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

[sobolevn]

• 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. dry-python/returns#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 Add @curry decorator dry-python/returns#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

[sobolevn]

I think that these lines are in charge of modifying the return type of __init__: https://github.com/python/mypy/blob/master/mypy/typeops.py#L103-L109

[sobolevn]

Is it possible to fix this behaviour? As I understand this code, we can try to preserve the overload structure and still maintain the original self type and perform class_callable transformation.

May I send a PR with the fix?

[msullivan]

Yeah I'll review a PR related to this if you send one up. This code path is pretty central so we might be a little conservative about the risk of unintended consequences, though.