"unresolved placeholder type None" on mypy 1.0.0

[jonathanslenders]

Bug Report

There is a bug that makes it impossible to typecheck any Python application/library that has an import of the asyncssh package. However, typechecking asyncssh directly doesn't result in this bug. (See below, for a sort of reproducer).

The error looks like this:

Traceback (most recent call last):
  File "/home/jonathan/.pyenv/versions/env/bin/mypy", line 8, in <module>
    sys.exit(console_entry())
  File "/home/jonathan/.pyenv/versions/3.9.13/envs/env/lib/python3.9/site-packages/mypy/__main__.py", line 15, in console_entry
    main()
  File "/home/jonathan/.pyenv/versions/3.9.13/envs/env/lib/python3.9/site-packages/mypy/main.py", line 95, in main
    res, messages, blockers = run_build(sources, options, fscache, t0, stdout, stderr)
  File "/home/jonathan/.pyenv/versions/3.9.13/envs/env/lib/python3.9/site-packages/mypy/main.py", line 174, in run_build
    res = build.build(sources, options, None, flush_errors, fscache, stdout, stderr)
  File "/home/jonathan/.pyenv/versions/3.9.13/envs/env/lib/python3.9/site-packages/mypy/build.py", line 194, in build
    result = _build(
  File "/home/jonathan/.pyenv/versions/3.9.13/envs/env/lib/python3.9/site-packages/mypy/build.py", line 277, in _build
    graph = dispatch(sources, manager, stdout)
  File "/home/jonathan/.pyenv/versions/3.9.13/envs/env/lib/python3.9/site-packages/mypy/build.py", line 2923, in dispatch
    process_graph(graph, manager)
  File "/home/jonathan/.pyenv/versions/3.9.13/envs/env/lib/python3.9/site-packages/mypy/build.py", line 3320, in process_graph
    process_stale_scc(graph, scc, manager)
  File "/home/jonathan/.pyenv/versions/3.9.13/envs/env/lib/python3.9/site-packages/mypy/build.py", line 3443, in process_stale_scc
    graph[id].write_cache()
  File "/home/jonathan/.pyenv/versions/3.9.13/envs/env/lib/python3.9/site-packages/mypy/build.py", line 2504, in write_cache
    new_interface_hash, self.meta = write_cache(
  File "/home/jonathan/.pyenv/versions/3.9.13/envs/env/lib/python3.9/site-packages/mypy/build.py", line 1574, in write_cache
    data = tree.serialize()
  File "/home/jonathan/.pyenv/versions/3.9.13/envs/env/lib/python3.9/site-packages/mypy/nodes.py", line 377, in serialize
    "names": self.names.serialize(self._fullname),
  File "/home/jonathan/.pyenv/versions/3.9.13/envs/env/lib/python3.9/site-packages/mypy/nodes.py", line 3841, in serialize
    data[key] = value.serialize(fullname, key)
  File "/home/jonathan/.pyenv/versions/3.9.13/envs/env/lib/python3.9/site-packages/mypy/nodes.py", line 3778, in serialize
    data["node"] = self.node.serialize()
  File "/home/jonathan/.pyenv/versions/3.9.13/envs/env/lib/python3.9/site-packages/mypy/nodes.py", line 3514, in serialize
    "target": self.target.serialize(),
  File "/home/jonathan/.pyenv/versions/3.9.13/envs/env/lib/python3.9/site-packages/mypy/types.py", line 2072, in serialize
    "ret_type": self.ret_type.serialize(),
  File "/home/jonathan/.pyenv/versions/3.9.13/envs/env/lib/python3.9/site-packages/mypy/types.py", line 605, in serialize
    "values": [v.serialize() for v in self.values],
  File "/home/jonathan/.pyenv/versions/3.9.13/envs/env/lib/python3.9/site-packages/mypy/types.py", line 605, in <listcomp>
    "values": [v.serialize() for v in self.values],
  File "/home/jonathan/.pyenv/versions/3.9.13/envs/env/lib/python3.9/site-packages/mypy/types.py", line 2876, in serialize
    assert False, f"Internal error: unresolved placeholder type {self.fullname}"
AssertionError: Internal error: unresolved placeholder type None

To Reproduce

Install asyncssh, then create a Python file with only this import and typecheck that file.

import asyncssh

Or:

mypy -c 'import asyncssh'

I tried to reduce the code surface as much as possible. Removing every file within asyncssh, except py.typed and __init__.py, and then write the following in __init__.py:

from typing import TypeVar, Callable
T = TypeVar('T', 'A', 'B')
Func = Callable[[], T]

With the above snippet it still happens.

Your Environment

• Mypy version used: 1.0.0
• Mypy command-line flags: --strict
• Mypy configuration options from mypy.ini (and other config files): /
• Python version used: 3.9.13

cc: @ronf

[wjurkowlaniec]

I have the same problem..

[hauntsaninja]

Thanks for the report! Confirming the crash and the regression from 0.991

And thank you for the minimal repro here!