[red-knot] Allow subclasses of Any to be assignable to Callable types (#17717)

## Summary

Fixes #17701.

## Test plan

New Markdown test.

---------

Co-authored-by: David Peter <mail@david-peter.de>

crates/red_knot_python_semantic/resources/mdtest/annotations/any.md

@@ -46,30 +46,27 @@ def f():
 y: Any = "not an Any"  # error: [invalid-assignment]
 ```
 
-## Subclass
+## Subclasses of `Any`
 
 The spec allows you to define subclasses of `Any`.
 
-`Subclass` has an unknown superclass, which might be `int`. The assignment to `x` should not be
+`SubclassOfAny` has an unknown superclass, which might be `int`. The assignment to `x` should not be
 allowed, even when the unknown superclass is `int`. The assignment to `y` should be allowed, since
 `Subclass` might have `int` as a superclass, and is therefore assignable to `int`.
 
 ```py
 from typing import Any
 
-class Subclass(Any): ...
+class SubclassOfAny(Any): ...
 
-reveal_type(Subclass.__mro__)  # revealed: tuple[Literal[Subclass], Any, Literal[object]]
+reveal_type(SubclassOfAny.__mro__)  # revealed: tuple[Literal[SubclassOfAny], Any, Literal[object]]
 
-x: Subclass = 1  # error: [invalid-assignment]
-y: int = Subclass()
-
-def _(s: Subclass):
-    reveal_type(s)  # revealed: Subclass
+x: SubclassOfAny = 1  # error: [invalid-assignment]
+y: int = SubclassOfAny()
 ```
 
-`Subclass` should not be assignable to a final class though, because `Subclass` could not possibly
-be a subclass of `FinalClass`:
+`SubclassOfAny` should not be assignable to a final class though, because `SubclassOfAny` could not
+possibly be a subclass of `FinalClass`:
 
 ```py
 from typing import final
@@ -77,11 +74,43 @@ from typing import final
 @final
 class FinalClass: ...
 
-f: FinalClass = Subclass()  # error: [invalid-assignment]
+f: FinalClass = SubclassOfAny()  # error: [invalid-assignment]
+
+@final
+class OtherFinalClass: ...
+
+f: FinalClass | OtherFinalClass = SubclassOfAny()  # error: [invalid-assignment]
 ```
 
-A use case where this comes up is with mocking libraries, where the mock object should be assignable
-to any type:
+A subclass of `Any` can also be assigned to arbitrary `Callable` types:
+
+```py
+from typing import Callable, Any
+
+def takes_callable1(f: Callable):
+    f()
+
+takes_callable1(SubclassOfAny())
+
+def takes_callable2(f: Callable[[int], None]):
+    f(1)
+
+takes_callable2(SubclassOfAny())
+```
+
+A subclass of `Any` cannot be assigned to literal types, since those can not be subclassed:
+
+```py
+from typing import Any, Literal
+
+class MockAny(Any):
+    pass
+
+x: Literal[1] = MockAny()  # error: [invalid-assignment]
+```
+
+A use case where subclasses of `Any` come up is in mocking libraries, where the mock object should
+be assignable to (almost) any type:
 
 ```py
 from unittest.mock import MagicMock

crates/red_knot_python_semantic/src/types.rs

@@ -1477,6 +1477,12 @@ impl<'db> Type<'db> {
                 self_callable.is_assignable_to(db, target_callable)
             }
 
+            (Type::NominalInstance(instance), Type::Callable(_))
+                if instance.class().is_subclass_of_any_or_unknown(db) =>
+            {
+                true
+            }
+
             (Type::NominalInstance(_) | Type::ProtocolInstance(_), Type::Callable(_)) => {
                 let call_symbol = self.member(db, "__call__").symbol;
                 match call_symbol {

crates/red_knot_python_semantic/src/types/class.rs

@@ -231,6 +231,16 @@ impl<'db> ClassType<'db> {
         class_literal.is_final(db)
     }
 
+    /// Is this class a subclass of `Any` or `Unknown`?
+    pub(crate) fn is_subclass_of_any_or_unknown(self, db: &'db dyn Db) -> bool {
+        self.iter_mro(db).any(|base| {
+            matches!(
+                base,
+                ClassBase::Dynamic(DynamicType::Any | DynamicType::Unknown)
+            )
+        })
+    }
+
     /// If `self` and `other` are generic aliases of the same generic class, returns their
     /// corresponding specializations.
     fn compatible_specializations(
@@ -310,13 +320,7 @@ impl<'db> ClassType<'db> {
             }
         }
 
-        if self.iter_mro(db).any(|base| {
-            matches!(
-                base,
-                ClassBase::Dynamic(DynamicType::Any | DynamicType::Unknown)
-            )
-        }) && !other.is_final(db)
-        {
+        if self.is_subclass_of_any_or_unknown(db) && !other.is_final(db) {
             return true;
         }