Fix crash when nested class appears in a protocol (#13489)

Fixes #6393

This is unspecified behavior in terms of PEP 544, so we just try to 
do something meaningful (see test case). At least we should not crash.

mypy/subtypes.py

@@ -61,6 +61,7 @@
     is_named_instance,
 )
 from mypy.typestate import SubtypeKind, TypeState
+from mypy.typevars import fill_typevars_with_any
 from mypy.typevartuples import extract_unpack, split_with_instance
 
 # Flags for detected protocol members
@@ -1060,6 +1061,10 @@ def find_member(
                     return getattr_type
         if itype.type.fallback_to_any:
             return AnyType(TypeOfAny.special_form)
+        if isinstance(v, TypeInfo):
+            # PEP 544 doesn't specify anything about such use cases. So we just try
+            # to do something meaningful (at least we should not crash).
+            return TypeType(fill_typevars_with_any(v))
     return None
 
 

test-data/unit/check-protocols.test

@@ -3118,3 +3118,23 @@ class P(Protocol):
 
 class A(P): ...
 A() # E: Cannot instantiate abstract class "A" with abstract attribute "f"
+
+[case testProtocolWithNestedClass]
+from typing import TypeVar, Protocol
+
+class Template(Protocol):
+    var: int
+    class Meta: ...
+
+class B:
+    var: int
+    class Meta: ...
+class C:
+    var: int
+    class Meta(Template.Meta): ...
+
+def foo(t: Template) -> None: ...
+foo(B())  # E: Argument 1 to "foo" has incompatible type "B"; expected "Template" \
+          # N: Following member(s) of "B" have conflicts: \
+          # N:     Meta: expected "Type[__main__.Template.Meta]", got "Type[__main__.B.Meta]"
+foo(C())  # OK