INTERNAL ERROR with `__hash__ = None`

[nzig]

Crash Report

Mypy crashed when run on the following reduced file. Setting __hash__ to None is the way to suppress Python's default hash method.

Traceback

> mypy mypy_crash.py --show-traceback
mypy_crash.py:2: error: Incompatible types in assignment (expression has type "None", base class "object" defined the type as "Callable[[object], int]")
mypy_crash.py:5: error: INTERNAL ERROR -- Please try using mypy master on Github:
https://mypy.rtfd.io/en/latest/common_issues.html#using-a-development-mypy-build
Please report a bug at https://github.com/python/mypy/issues
version: 0.820+dev.15bd486a67a3214fdb854ba2f1d6d524113da6c1
Traceback (most recent call last):
  File "/home/user/.virtualenvs/mypy/bin/mypy", line 8, in <module>
    sys.exit(console_entry())
  File "/home/user/.virtualenvs/mypy/lib/python3.6/site-packages/mypy/__main__.py", line 11, in console_entry
    main(None, sys.stdout, sys.stderr)
  File "/home/user/.virtualenvs/mypy/lib/python3.6/site-packages/mypy/main.py", line 98, in main
    res = build.build(sources, options, None, flush_errors, fscache, stdout, stderr)
  File "/home/user/.virtualenvs/mypy/lib/python3.6/site-packages/mypy/build.py", line 180, in build
    sources, options, alt_lib_path, flush_errors, fscache, stdout, stderr, extra_plugins
  File "/home/user/.virtualenvs/mypy/lib/python3.6/site-packages/mypy/build.py", line 253, in _build
    graph = dispatch(sources, manager, stdout)
  File "/home/user/.virtualenvs/mypy/lib/python3.6/site-packages/mypy/build.py", line 2688, in dispatch
    process_graph(graph, manager)
  File "/home/user/.virtualenvs/mypy/lib/python3.6/site-packages/mypy/build.py", line 3012, in process_graph
    process_stale_scc(graph, scc, manager)
  File "/home/user/.virtualenvs/mypy/lib/python3.6/site-packages/mypy/build.py", line 3119, in process_stale_scc
    if not graph[id].type_check_second_pass():
  File "/home/user/.virtualenvs/mypy/lib/python3.6/site-packages/mypy/build.py", line 2181, in type_check_second_pass
    return self.type_checker().check_second_pass()
  File "/home/user/.virtualenvs/mypy/lib/python3.6/site-packages/mypy/checker.py", line 344, in check_second_pass
    self.check_partial(node)
  File "/home/user/.virtualenvs/mypy/lib/python3.6/site-packages/mypy/checker.py", line 356, in check_partial
    self.accept(node)
  File "/home/user/.virtualenvs/mypy/lib/python3.6/site-packages/mypy/checker.py", line 401, in accept
    stmt.accept(self)
  File "/home/user/.virtualenvs/mypy/lib/python3.6/site-packages/mypy/nodes.py", line 687, in accept
    return visitor.visit_func_def(self)
  File "/home/user/.virtualenvs/mypy/lib/python3.6/site-packages/mypy/checker.py", line 726, in visit_func_def
    self._visit_func_def(defn)
  File "/home/user/.virtualenvs/mypy/lib/python3.6/site-packages/mypy/checker.py", line 737, in _visit_func_def
    self.check_method_override(defn)
  File "/home/user/.virtualenvs/mypy/lib/python3.6/site-packages/mypy/checker.py", line 1420, in check_method_override
    if self.check_method_or_accessor_override_for_base(defn, base):
  File "/home/user/.virtualenvs/mypy/lib/python3.6/site-packages/mypy/checker.py", line 1448, in check_method_or_accessor_override_for_base
    if self.check_method_override_for_base_with_name(defn, name, base):
  File "/home/user/.virtualenvs/mypy/lib/python3.6/site-packages/mypy/checker.py", line 1509, in check_method_override_for_base_with_name
    assert False, str(base_attr.node)
AssertionError: Var(__hash__)
mypy_crash.py:5: : note: use --pdb to drop into pdb

To Reproduce

Contents of mypy_crash.py:

class Base:
    __hash__ = None

class Dervied(Base):
    def __hash__(self) -> int:
        return 0

Your Environment

• Mypy version used: master (0.820+dev.15bd486a67a3214fdb854ba2f1d6d524113da6c1)
• Mypy command-line flags: None
• Mypy configuration options from mypy.ini (and other config files): None
• Python version used: 3.6.12 (compiled from source with pyenv)
• Operating system and version: Ubuntu 20.04