`ChainMap` has wrong or unintuitive type after #6042

#6042/#6044 gives me new mypy complaints on my code base.

ChainMap only ever writes to the first mapping in maps. That's my understanding. For example ChainMap({}, defaults) is a typical use-case where the user gets a dict-like and cannot mutate the defaults. That's the typical stacking property of the ChainMap, you only ever mutate the top, most recent mapping.

From my understanding:

def __init__(self):
def __init__(self, __map: MutableMapping):
def __init__(self, __map: MutableMapping, *maps: ImmutableMapping):

I don't think we can have correct hints for the maps member, and new_child of course expects a mutable mapping because iirc it is exactly the next top most thing on the stack.

Originally posted by @kaste in https://github.com/python/typeshed/issues/6042#issuecomment-1186610570

(I started this as a comment on the closed issue but closed issues don't move and I got initial supportive feedback on my comment from @hauntsaninja.)

从 ChainMap 类型的入口点开始，查看 issue #6042 和 #6044 中的上下文。将提议的构造函数和 new_child 类型与当前的存根进行比较；当 ChainMap({}, defaults) 支持对第一个映射进行变更，且不会针对 defaults 映射产生报告中的 mypy 报错时，即视为完成。