[red-knot] Several failing tests for generics (#16509)

To kick off the work of supporting generics, this adds many new
(currently failing) tests, showing the behavior we plan to support.

This is still missing a lot!  Not included:

- typevar tuples
- param specs
- variance
- `Self`

But it's a good start! We can add more failing tests for those once we
tackle these.

---------

Co-authored-by: Carl Meyer <[email protected]>

Author: dcreager

crates/red_knot_python_semantic/resources/mdtest/generics.md

@@ -0,0 +1,186 @@
+# Generic classes
+
+## PEP 695 syntax
+
+TODO: Add a `red_knot_extension` function that asserts whether a function or class is generic.
+
+This is a generic class defined using PEP 695 syntax:
+
+```py
+class C[T]: ...
+```
+
+A class that inherits from a generic class, and fills its type parameters with typevars, is generic:
+
+```py
+# TODO: no error
+# error: [non-subscriptable]
+class D[U](C[U]): ...
+```
+
+A class that inherits from a generic class, but fills its type parameters with concrete types, is
+_not_ generic:
+
+```py
+# TODO: no error
+# error: [non-subscriptable]
+class E(C[int]): ...
+```
+
+A class that inherits from a generic class, and doesn't fill its type parameters at all, implicitly
+uses the default value for the typevar. In this case, that default type is `Unknown`, so `F`
+inherits from `C[Unknown]` and is not itself generic.
+
+```py
+class F(C): ...
+```
+
+## Legacy syntax
+
+This is a generic class defined using the legacy syntax:
+
+```py
+from typing import Generic, TypeVar
+
+T = TypeVar("T")
+
+# TODO: no error
+# error: [invalid-base]
+class C(Generic[T]): ...
+```
+
+A class that inherits from a generic class, and fills its type parameters with typevars, is generic.
+
+```py
+class D(C[T]): ...
+```
+
+(Examples `E` and `F` from above do not have analogues in the legacy syntax.)
+
+## Inferring generic class parameters
+
+The type parameter can be specified explicitly:
+
+```py
+class C[T]:
+    x: T
+
+# TODO: no error
+# TODO: revealed: C[int]
+# error: [non-subscriptable]
+reveal_type(C[int]())  # revealed: Unknown
+```
+
+We can infer the type parameter from a type context:
+
+```py
+c: C[int] = C()
+# TODO: revealed: C[int]
+reveal_type(c)  # revealed: C
+```
+
+The typevars of a fully specialized generic class should no longer be visible:
+
+```py
+# TODO: revealed: int
+reveal_type(c.x)  # revealed: T
+```
+
+If the type parameter is not specified explicitly, and there are no constraints that let us infer a
+specific type, we infer the typevar's default type:
+
+```py
+class D[T = int]: ...
+
+# TODO: revealed: D[int]
+reveal_type(D())  # revealed: D
+```
+
+If a typevar does not provide a default, we use `Unknown`:
+
+```py
+# TODO: revealed: C[Unknown]
+reveal_type(C())  # revealed: C
+```
+
+If the type of a constructor parameter is a class typevar, we can use that to infer the type
+parameter:
+
+```py
+class E[T]:
+    def __init__(self, x: T) -> None: ...
+
+# TODO: revealed: E[int] or E[Literal[1]]
+reveal_type(E(1))  # revealed: E
+```
+
+The types inferred from a type context and from a constructor parameter must be consistent with each
+other:
+
+```py
+# TODO: error
+wrong_innards: E[int] = E("five")
+```
+
+## Generic subclass
+
+When a generic subclass fills its superclass's type parameter with one of its own, the actual types
+propagate through:
+
+```py
+class Base[T]:
+    x: T
+
+# TODO: no error
+# error: [non-subscriptable]
+class Sub[U](Base[U]): ...
+
+# TODO: no error
+# TODO: revealed: int
+# error: [non-subscriptable]
+reveal_type(Base[int].x)  # revealed: Unknown
+# TODO: revealed: int
+reveal_type(Sub[int].x)  # revealed: Unknown
+```
+
+## Cyclic class definition
+
+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).
+
+`stub.pyi`:
+
+```pyi
+class Base[T]: ...
+# TODO: no error
+# error: [non-subscriptable]
+class Sub(Base[Sub]): ...
+
+reveal_type(Sub)  # revealed: Literal[Sub]
+```
+
+`string_annotation.py`:
+
+```py
+class Base[T]: ...
+
+# TODO: no error
+# error: [non-subscriptable]
+class Sub(Base["Sub"]): ...
+
+reveal_type(Sub)  # revealed: Literal[Sub]
+```
+
+`bare_annotation.py`:
+
+```py
+class Base[T]: ...
+
+# TODO: error: [unresolved-reference]
+class Sub(Base[Sub]): ...
+```
+
+[crtp]: https://en.wikipedia.org/wiki/Curiously_recurring_template_pattern
+[f-bound]: https://en.wikipedia.org/wiki/Bounded_quantification#F-bounded_quantification