Be stricter about access to generic vars from class

This behaviour was intentionally chosen in python#6418

I agree with Ivan's comment there that it's similar to how mypy allows
instantiation of `type[Abstract]` -- but that's a thing that I want to
explore disallowing.

Let's see primer

mypy/checkmember.py

@@ -1087,13 +1087,11 @@ def analyze_class_attribute_access(
                 def_vars.add(node.node.info.self_type)
             typ_vars = set(get_type_vars(t))
             if def_vars & typ_vars:
-                # Exception: access on Type[...], including first argument of class methods is OK.
-                if not isinstance(get_proper_type(mx.original_type), TypeType) or node.implicit:
-                    if node.node.is_classvar:
-                        message = message_registry.GENERIC_CLASS_VAR_ACCESS
-                    else:
-                        message = message_registry.GENERIC_INSTANCE_VAR_CLASS_ACCESS
-                    mx.msg.fail(message, mx.context)
+                if node.node.is_classvar:
+                    message = message_registry.GENERIC_CLASS_VAR_ACCESS
+                else:
+                    message = message_registry.GENERIC_INSTANCE_VAR_CLASS_ACCESS
+                mx.msg.fail(message, mx.context)
             t = expand_self_type_if_needed(t, mx, node.node, itype, is_class=True)
             # Erase non-mapped variables, but keep mapped ones, even if there is an error.
             # In the above example this means that we infer following types:

test-data/unit/check-classvar.test

@@ -282,17 +282,27 @@ main:2: note: Revealed type is "builtins.int"
 main:3: error: Cannot assign to class variable "x" via instance
 
 [case testClassVarWithGeneric]
-from typing import ClassVar, Generic, TypeVar
+from typing import ClassVar, Generic, TypeVar, Type
 T = TypeVar('T')
 class A(Generic[T]):
     x: ClassVar[T]  # E: ClassVar cannot contain type variables
     @classmethod
     def foo(cls) -> T:
-        return cls.x  # OK
+        return cls.x  # E: Access to generic class variables is ambiguous
 
-A.x  # E: Access to generic class variables is ambiguous
-A.x = 1  # E: Access to generic class variables is ambiguous
-A[int].x  # E: Access to generic class variables is ambiguous
+def main(A_T: Type[A]):
+    A.x  # E: Access to generic class variables is ambiguous
+    A.x = 1  # E: Access to generic class variables is ambiguous
+    A[int].x  # E: Access to generic class variables is ambiguous
+
+    A_T.x  # E: Access to generic class variables is ambiguous
+    A_T.x = 1  # E: Access to generic class variables is ambiguous
+    A_T[int].x  # E: Value of type "Type[A[Any]]" is not indexable
+
+    a = A
+    a.x  # E: Access to generic class variables is ambiguous
+    a.x = 1  # E: Access to generic class variables is ambiguous
+    a[int].x  # E: The type "Type[A[Any]]" is not generic and not indexable
 
 class Bad(A[int]):
     pass
@@ -311,7 +321,7 @@ class A(Generic[T, U]):
     x: ClassVar[Union[T, Tuple[U, Type[U]]]]  # E: ClassVar cannot contain type variables
     @classmethod
     def foo(cls) -> Union[T, Tuple[U, Type[U]]]:
-        return cls.x  # OK
+        return cls.x  # E: Access to generic class variables is ambiguous
 
 A.x  # E: Access to generic class variables is ambiguous
 A.x = 1  # E: Access to generic class variables is ambiguous

test-data/unit/check-generics.test

@@ -2225,7 +2225,7 @@ class C(Generic[T]):
     x: T
     @classmethod
     def get(cls) -> T:
-        return cls.x  # OK
+        return cls.x  # E: Access to generic instance variables via class is ambiguous
 
 x = C.x  # E: Access to generic instance variables via class is ambiguous
 reveal_type(x)  # N: Revealed type is "Any"
@@ -2288,7 +2288,7 @@ reveal_type(b)  # N: Revealed type is "__main__.B"
 
 def g(t: Type[Maker[T]]) -> T:
     if bool():
-        return t.x
+        return t.x  # E: Access to generic instance variables via class is ambiguous
     return t.get()
 bb = g(B)
 reveal_type(bb)  # N: Revealed type is "__main__.B"