class 'C'>.__init_subclass__() -> None
         __instancecheck__ :: bound method <class 'C'>.__instancecheck__(instance: Any, /) -> bool
         __itemsize__ :: int
         __module__ :: str
@@ -1575,7 +1575,7 @@ Meta.<CURSOR>
                 __getstate__ :: def __getstate__(self) -> object
                 __hash__ :: def __hash__(self) -> int
                 __init__ :: Overload[(self, o: object, /) -> None, (self, name: str, bases: tuple[type, ...], dict: dict[str, Any], /, **kwds: Any) -> None]
-                __init_subclass__ :: def __init_subclass__(cls) -> None
+                __init_subclass__ :: bound method <class 'Meta'>.__init_subclass__() -> None
                 __instancecheck__ :: def __instancecheck__(self, instance: Any, /) -> bool
                 __itemsize__ :: int
                 __module__ :: str
@@ -1682,7 +1682,7 @@ Quux.<CURSOR>
         __getstate__ :: def __getstate__(self) -> object
         __hash__ :: def __hash__(self) -> int
         __init__ :: def __init__(self) -> Unknown
-        __init_subclass__ :: def __init_subclass__(cls) -> None
+        __init_subclass__ :: bound method <class 'Quux'>.__init_subclass__() -> None
         __instancecheck__ :: bound method <class 'Quux'>.__instancecheck__(instance: Any, /) -> bool
         __itemsize__ :: int
         __module__ :: str
@@ -1756,7 +1756,7 @@ Answer.<CURSOR>
                 __getstate__ :: def __getstate__(self) -> object
                 __hash__ :: def __hash__(self) -> int
                 __init__ :: def __init__(self) -> None
-                __init_subclass__ :: def __init_subclass__(cls) -> None
+                __init_subclass__ :: bound method <class 'Answer'>.__init_subclass__() -> None
                 __instancecheck__ :: bound method <class 'Answer'>.__instancecheck__(instance: Any, /) -> bool
                 __itemsize__ :: int
                 __iter__ :: bound method <class 'Answer'>.__iter__[_EnumMemberT]() -> Iterator[_EnumMemberT@__iter__]

crates/ty_python_semantic/resources/mdtest/call/methods.md

@@ -462,6 +462,22 @@ reveal_type(C.f2(1))  # revealed: str
 reveal_type(C().f2(1))  # revealed: str
 ```
 
+### `__init_subclass__`
+
+The [`__init_subclass__`] method is implicitly a classmethod:
+
+```py
+class Base:
+    def __init_subclass__(cls, **kwargs):
+        super().__init_subclass__(**kwargs)
+        cls.custom_attribute: int = 0
+
+class Derived(Base):
+    pass
+
+reveal_type(Derived.custom_attribute)  # revealed: int
+```
+
 ## `@staticmethod`
 
 ### Basic
@@ -571,3 +587,4 @@ reveal_type(C().f2(1))  # revealed: str
 ```
 
 [functions and methods]: https://docs.python.org/3/howto/descriptor.html#functions-and-methods
+[`__init_subclass__`]: https://docs.python.org/3/reference/datamodel.html#object.__init_subclass__

crates/ty_python_semantic/src/types.rs

@@ -46,7 +46,7 @@ use crate::types::diagnostic::{INVALID_AWAIT, INVALID_TYPE_FORM, UNSUPPORTED_BOO
 pub use crate::types::display::DisplaySettings;
 use crate::types::enums::{enum_metadata, is_single_member_enum};
 use crate::types::function::{
-    DataclassTransformerParams, FunctionDecorators, FunctionSpans, FunctionType, KnownFunction,
+    DataclassTransformerParams, FunctionSpans, FunctionType, KnownFunction,
 };
 use crate::types::generics::{
     GenericContext, PartialSpecialization, Specialization, bind_typevar, walk_generic_context,
@@ -8819,10 +8819,7 @@ impl<'db> BoundMethodType<'db> {
     /// a `@classmethod`, then it should be an instance of that bound-instance type.
     pub(crate) fn typing_self_type(self, db: &'db dyn Db) -> Type<'db> {
         let mut self_instance = self.self_instance(db);
-        if self
+        if self.function(db).is_classmethod(db) {
-            .function(db)
-            .has_known_decorator(db, FunctionDecorators::CLASSMETHOD)
-        {
             self_instance = self_instance.to_instance(db).unwrap_or_else(Type::unknown);
         }
         self_instance

crates/ty_python_semantic/src/types/call/bind.rs

@@ -272,7 +272,7 @@ impl<'db> Bindings<'db> {
             for (overload_index, overload) in binding.matching_overloads_mut() {
                 match binding_type {
                     Type::MethodWrapper(MethodWrapperKind::FunctionTypeDunderGet(function)) => {
-                        if function.has_known_decorator(db, FunctionDecorators::CLASSMETHOD) {
+                        if function.is_classmethod(db) {
                             match overload.parameter_types() {
                                 [_, Some(owner)] => {
                                     overload.set_return_type(Type::BoundMethod(
@@ -308,7 +308,7 @@ impl<'db> Bindings<'db> {
                         if let [Some(function_ty @ Type::FunctionLiteral(function)), ..] =
                             overload.parameter_types()
                         {
-                            if function.has_known_decorator(db, FunctionDecorators::CLASSMETHOD) {
+                            if function.is_classmethod(db) {
                                 match overload.parameter_types() {
                                     [_, _, Some(owner)] => {
                                         overload.set_return_type(Type::BoundMethod(

crates/ty_python_semantic/src/types/class.rs

@@ -1258,7 +1258,7 @@ pub(super) enum MethodDecorator {
 impl MethodDecorator {
     fn try_from_fn_type(db: &dyn Db, fn_type: FunctionType) -> Result<Self, ()> {
         match (
-            fn_type.has_known_decorator(db, FunctionDecorators::CLASSMETHOD),
+            fn_type.is_classmethod(db),
             fn_type.has_known_decorator(db, FunctionDecorators::STATICMETHOD),
         ) {
             (true, true) => Err(()), // A method can't be static and class method at the same time.

crates/ty_python_semantic/src/types/function.rs

@@ -721,6 +721,13 @@ impl<'db> FunctionType<'db> {
         self.literal(db).has_known_decorator(db, decorator)
     }
 
+    /// Returns true if this method is decorated with `@classmethod`, or if it is implicitly a
+    /// classmethod.
+    pub(crate) fn is_classmethod(self, db: &'db dyn Db) -> bool {
+        self.has_known_decorator(db, FunctionDecorators::CLASSMETHOD)
+            || self.name(db) == "__init_subclass__"
+    }
+
     /// If the implementation of this function is deprecated, returns the `@warnings.deprecated`.
     ///
     /// Checking if an overload is deprecated requires deeper call analysis.