Hang when type checking in weak global mode

[JukkaL]

This results in a hang, when type checked in --py2 mode (but only then):

# mypy: weak=global
ans = set()
ans = ans.union([])
for i in range(10):
    ans = ans.union([])

Traceback:

Traceback (most recent call last):
  File "/Users/jukka/src/mypy/scripts/mypy", line 184, in <module>
    main()
  File "/Users/jukka/src/mypy/scripts/mypy", line 36, in main
    type_check_only(path, module, bin_dir, options)
  File "/Users/jukka/src/mypy/scripts/mypy", line 80, in type_check_only
    python_path=options.python_path)
  File "/Users/jukka/src/mypy/mypy/build.py", line 160, in build
    result = manager.process(UnprocessedFile(info, program_text))
  File "/Users/jukka/src/mypy/mypy/build.py", line 344, in process
    next.process()
  File "/Users/jukka/src/mypy/mypy/build.py", line 796, in process
    self.type_checker().visit_file(self.tree, self.tree.path)
  File "/Users/jukka/src/mypy/mypy/checker.py", line 359, in visit_file
    self.accept(d)
  File "/Users/jukka/src/mypy/mypy/checker.py", line 366, in accept
    typ = node.accept(self)
  File "/Users/jukka/src/mypy/mypy/nodes.py", line 585, in accept
    return visitor.visit_for_stmt(self)
  File "/Users/jukka/src/mypy/mypy/checker.py", line 1602, in visit_for_stmt
    self.accept_in_frame(s.body, repeat_till_fixed=True)
  File "/Users/jukka/src/mypy/mypy/checker.py", line 380, in accept_in_frame
    changed, _ = self.binder.pop_frame(True, True)
  File "/Users/jukka/src/mypy/mypy/checker.py", line 189, in pop_frame
    changed = self.update_from_options(options)
  File "/Users/jukka/src/mypy/mypy/checker.py", line 144, in update_from_options
    type = join_simple(self.frames[0][key], type, other)
  File "/Users/jukka/src/mypy/mypy/join.py", line 26, in join_simple
    if is_subtype(s, t):
  File "/Users/jukka/src/mypy/mypy/subtypes.py", line 47, in is_subtype
    return left.accept(SubtypeVisitor(right, type_parameter_checker))
  File "/Users/jukka/src/mypy/mypy/types.py", line 178, in accept
    return visitor.visit_instance(self)
  File "/Users/jukka/src/mypy/mypy/subtypes.py", line 108, in visit_instance
    zip(t.args, right.args, right.type.defn.type_vars))
  File "/Users/jukka/src/mypy/mypy/subtypes.py", line 107, in <genexpr>
    for lefta, righta, tvar in
  File "/Users/jukka/src/mypy/mypy/subtypes.py", line 20, in check_type_parameter
    return is_subtype(lefta, righta, check_type_parameter)
  File "/Users/jukka/src/mypy/mypy/subtypes.py", line 47, in is_subtype
    return left.accept(SubtypeVisitor(right, type_parameter_checker))
  File "/Users/jukka/src/mypy/mypy/types.py", line 459, in accept
    return visitor.visit_union_type(self)
  File "/Users/jukka/src/mypy/mypy/subtypes.py", line 180, in visit_union_type
    for item in left.items)
  File "/Users/jukka/src/mypy/mypy/subtypes.py", line 180, in <genexpr>
    for item in left.items)
  File "/Users/jukka/src/mypy/mypy/subtypes.py", line 47, in is_subtype
    return left.accept(SubtypeVisitor(right, type_parameter_checker))
  File "/Users/jukka/src/mypy/mypy/types.py", line 459, in accept
    return visitor.visit_union_type(self)
  File "/Users/jukka/src/mypy/mypy/subtypes.py", line 180, in visit_union_type
    for item in left.items)
  File "/Users/jukka/src/mypy/mypy/subtypes.py", line 180, in <genexpr>
    for item in left.items)
...
  File "/Users/jukka/src/mypy/mypy/subtypes.py", line 45, in is_subtype
    for item in cast(UnionType, right).items)
  File "/Users/jukka/src/mypy/mypy/subtypes.py", line 45, in <genexpr>
    for item in cast(UnionType, right).items)
  File "/Users/jukka/src/mypy/mypy/subtypes.py", line 45, in is_subtype
    for item in cast(UnionType, right).items)
  File "/Users/jukka/src/mypy/mypy/subtypes.py", line 45, in <genexpr>
    for item in cast(UnionType, right).items)
  File "/Users/jukka/src/mypy/mypy/subtypes.py", line 45, in is_subtype
    for item in cast(UnionType, right).items)
  File "/Users/jukka/src/mypy/mypy/subtypes.py", line 45, in <genexpr>
    for item in cast(UnionType, right).items)
  File "/Users/jukka/src/mypy/mypy/subtypes.py", line 45, in is_subtype
    for item in cast(UnionType, right).items)
  File "/Users/jukka/src/mypy/mypy/subtypes.py", line 45, in <genexpr>
    for item in cast(UnionType, right).items)
  File "/Users/jukka/src/mypy/mypy/subtypes.py", line 45, in is_subtype
    for item in cast(UnionType, right).items)
  File "/Users/jukka/src/mypy/mypy/subtypes.py", line 45, in <genexpr>
    for item in cast(UnionType, right).items)
  File "/Users/jukka/src/mypy/mypy/subtypes.py", line 45, in is_subtype
    for item in cast(UnionType, right).items)
  File "/Users/jukka/src/mypy/mypy/subtypes.py", line 45, in <genexpr>
    for item in cast(UnionType, right).items)
  File "/Users/jukka/src/mypy/mypy/subtypes.py", line 45, in is_subtype
    for item in cast(UnionType, right).items)
  File "/Users/jukka/src/mypy/mypy/subtypes.py", line 45, in <genexpr>
    for item in cast(UnionType, right).items)
  File "/Users/jukka/src/mypy/mypy/subtypes.py", line 40, in is_subtype
    if (isinstance(right, AnyType) or isinstance(right, UnboundType)
  File "/Users/jukka/venv/mypy/bin/../lib/python3.4/abc.py", line 182, in __instancecheck__
    if subclass in cls._abc_cache:
  File "/Users/jukka/venv/mypy/bin/../lib/python3.4/_weakrefset.py", line 72, in __contains__
    wr = ref(item)
KeyboardInterrupt