[ty] propagate classmethod-ness through decorators returning Callables

crates/ruff_benchmark/benches/ty_walltime.rs

@@ -194,7 +194,7 @@ static SYMPY: Benchmark = Benchmark::new(
         max_dep_date: "2025-06-17",
         python_version: PythonVersion::PY312,
     },
-    13030,
+    13100,
 );
 
 static TANJUN: Benchmark = Benchmark::new(

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

@@ -113,9 +113,7 @@ intention that it shouldn't influence the method's descriptor behavior. For exam
 ```py
 from typing import Callable
 
-# TODO: this could use a generic signature, but we don't support
-# `ParamSpec` and solving of typevars inside `Callable` types yet.
-def memoize(f: Callable[[C1, int], str]) -> Callable[[C1, int], str]:
+def memoize[**P, R](f: Callable[P, R]) -> Callable[P, R]:
     raise NotImplementedError
 
 class C1:
@@ -259,4 +257,37 @@ reveal_type(MyClass().method)  # revealed: (...) -> int
 reveal_type(MyClass().method.__name__)  # revealed: str
 ```
 
+## classmethods passed through Callable-returning decorators
+
+The behavior described above is also applied to classmethods. If a method is decorated with
+`@classmethod`, and also with another decorator which returns a Callable type, we make the
+assumption that the decorator returns a callable which still has the classmethod descriptor
+behavior.
+
+```py
+from typing import Callable
+
+def callable_identity[**P, R](func: Callable[P, R]) -> Callable[P, R]:
+    return func
+
+class C:
+    @callable_identity
+    @classmethod
+    def f1(cls, x: int) -> str:
+        return "a"
+
+    @classmethod
+    @callable_identity
+    def f2(cls, x: int) -> str:
+        return "a"
+
+# error: [too-many-positional-arguments]
+# error: [invalid-argument-type]
+C.f1(C, 1)
+C.f1(1)
+C().f1(1)
+C.f2(1)
+C().f2(1)
+```
+
 [`tensorbase`]: https://github.com/pytorch/pytorch/blob/f3913ea641d871f04fa2b6588a77f63efeeb9f10/torch/_tensor.py#L1084-L1092

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

@@ -444,20 +444,18 @@ When a `@classmethod` is additionally decorated with another decorator, it is st
 class method:
 
 ```py
-from __future__ import annotations
-
 def does_nothing[T](f: T) -> T:
     return f
 
 class C:
     @classmethod
     @does_nothing
-    def f1(cls: type[C], x: int) -> str:
+    def f1(cls, x: int) -> str:
         return "a"
 
     @does_nothing
     @classmethod
-    def f2(cls: type[C], x: int) -> str:
+    def f2(cls, x: int) -> str:
         return "a"
 
 reveal_type(C.f1(1))  # revealed: str

crates/ty_python_semantic/src/types.rs

@@ -4601,15 +4601,18 @@ impl<'db> Type<'db> {
             owner.display(db)
         );
         match self {
-            Type::Callable(callable) if callable.is_function_like(db) => {
-                // For "function-like" callables, model the behavior of `FunctionType.__get__`.
+            Type::Callable(callable)
+                if callable.is_function_like(db) || callable.is_classmethod_like(db) =>
+            {
+                // For "function-like" or "classmethod-like" callables, model the behavior of
+                // `FunctionType.__get__` or `classmethod.__get__`.
                 //
                 // It is a shortcut to model this in `try_call_dunder_get`. If we want to be really precise,
                 // we should instead return a new method-wrapper type variant for the synthesized `__get__`
                 // method of these synthesized functions. The method-wrapper would then be returned from
                 // `find_name_in_mro` when called on function-like `Callable`s. This would allow us to
                 // correctly model the behavior of *explicit* `SomeDataclass.__init__.__get__` calls.
-                return if instance.is_none(db) {
+                return if instance.is_none(db) && callable.is_function_like(db) {
                     Some((self, AttributeKind::NormalOrNonDataDescriptor))
                 } else {
                     Some((
@@ -12243,6 +12246,10 @@ pub enum CallableTypeKind {
     /// instances, i.e. they bind `self`.
     FunctionLike,
 
+    /// A callable type that we believe represents a classmethod (i.e. it will unconditionally bind
+    /// the first argument on `__get__`).
+    ClassMethodLike,
+
     /// Represents the value bound to a `typing.ParamSpec` type variable.
     ParamSpecValue,
 }
@@ -12339,6 +12346,10 @@ impl<'db> CallableType<'db> {
         matches!(self.kind(db), CallableTypeKind::FunctionLike)
     }
 
+    pub(crate) fn is_classmethod_like(self, db: &'db dyn Db) -> bool {
+        matches!(self.kind(db), CallableTypeKind::ClassMethodLike)
+    }
+
     pub(crate) fn bind_self(
         self,
         db: &'db dyn Db,

crates/ty_python_semantic/src/types/function.rs

@@ -1087,7 +1087,12 @@ impl<'db> FunctionType<'db> {
 
     /// Convert the `FunctionType` into a [`CallableType`].
     pub(crate) fn into_callable_type(self, db: &'db dyn Db) -> CallableType<'db> {
-        CallableType::new(db, self.signature(db), CallableTypeKind::FunctionLike)
+        let kind = if self.is_classmethod(db) {
+            CallableTypeKind::ClassMethodLike
+        } else {
+            CallableTypeKind::FunctionLike
+        };
+        CallableType::new(db, self.signature(db), kind)
     }
 
     /// Convert the `FunctionType` into a [`BoundMethodType`].

crates/ty_python_semantic/src/types/infer/builder.rs

@@ -2408,37 +2408,43 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
                 .map(|bindings| bindings.return_type(self.db()))
             {
                 Ok(return_ty) => {
-                    fn into_function_like_callable<'d>(
+                    fn propagate_callable_kind<'d>(
                         db: &'d dyn Db,
                         ty: Type<'d>,
+                        kind: CallableTypeKind,
                     ) -> Option<Type<'d>> {
                         match ty {
                             Type::Callable(callable) => Some(Type::Callable(CallableType::new(
                                 db,
                                 callable.signatures(db),
-                                CallableTypeKind::FunctionLike,
+                                kind,
                             ))),
                             Type::Union(union) => union
-                                .try_map(db, |element| into_function_like_callable(db, *element)),
+                                .try_map(db, |element| propagate_callable_kind(db, *element, kind)),
                             // Intersections are currently not handled here because that would require
                             // the decorator to be explicitly annotated as returning an intersection.
                             _ => None,
                         }
                     }
 
-                    let is_input_function_like = inferred_ty
+                    let propagatable_kind = inferred_ty
                         .try_upcast_to_callable(self.db())
                         .and_then(CallableTypes::exactly_one)
-                        .is_some_and(|callable| callable.is_function_like(self.db()));
+                        .and_then(|callable| match callable.kind(self.db()) {
+                            kind @ (CallableTypeKind::FunctionLike
+                            | CallableTypeKind::ClassMethodLike) => Some(kind),
+                            _ => None,
+                        });
 
-                    if is_input_function_like
-                        && let Some(return_ty_function_like) =
-                            into_function_like_callable(self.db(), return_ty)
+                    if let Some(return_ty_modified) = propagatable_kind
+                        .and_then(|kind| propagate_callable_kind(self.db(), return_ty, kind))
                     {
-                        // When a method on a class is decorated with a function that returns a `Callable`, assume that
-                        // the returned callable is also function-like. See "Decorating a method with a `Callable`-typed
-                        // decorator" in `callables_as_descriptors.md` for the extended explanation.
-                        return_ty_function_like
+                        // When a method on a class is decorated with a function that returns a
+                        // `Callable`, assume that the returned callable is also function-like (or
+                        // classmethod-like). See "Decorating a method with a `Callable`-typed
+                        // decorator" in `callables_as_descriptors.md` for the extended
+                        // explanation.
+                        return_ty_modified
                     } else {
                         return_ty
                     }