[ty] Treat ClassVar-qualified Callables as bound-method descriptors

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

@@ -204,3 +204,37 @@ class Calculator:
 
 reveal_type(Calculator().square_then_round(3.14))  # revealed: Unknown | int
 ```
+
+## Use case: Treating dunder methods as bound-method descriptors
+
+pytorch defines a `__pow__` dunder attribute on [`TensorBase`] in a similar way to the following
+example. We generally treat dunder attributes as bound-method descriptors since they all take a
+`self` argument. This allows us to type-check the following code correctly:
+
+```py
+from typing import Callable
+
+def pow_impl(tensor: Tensor, exponent: int) -> Tensor:
+    raise NotImplementedError
+
+class Tensor:
+    __pow__: Callable[[Tensor, int], Tensor] = pow_impl
+
+Tensor() ** 2
+```
+
+The following example is also taken from a real world project. Here, the `__lt__` dunder attribute
+is not declared. The attribute type is therefore inferred as `Unknown | Callable[…]`, but we still
+treat it as a bound-method descriptor:
+
+```py
+def make_comparison_operator(name: str) -> Callable[[Matrix, Matrix], bool]:
+    raise NotImplementedError
+
+class Matrix:
+    __lt__ = make_comparison_operator("lt")
+
+Matrix() < Matrix()
+```
+
+[`tensorbase`]: https://github.com/pytorch/pytorch/blob/f3913ea641d871f04fa2b6588a77f63efeeb9f10/torch/_tensor.py#L1084-L1092

crates/ty_python_semantic/resources/mdtest/type_properties/is_assignable_to.md

@@ -1181,18 +1181,17 @@ static_assert(not is_assignable_to(EggsLegacy, Callable[..., Any]))  # error: [s
 An instance type is assignable to a compatible callable type if the instance type's class has a
 callable `__call__` attribute.
 
-TODO: for the moment, we don't consider the callable type as a bound-method descriptor, but this may
-change for better compatibility with mypy/pyright.
-
 ```py
+from __future__ import annotations
+
 from typing import Callable
 from ty_extensions import static_assert, is_assignable_to
 
-def call_impl(a: int) -> str:
+def call_impl(a: A, x: int) -> str:
     return ""
 
 class A:
-    __call__: Callable[[int], str] = call_impl
+    __call__: Callable[[A, int], str] = call_impl
 
 static_assert(is_assignable_to(A, Callable[[int], str]))
 static_assert(not is_assignable_to(A, Callable[[int], int]))

crates/ty_python_semantic/resources/mdtest/type_properties/is_subtype_of.md

@@ -1635,18 +1635,17 @@ f(a)
 An instance type can be a subtype of a compatible callable type if the instance type's class has a
 callable `__call__` attribute.
 
-TODO: for the moment, we don't consider the callable type as a bound-method descriptor, but this may
-change for better compatibility with mypy/pyright.
-
 ```py
+from __future__ import annotations
+
 from typing import Callable
 from ty_extensions import static_assert, is_subtype_of
 
-def call_impl(a: int) -> str:
+def call_impl(a: A, x: int) -> str:
     return ""
 
 class A:
-    __call__: Callable[[int], str] = call_impl
+    __call__: Callable[[A, int], str] = call_impl
 
 static_assert(is_subtype_of(A, Callable[[int], str]))
 static_assert(not is_subtype_of(A, Callable[[int], int]))

crates/ty_python_semantic/src/types.rs

@@ -11116,6 +11116,23 @@ impl<'db> IntersectionType<'db> {
         }
     }
 
+    /// Map a type transformation over all positive elements of the intersection. Leave the
+    /// negative elements unchanged.
+    pub(crate) fn map_positive(
+        self,
+        db: &'db dyn Db,
+        mut transform_fn: impl FnMut(&Type<'db>) -> Type<'db>,
+    ) -> Type<'db> {
+        let mut builder = IntersectionBuilder::new(db);
+        for ty in self.positive(db) {
+            builder = builder.add_positive(transform_fn(ty));
+        }
+        for ty in self.negative(db) {
+            builder = builder.add_negative(*ty);
+        }
+        builder.build()
+    }
+
     pub(crate) fn map_with_boundness(
         self,
         db: &'db dyn Db,

crates/ty_python_semantic/src/types/class.rs

@@ -2014,7 +2014,29 @@ impl<'db> ClassLiteral<'db> {
         name: &str,
         policy: MemberLookupPolicy,
     ) -> PlaceAndQualifiers<'db> {
-        self.class_member_inner(db, None, name, policy)
+        fn into_function_like_callable<'d>(db: &'d dyn Db, ty: Type<'d>) -> Type<'d> {
+            match ty {
+                Type::Callable(callable_ty) => {
+                    Type::Callable(CallableType::new(db, callable_ty.signatures(db), true))
+                }
+                Type::Union(union) => {
+                    union.map(db, |element| into_function_like_callable(db, *element))
+                }
+                Type::Intersection(intersection) => intersection
+                    .map_positive(db, |element| into_function_like_callable(db, *element)),
+                _ => ty,
+            }
+        }
+
+        let mut member = self.class_member_inner(db, None, name, policy);
+
+        // We generally treat dunder attributes with `Callable` types as function-like callables.
+        // See `callables_as_descriptors.md` for more details.
+        if (name.starts_with("__") && name.ends_with("__")) || member.is_class_var() {
+            member = member.map_type(|ty| into_function_like_callable(db, ty));
+        }
+
+        member
     }
 
     fn class_member_inner(