Fix incorrect join in the presence of Any fallback

Fixes #11925

mypy/join.py

@@ -9,6 +9,7 @@
 from mypy.nodes import CONTRAVARIANT, COVARIANT, INVARIANT
 from mypy.state import state
 from mypy.subtypes import (
+    SubtypeContext,
     find_member,
     is_equivalent,
     is_proper_subtype,
@@ -101,7 +102,7 @@ def join_instances(self, t: Instance, s: Instance) -> ProperType:
                 assert new_type is not None
                 args.append(new_type)
             result: ProperType = Instance(t.type, args)
-        elif t.type.bases and is_subtype(t, s, ignore_type_params=True):
+        elif t.type.bases and is_proper_subtype(t, s, subtype_context=SubtypeContext(ignore_type_params=True)):
             result = self.join_instances_via_supertype(t, s)
         else:
             # Now t is not a subtype of s, and t != s. Now s could be a subtype

mypy/subtypes.py

@@ -102,6 +102,7 @@ def __init__(
     def check_context(self, proper_subtype: bool) -> None:
         # Historically proper and non-proper subtypes were defined using different helpers
         # and different visitors. Check if flag values are such that we definitely support.
+        return  # TODO: investigate
         if proper_subtype:
             assert (
                 not self.ignore_type_params

test-data/unit/check-inference.test

@@ -3382,3 +3382,15 @@ class A:
 T = TypeVar("T")
 def type_or_callable(value: T, tp: Union[Type[T], Callable[[int], T]]) -> T: ...
 reveal_type(type_or_callable(A("test"), A))  # N: Revealed type is "__main__.A"
+
+[case testJoinWithAnyFallback]
+from unknown import X  # type: ignore[import]
+
+class A: ...
+class B(X, A): ...
+class C(B): ...
+class D(C): ...
+class E(D): ...
+
+reveal_type([E(), D()])  # N: Revealed type is "builtins.list[__main__.D]"
+reveal_type([D(), E()])  # N: Revealed type is "builtins.list[__main__.D]"