[red-knot] GenericAlias instances as a base class (#17575)

## Summary

We currently emit a diagnostic for code like the following:
```py
from typing import Any

# error: Invalid class base with type `GenericAlias` (all bases must be a class, `Any`, `Unknown` or `Todo`)
class C(tuple[Any, ...]): ...
```

The changeset here silences this diagnostic by recognizing instances of
`GenericAlias` in `ClassBase::try_from_type`, and inferring a `@Todo`
type for them. This is a change in preparation for #17557, because `C`
previously had `Unknown` in its MRO …
```py
reveal_type(C.__mro__)  # tuple[Literal[C], Unknown, Literal[object]]
```
… which would cause us to think that `C` is assignable to everything.

The changeset also removes some false positive `invalid-base`
diagnostics across the ecosystem.

## Test Plan

Updated Markdown tests.

Author: sharkdp

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

@@ -106,13 +106,13 @@ reveal_type(ChainMapSubclass.__mro__)
 class CounterSubclass(typing.Counter): ...
 
 # TODO: Should be (CounterSubclass, Counter, dict, MutableMapping, Mapping, Collection, Sized, Iterable, Container, Generic, object)
-# revealed: tuple[Literal[CounterSubclass], Literal[Counter], Unknown, Literal[object]]
+# revealed: tuple[Literal[CounterSubclass], Literal[Counter], @Todo(GenericAlias instance), @Todo(`Generic[]` subscript), Literal[object]]
 reveal_type(CounterSubclass.__mro__)
 
 class DefaultDictSubclass(typing.DefaultDict): ...
 
 # TODO: Should be (DefaultDictSubclass, defaultdict, dict, MutableMapping, Mapping, Collection, Sized, Iterable, Container, Generic, object)
-# revealed: tuple[Literal[DefaultDictSubclass], Literal[defaultdict], Unknown, Literal[object]]
+# revealed: tuple[Literal[DefaultDictSubclass], Literal[defaultdict], @Todo(GenericAlias instance), Literal[object]]
 reveal_type(DefaultDictSubclass.__mro__)
 
 class DequeSubclass(typing.Deque): ...
@@ -124,6 +124,6 @@ reveal_type(DequeSubclass.__mro__)
 class OrderedDictSubclass(typing.OrderedDict): ...
 
 # TODO: Should be (OrderedDictSubclass, OrderedDict, dict, MutableMapping, Mapping, Collection, Sized, Iterable, Container, Generic, object)
-# revealed: tuple[Literal[OrderedDictSubclass], Literal[OrderedDict], Unknown, Literal[object]]
+# revealed: tuple[Literal[OrderedDictSubclass], Literal[OrderedDict], @Todo(GenericAlias instance), Literal[object]]
 reveal_type(OrderedDictSubclass.__mro__)
 ```

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

@@ -326,8 +326,6 @@ class Sub(Base[Sub]): ...
 ## Another cyclic case
 
 ```pyi
-# TODO no error (generics)
-# error: [invalid-base]
 class Derived[T](list[Derived[T]]): ...
 ```
 

crates/red_knot_python_semantic/resources/mdtest/subscript/tuple.md

@@ -81,13 +81,11 @@ python-version = "3.9"
 ```
 
 ```py
-# TODO: `tuple[int, str]` is a valid base (generics)
-# error: [invalid-base] "Invalid class base with type `GenericAlias` (all bases must be a class, `Any`, `Unknown` or `Todo`)"
 class A(tuple[int, str]): ...
 
 # Runtime value: `(A, tuple, object)`
 # TODO: Generics
-reveal_type(A.__mro__)  # revealed: tuple[Literal[A], Unknown, Literal[object]]
+reveal_type(A.__mro__)  # revealed: tuple[Literal[A], @Todo(GenericAlias instance), Literal[object]]
 ```
 
 ## `typing.Tuple`

crates/red_knot_python_semantic/resources/mdtest/type_of/basic.md

@@ -145,12 +145,10 @@ _: type[A, B]
 ## As a base class
 
 ```py
-# TODO: this is a false positive
-# error: [invalid-base] "Invalid class base with type `GenericAlias` (all bases must be a class, `Any`, `Unknown` or `Todo`)"
 class Foo(type[int]): ...
 
 # TODO: should be `tuple[Literal[Foo], Literal[type], Literal[object]]
-reveal_type(Foo.__mro__)  # revealed: tuple[Literal[Foo], Unknown, Literal[object]]
+reveal_type(Foo.__mro__)  # revealed: tuple[Literal[Foo], @Todo(GenericAlias instance), Literal[object]]
 ```
 
 ## `@final` classes

crates/red_knot_python_semantic/src/types/class_base.rs

@@ -78,6 +78,9 @@ impl<'db> ClassBase<'db> {
                 Self::Class(literal.default_specialization(db))
             }),
             Type::GenericAlias(generic) => Some(Self::Class(ClassType::Generic(generic))),
+            Type::Instance(instance) if instance.class().is_known(db, KnownClass::GenericAlias) => {
+                Self::try_from_type(db, todo_type!("GenericAlias instance"))
+            }
             Type::Union(_) => None, // TODO -- forces consideration of multiple possible MROs?
             Type::Intersection(_) => None, // TODO -- probably incorrect?
             Type::Instance(_) => None, // TODO -- handle `__mro_entries__`?