Strange custom type for `__init__` behaviour [@curry plugin]

使用链接的 returns pull request 重现该问题：使用 Poetry 安装它，将示例保存为 ex.py，然后运行 poetry run mypy ex.py。阅读 returns/curry.py、returns/contrib/mypy/decorator_plugin.py 以及链接的 curry typetest，以比较 __init__ 和 some。完成标准是 Test 类型保留 plugin 生成的 curried 签名，而不是错误的构造函数重载。

• Are you reporting a bug, or opening a feature request?

Bug report / support question.

• Please insert below the code you are checking with mypy,
  or a mock-up repro if the source is private. We would appreciate
  if you try to simplify your case to a minimal repro.

I am working on @curry typed decorator support. https://github.com/dry-python/returns/pull/350
It works fine for all cases right now. Except for the case when __init__ method is curried.
In this particular case it changes our correct return type (returned from our custom plugin) to some strange incorrect type.

from returns.curry import curry

class Test(object):
    @curry
    def __init__(self, a: int, b: str) -> None:
        ...

    @curry
    def some(self, a: int, b: str) -> None:
        ...

t: Test

# Works fine!
reveal_type(t.some)
# => Revealed type is 'Overload(def () -> Overload(def (a: builtins.int, b: builtins.str), def (a: builtins.int) -> def (b: builtins.str)), def (a: builtins.int) -> def (b: builtins.str), def (a: builtins.int, b: builtins.str))'

# Modifies the return type from our plugin.
reveal_type(Test)
# => Revealed type is 'Overload(def () -> ex.Test, def (a: builtins.int) -> ex.Test, def (a: builtins.int, b: builtins.str) -> ex.Test)'

Full reproduction:

1. Clone https://github.com/dry-python/returns/pull/350
2. poetry install
3. Drop the code from this issue into ./ex.py
4. poetry run mypy ex.py

• What is the actual behavior/output?

Let's focus on the incorrect output we are getting for __init__ methods:

reveal_type(Test)
# => Revealed type is 'Overload(def () -> ex.Test, def (a: builtins.int) -> ex.Test, def (a: builtins.int, b: builtins.str) -> ex.Test)'

• What is the behavior/output you expect?

I expect it to have the exact same type our plugin produces:

Overload(def (self: ex.Test) -> Overload(def (a: builtins.int, b: builtins.str), def (a: builtins.int) -> def (b: builtins.str)), def (self: ex.Test, a: builtins.int) -> def (b: builtins.str), def (self: ex.Test, a: builtins.int, b: builtins.str))

The thing is this type is produced for both .some() and __init__ methods in this example. But, __init__ does change it for some reason. And .some() works correctly.

• What are the versions of mypy and Python you are using?
  Do you see the same issue after installing mypy from Git master?

• mypy: 0.770
• any python from 3.6 to 3.8

• What are the mypy flags you are using? (For example --strict-optional)

Full list: https://github.com/dry-python/returns/blob/master/setup.cfg#L101

Useful links:

1. Failing CI with the helpful error message: https://github.com/dry-python/returns/runs/666196191?check_suite_focus=true#step:7:660
2. Failing typetest source: https://github.com/orsinium-forks/returns/blob/curry/typesafety/test_curry/test_curry/test_curry_arguments.yml#L20
3. Implementation: https://github.com/orsinium-forks/returns/blob/curry/returns/curry.py#L38
4. Plugin source: https://github.com/orsinium-forks/returns/blob/curry/returns/contrib/mypy/decorator_plugin.py#L116
5. Upstream: https://github.com/dry-python/returns