[red-knot] Add callable subtyping for callable instances and bound methods (#17105)

## Summary

Trying to improve #17005
Partially fixes #16953

## Test Plan

Update is_subtype_of.md

---------

Co-authored-by: Carl Meyer <carl@astral.sh>

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

@@ -1099,5 +1099,54 @@ static_assert(is_subtype_of(TypeOf[C.foo], object))
 static_assert(not is_subtype_of(object, TypeOf[C.foo]))
 ```
 
+### Classes with `__call__`
+
+```py
+from typing import Callable
+from knot_extensions import TypeOf, is_subtype_of, static_assert, is_assignable_to
+
+class A:
+    def __call__(self, a: int) -> int:
+        return a
+
+a = A()
+
+static_assert(is_subtype_of(A, Callable[[int], int]))
+static_assert(not is_subtype_of(A, Callable[[], int]))
+static_assert(not is_subtype_of(Callable[[int], int], A))
+
+def f(fn: Callable[[int], int]) -> None: ...
+
+f(a)
+```
+
+### Bound methods
+
+```py
+from typing import Callable
+from knot_extensions import TypeOf, static_assert, is_subtype_of
+
+class A:
+    def f(self, a: int) -> int:
+        return a
+
+    @classmethod
+    def g(cls, a: int) -> int:
+        return a
+
+a = A()
+
+static_assert(is_subtype_of(TypeOf[a.f], Callable[[int], int]))
+static_assert(is_subtype_of(TypeOf[a.g], Callable[[int], int]))
+static_assert(is_subtype_of(TypeOf[A.g], Callable[[int], int]))
+
+static_assert(not is_subtype_of(TypeOf[a.f], Callable[[float], int]))
+static_assert(not is_subtype_of(TypeOf[A.g], Callable[[], int]))
+
+# TODO: This assertion should be true
+# error: [static-assert-error] "Static assertion error: argument evaluates to `False`"
+static_assert(is_subtype_of(TypeOf[A.f], Callable[[A, int], int]))
+```
+
 [special case for float and complex]: https://typing.readthedocs.io/en/latest/spec/special-types.html#special-cases-for-float-and-complex
 [typing documentation]: https://typing.readthedocs.io/en/latest/spec/concepts.html#subtype-supertype-and-type-equivalence

crates/red_knot_python_semantic/src/types.rs

@@ -727,6 +727,10 @@ impl<'db> Type<'db> {
                     .is_subtype_of(db, target)
             }
 
+            (Type::BoundMethod(self_bound_method), Type::Callable(_)) => self_bound_method
+                .into_callable_type(db)
+                .is_subtype_of(db, target),
+
             // A `FunctionLiteral` type is a single-valued type like the other literals handled above,
             // so it also, for now, just delegates to its instance fallback.
             (Type::FunctionLiteral(_), _) => KnownClass::FunctionType
@@ -833,6 +837,16 @@ impl<'db> Type<'db> {
                 self_instance.is_subtype_of(db, target_instance)
             }
 
+            (Type::Instance(_), Type::Callable(_)) => {
+                let call_symbol = self.member(db, "__call__").symbol;
+                match call_symbol {
+                    Symbol::Type(Type::BoundMethod(call_function), _) => call_function
+                        .into_callable_type(db)
+                        .is_subtype_of(db, target),
+                    _ => false,
+                }
+            }
+
             // Other than the special cases enumerated above,
             // `Instance` types are never subtypes of any other variants
             (Type::Instance(_), _) => false,
@@ -4414,6 +4428,15 @@ pub struct BoundMethodType<'db> {
     self_instance: Type<'db>,
 }
 
+impl<'db> BoundMethodType<'db> {
+    pub(crate) fn into_callable_type(self, db: &'db dyn Db) -> Type<'db> {
+        Type::Callable(CallableType::new(
+            db,
+            self.function(db).signature(db).bind_self(),
+        ))
+    }
+}
+
 /// This type represents the set of all callable objects with a certain signature.
 /// It can be written in type expressions using `typing.Callable`.
 /// `lambda` expressions are inferred directly as `CallableType`s; all function-literal types

crates/red_knot_python_semantic/src/types/signatures.rs

@@ -265,6 +265,13 @@ impl<'db> Signature<'db> {
     pub(crate) fn parameters(&self) -> &Parameters<'db> {
         &self.parameters
     }
+
+    pub(crate) fn bind_self(&self) -> Self {
+        Self {
+            parameters: Parameters::new(self.parameters().iter().skip(1).cloned()),
+            return_ty: self.return_ty,
+        }
+    }
 }
 
 #[derive(Clone, Debug, PartialEq, Eq, Hash, salsa::Update)]