reachable code reported as unreachable using __init__ vs Protocol vs decorators

從 pp.py 的重現開始，並使用所示的 Protocol、裝飾器和 wrapped 檢查執行 mypy pp.py --warn-unreachable。將執行階段輸出與 mypy 對無法到達陳述式的診斷進行比較；完成的標準是，已到達的 Protocol 分支不再被報告為無法到達。

Please provide more information to help us understand the issue:

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

bug

• 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.

This code decorates init with typeguard, which gives the decorated init a wrapped attribute referring to the original init. This code attempts to unwrap that to get to the original init definition (in my real world use case, to inspect the argument definitions for constructing an instance of a class). At runtime, a Protocol can be used to detect if init has a __wrapped__ attribute, but this does not match up with what mypy believes.

from typing import cast, Any
from typing_extensions import Protocol, runtime_checkable
import typeguard

class Foo():

  @typeguard.typechecked
  def __init__(self, x: int, y: int,z: int = 3):
    self.x = x
    self.y = y
    self.z = z
    print("init")

f = Foo(9,10)
print(f)

func = type(f).__init__
print("__init__ func is {}".format(func))

if hasattr(func, '__wrapped__'):
  print("Path 1: is wrapped according to hasattr")
  print(cast(Any, type(f).__init__).__wrapped__)
else:
  print("Path 1: is not wrapped")


@runtime_checkable
class W(Protocol):
  __wrapped__: object

if isinstance(func, W):
  print("Path 2: is wrapped according to Protocol - mypy says this is unreachable")
  print(type(f).__init__.__wrapped__)
else:
  print("Path 2: is not wrapped")

• What is the actual behavior/output?
  Execution prints my code's message: Path 2: is wrapped according to Protocol - mypy says this is unreachable from line 32.
  mypy reports: pp.py:32: error: Statement is unreachable for that print statement.

Mypy is reporting a statement as unreachable which is actually reached.

• What is the behavior/output you expect?
  Mypy should not report that line as unreachable.

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

I have tried mypy 0.770 and git master:

$ mypy --version
mypy 0.770+dev.52c0a63f4604a2bfa8c0edd6589ab25fab4bdaec

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

$ mypy pp.py --warn-unreachable

with no mypy.ini