Don't serialize redefined symbol nodes (#7499)

Redefined nodes sometimes have busted internal references (like a self
argument that has the class as its type even though it won't be able
to look it up) which can cause crashes. Since they can't have
references from outside themselves, we don't lose anything by not
serializing them.

Fixes a crash I observed at Dropbox.

This partially reverts #7413, which introduced this crash (while
fixing another), but leaves the test case it added.

mypy/fixup.py

@@ -57,12 +57,8 @@ def visit_type_info(self, info: TypeInfo) -> None:
             if info.metaclass_type:
                 info.metaclass_type.accept(self.type_fixer)
             if info._mro_refs:
-                # If the class is a "-redefinition", then its
-                # reference to itself might be busted, so just use the
-                # info instead of looking up the first element. Ew.
-                info.mro = [info] + [
-                    lookup_qualified_typeinfo(self.modules, name, self.allow_missing)
-                    for name in info._mro_refs[1:]]
+                info.mro = [lookup_qualified_typeinfo(self.modules, name, self.allow_missing)
+                            for name in info._mro_refs]
                 info._mro_refs = None
         finally:
             self.current_info = save_info

mypy/nodes.py

@@ -2996,8 +2996,6 @@ def serialize(self, prefix: str, name: str) -> JsonDict:
                 fullname = self.node.fullname()
                 if (fullname is not None and '.' in fullname
                         and fullname != prefix + '.' + name
-                        # If it only doesn't match because of -redefinition, that is OK
-                        and fullname != prefix + '.' + name.split('-redefinition')[0]
                         and not (isinstance(self.node, Var)
                                  and self.node.from_module_getattr)):
                     data['cross_ref'] = fullname

mypy/semanal.py

@@ -4245,6 +4245,11 @@ def add_redefinition(self,
         redefinitions (such as e.g. variable redefined as a class).
         """
         i = 1
+        # Don't serialize redefined nodes. They are likely to have
+        # busted internal references which can cause problems with
+        # serialization and they can't have any external references to
+        # them.
+        symbol.no_serialize = True
         while True:
             if i == 1:
                 new_name = '{}-redefinition'.format(name)

test-data/unit/check-incremental.test

@@ -5094,3 +5094,16 @@ tmp/a.py:4: error: Cannot determine type of 'foo'
 [out2]
 tmp/a.py:3: error: Cannot determine type of 'foo'
 tmp/a.py:4: error: Cannot determine type of 'foo'
+
+[case testRedefinitionClass]
+import b
+[file a.py]
+from whatever import Foo  # type: ignore
+
+class Foo:  # type: ignore
+    def f(self) -> None:
+        pass
+[file b.py]
+import a
+[file b.py.2]
+import a # a change