[red-knot] fix inheritance-cycle detection for generic classes (#17620)

## Summary

The `ClassLiteralType::inheritance_cycle` method is intended to detect
inheritance cycles that would result in cyclic MROs, emit a diagnostic,
and skip actually trying to create the cyclic MRO, falling back to an
"error" MRO instead with just `Unknown` and `object`.

This method didn't work properly for generic classes. It used
`fully_static_explicit_bases`, which filter-maps `explicit_bases` over
`Type::into_class_type`, which returns `None` for an unspecialized
generic class literal. So in a case like `class C[T](C): ...`, because
the explicit base is an unspecialized generic, we just skipped it, and
failed to detect the class as cyclically defined.

Instead, iterate directly over all `explicit_bases`, and explicitly
handle both the specialized (`GenericAlias`) and unspecialized
(`ClassLiteral`) cases, so that we check all bases and correctly detect
cyclic inheritance.

## Test Plan

Added mdtests.

crates/red_knot_python_semantic/resources/mdtest/generics/classes.md

@@ -275,14 +275,16 @@ c: C[int] = C[int]()
 reveal_type(c.method("string"))  # revealed: Literal["string"]
 ```
 
-## Cyclic class definition
+## Cyclic class definitions
+
+### F-bounded quantification
 
 A class can use itself as the type parameter of one of its superclasses. (This is also known as the
 [curiously recurring template pattern][crtp] or [F-bounded quantification][f-bound].)
 
-Here, `Sub` is not a generic class, since it fills its superclass's type parameter (with itself).
+#### In a stub file
 
-`stub.pyi`:
+Here, `Sub` is not a generic class, since it fills its superclass's type parameter (with itself).
 
 ```pyi
 class Base[T]: ...
@@ -291,9 +293,9 @@ class Sub(Base[Sub]): ...
 reveal_type(Sub)  # revealed: Literal[Sub]
 ```
 
-A similar case can work in a non-stub file, if forward references are stringified:
+#### With string forward references
 
-`string_annotation.py`:
+A similar case can work in a non-stub file, if forward references are stringified:
 
 ```py
 class Base[T]: ...
@@ -302,9 +304,9 @@ class Sub(Base["Sub"]): ...
 reveal_type(Sub)  # revealed: Literal[Sub]
 ```
 
-In a non-stub file, without stringified forward references, this raises a `NameError`:
+#### Without string forward references
 
-`bare_annotation.py`:
+In a non-stub file, without stringified forward references, this raises a `NameError`:
 
 ```py
 class Base[T]: ...
@@ -313,11 +315,23 @@ class Base[T]: ...
 class Sub(Base[Sub]): ...
 ```
 
-## Another cyclic case
+### Cyclic inheritance as a generic parameter
 
 ```pyi
 class Derived[T](list[Derived[T]]): ...
 ```
 
+### Direct cyclic inheritance
+
+Inheritance that would result in a cyclic MRO is detected as an error.
+
+```py
+# error: [cyclic-class-definition]
+class C[T](C): ...
+
+# error: [cyclic-class-definition]
+class D[T](D[int]): ...
+```
+
 [crtp]: https://en.wikipedia.org/wiki/Curiously_recurring_template_pattern
 [f-bound]: https://en.wikipedia.org/wiki/Bounded_quantification#F-bounded_quantification

crates/red_knot_python_semantic/src/types/class.rs

@@ -174,6 +174,10 @@ impl<'db> GenericAlias<'db> {
     pub(crate) fn definition(self, db: &'db dyn Db) -> Definition<'db> {
         self.origin(db).class(db).definition(db)
     }
+
+    pub(crate) fn class_literal(self, db: &'db dyn Db) -> ClassLiteralType<'db> {
+        ClassLiteralType::Generic(self.origin(db))
+    }
 }
 
 impl<'db> From<GenericAlias<'db>> for Type<'db> {
@@ -1690,8 +1694,12 @@ impl<'db> ClassLiteralType<'db> {
             visited_classes: &mut IndexSet<ClassLiteralType<'db>>,
         ) -> bool {
             let mut result = false;
-            for explicit_base_class in class.fully_static_explicit_bases(db) {
-                let (explicit_base_class_literal, _) = explicit_base_class.class_literal(db);
+            for explicit_base in class.explicit_bases(db) {
+                let explicit_base_class_literal = match explicit_base {
+                    Type::ClassLiteral(class_literal) => *class_literal,
+                    Type::GenericAlias(generic_alias) => generic_alias.class_literal(db),
+                    _ => continue,
+                };
                 if !classes_on_stack.insert(explicit_base_class_literal) {
                     return true;
                 }
@@ -1705,7 +1713,6 @@ impl<'db> ClassLiteralType<'db> {
                         visited_classes,
                     );
                 }
-
                 classes_on_stack.pop();
             }
             result