[ty] Treat Callable dunder members as bound method descriptors (#20860)

## Summary

Dunder methods (at least the ones defined in the standard library)
always take an instance of the class as the first parameter. So it seems
reasonable to generally treat them as bound method descriptors if they
are defined via a `Callable` type.

This removes just a few false positives from the ecosystem, but solves
three user-reported issues:

closes https://github.com/astral-sh/ty/issues/908
closes https://github.com/astral-sh/ty/issues/1143
closes https://github.com/astral-sh/ty/issues/1209

In addition to the change here, I also considered [making `ClassVar`s
bound method descriptors](https://github.com/astral-sh/ruff/pull/20861).
However, there was zero ecosystem impact. So I think we can also close
https://github.com/astral-sh/ty/issues/491 with this PR.

closes https://github.com/astral-sh/ty/issues/491

## Test Plan

Added regression test

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 = member.map_type(|ty| into_function_like_callable(db, ty));
+        }
+
+        member
     }
 
     fn class_member_inner(