Don't serialize redefined symbol nodes

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