Mypy reports internal error

[hackaugusto]

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.

Here is the minimal reproducible example

from dataclasses import dataclass
from typing import Generic, TypeVar

T = TypeVar("T", bound="B")

class A:
    pass

@dataclass
class Transaction(Generic[T]):
    a: A

• What is the actual behavior/output?

mypy_bug.py:4: error: Name 'B' is not defined
mypy_bug.py:12: 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: 0.720
Traceback (most recent call last):
  File "mypy/newsemanal/semanal.py", line 4550, in accept
  File "mypy/nodes.py", line 662, in accept__Node_glue
  File "mypy/nodes.py", line 663, in accept
  File "mypy/newsemanal/semanal.py", line 519, in visit_func_def__StatementVisitor_glue
  File "mypy/newsemanal/semanal.py", line 539, in visit_func_def
  File "mypy/newsemanal/semanal.py", line 571, in analyze_func_def
  File "mypy/newsemanal/semanal.py", line 4299, in defer
AssertionError: Must not defer during final iteration
mypy_bug.py:12: : note: use --pdb to drop into pdb

mypy_bug.py:12: 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: 0.730+dev.8782ae7f789ad5b8cab97d3c8419f9f98b6eb285
Traceback (most recent call last):
  File "/home/hack/work/brainbot/pyvenv3-dev/bin/mypy", line 11, in <module>
    load_entry_point('mypy', 'console_scripts', 'mypy')()
  File "/home/hack/work/brainbot/mypy/mypy/__main__.py", line 8, in console_entry
    main(None, sys.stdout, sys.stderr)
  File "/home/hack/work/brainbot/mypy/mypy/main.py", line 88, in main
    res = build.build(sources, options, None, flush_errors, fscache, stdout, stderr)
  File "/home/hack/work/brainbot/mypy/mypy/build.py", line 164, in build
    result = _build(sources, options, alt_lib_path, flush_errors, fscache, stdout, stderr)
  File "/home/hack/work/brainbot/mypy/mypy/build.py", line 229, in _build
    graph = dispatch(sources, manager, stdout)
  File "/home/hack/work/brainbot/mypy/mypy/build.py", line 2566, in dispatch
    process_graph(graph, manager)
  File "/home/hack/work/brainbot/mypy/mypy/build.py", line 2875, in process_graph
    process_stale_scc(graph, scc, manager)
  File "/home/hack/work/brainbot/mypy/mypy/build.py", line 2968, in process_stale_scc
    mypy.semanal_main.semantic_analysis_for_scc(graph, scc, manager.errors)
  File "/home/hack/work/brainbot/mypy/mypy/semanal_main.py", line 78, in semantic_analysis_for_scc
    process_functions(graph, scc, patches)
  File "/home/hack/work/brainbot/mypy/mypy/semanal_main.py", line 238, in process_functions
    patches)
  File "/home/hack/work/brainbot/mypy/mypy/semanal_main.py", line 274, in process_top_level_function
    final_iteration, patches)
  File "/home/hack/work/brainbot/mypy/mypy/semanal_main.py", line 330, in semantic_analyze_target
    active_type=active_type)
  File "/home/hack/work/brainbot/mypy/mypy/semanal.py", line 380, in refresh_partial
    self.accept(node)
  File "/home/hack/work/brainbot/mypy/mypy/semanal.py", line 4624, in accept
    node.accept(self)
  File "/home/hack/work/brainbot/mypy/mypy/nodes.py", line 668, in accept
    return visitor.visit_func_def(self)
  File "/home/hack/work/brainbot/mypy/mypy/semanal.py", line 556, in visit_func_def
    self.analyze_func_def(defn)
  File "/home/hack/work/brainbot/mypy/mypy/semanal.py", line 588, in analyze_func_def
    self.defer(defn)
  File "/home/hack/work/brainbot/mypy/mypy/semanal.py", line 4358, in defer
    assert not self.final_iteration, 'Must not defer during final iteration'
AssertionError: Must not defer during final iteration
mypy_bug.py:12: : note: use --pdb to drop into pdb

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

Python 3.7.4
mypy 0.720 and 8782ae7

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

Executed with mypy bug.py --show-traceback

[ilevkivskyi]

Thanks for reporting! This looks pretty bad, also the same crash exists for attrs, see #7129 (comment)

[jjlee]

I get the same assertion failure from this (python 3.7.5, mypy version 0.770+dev.35b50397cabd36066d2dd92b979794b94f1741d3 -- i.e. git master as of today):

from dataclasses import dataclass, field

@dataclass
class Base:
    foo: bool = field(default=False, init=False)

@dataclass
class Derived(Base):
    @property
    def foo(self):
        pass

[msullivan]

@jjlee at least is related to #6809. I'm taking a look at this cluster

[jjlee]

@msullivan Both my example and the original case were fixed by this lovely PR: #8271 (07a20a6), so I think this bug can be closed. Thanks!