del partial_types[var] fails with KeyError: <Var 'errors' at 0x...>

Crash Report

Followed the instructions after getting the internal error:

src/foo/foo.py:17: error: INTERNAL ERROR -- Please try using mypy master on GitHub:
https://mypy.readthedocs.io/en/stable/common_issues.html#using-a-development-mypy-build
Please report a bug at https://github.com/python/mypy/issues
version: 1.20.0+dev.92a78583b1cd0c9d0cc3966dfc534020128888ca

Traceback

Traceback (most recent call last):
  File "/path/to/.venv/bin/mypy", line 10, in <module>
    sys.exit(console_entry())
  File "/path/to/.venv/lib/python3.14/site-packages/mypy/__main__.py", line 15, in console_entry
    main()
  File "/path/to/.venv/lib/python3.14/site-packages/mypy/main.py", line 135, in main
    res, messages, blockers = run_build(sources, options, fscache, t0, stdout, stderr)
  File "/path/to/.venv/lib/python3.14/site-packages/mypy/main.py", line 219, in run_build
    res = build.build(sources, options, None, flush_errors, fscache, stdout, stderr)
  File "/path/to/.venv/lib/python3.14/site-packages/mypy/build.py", line 371, in build
    result = build_inner(
  File "/path/to/.venv/lib/python3.14/site-packages/mypy/build.py", line 468, in build_inner
    graph = dispatch(sources, manager, stdout)
  File "/path/to/.venv/lib/python3.14/site-packages/mypy/build.py", line 3582, in dispatch
    process_graph(graph, manager)
  File "/path/to/.venv/lib/python3.14/site-packages/mypy/build.py", line 4047, in process_graph
    done, still_working, results = manager.wait_for_done(graph)
  File "/path/to/.venv/lib/python3.14/site-packages/mypy/build.py", line 1189, in wait_for_done
    process_stale_scc(graph, next_scc, self)
  File "/path/to/.venv/lib/python3.14/site-packages/mypy/build.py", line 4215, in process_stale_scc
    graph[id].type_check_first_pass()
  File "/path/to/.venv/lib/python3.14/site-packages/mypy/build.py", line 2904, in type_check_first_pass
    self.type_checker().check_first_pass()
  File "/path/to/.venv/lib/python3.14/site-packages/mypy/checker.py", line 537, in check_first_pass
    self.accept(d)
  File "/path/to/.venv/lib/python3.14/site-packages/mypy/checker.py", line 648, in accept
    stmt.accept(self)
  File "/path/to/.venv/lib/python3.14/site-packages/mypy/nodes.py", line 1004, in accept
    return visitor.visit_func_def(self)
  File "/path/to/.venv/lib/python3.14/site-packages/mypy/checker.py", line 1197, in visit_func_def
    self.check_func_item(defn, name=defn.name)
  File "/path/to/.venv/lib/python3.14/site-packages/mypy/checker.py", line 1233, in check_func_item
    self.check_func_def(defn, typ, name, allow_empty)
  File "/path/to/.venv/lib/python3.14/site-packages/mypy/checker.py", line 1437, in check_func_def
    self.accept(item.body)
  File "/path/to/.venv/lib/python3.14/site-packages/mypy/checker.py", line 648, in accept
    stmt.accept(self)
  File "/path/to/.venv/lib/python3.14/site-packages/mypy/nodes.py", line 1582, in accept
    return visitor.visit_block(self)
  File "/path/to/.venv/lib/python3.14/site-packages/mypy/checker.py", line 3164, in visit_block
    self.accept(s)
  File "/path/to/.venv/lib/python3.14/site-packages/mypy/checker.py", line 648, in accept
    stmt.accept(self)
  File "/path/to/.venv/lib/python3.14/site-packages/mypy/nodes.py", line 1912, in accept
    return visitor.visit_try_stmt(self)
  File "/path/to/.venv/lib/python3.14/site-packages/mypy/checker.py", line 5216, in visit_try_stmt
    self.visit_try_without_finally(s, try_frame=bool(s.finally_body))
  File "/path/to/.venv/lib/python3.14/site-packages/mypy/checker.py", line 5285, in visit_try_without_finally
    self.accept(s.else_body)
  File "/path/to/.venv/lib/python3.14/site-packages/mypy/checker.py", line 648, in accept
    stmt.accept(self)
  File "/path/to/.venv/lib/python3.14/site-packages/mypy/nodes.py", line 1582, in accept
    return visitor.visit_block(self)
  File "/path/to/.venv/lib/python3.14/site-packages/mypy/checker.py", line 3164, in visit_block
    self.accept(s)
  File "/path/to/.venv/lib/python3.14/site-packages/mypy/checker.py", line 648, in accept
    stmt.accept(self)
    ~~~~~~~~~~~^^^^^^
  File "/path/to/.venv/lib/python3.14/site-packages/mypy/nodes.py", line 1669, in accept
    return visitor.visit_assignment_stmt(self)
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
  File "/path/to/.venv/lib/python3.14/site-packages/mypy/checker.py", line 3215, in visit_assignment_stmt
    self.check_assignment(s.lvalues[-1], s.rvalue, s.type is None, s.new_syntax)
    ~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/path/to/.venv/lib/python3.14/site-packages/mypy/checker.py", line 3275, in check_assignment
    self.try_infer_partial_generic_type_from_assignment(lvalue, rvalue, "=")
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^
  File "/path/to/.venv/lib/python3.14/site-packages/mypy/checker.py", line 3547, in try_infer_partial_generic_type_from_assignment
    del partial_types[var]
        ~~~~~~~~~~~~~^^^^^
KeyError: <Var 'errors' at 0x105f66200>
src/foo/foo.py:17: note: use --pdb to drop into pdb

To Reproduce

Stand-alone example. Package needed:

first==2.0.2

Source code:

import logging
from typing import Any

from first import first

logger = logging.getLogger()
logger.setLevel(logging.INFO)


def update_rows(client, rows: list[dict[str, Any]]) -> None:
    response = client.update(rows)
    try:
        response_errors = response.errors
    except AttributeError:
        errors = []
    else:
        errors = first(
            (
                sorted(errors, key=int)
                for error in response_errors
                if (errors := error.context.get("objectWriteTraceId")) is not None
            ),
            default=[],
        )
    logger.info(
        "Number of contacts updated: %s, errors: %s", len(response.results), errors
    )

Your Environment

• Mypy version used: 1.20.0+dev.92a78583b1cd0c9d0cc3966dfc534020128888ca (master)
• Mypy command-line flags: --show-traceback
• Mypy configuration options from mypy.ini (and other config files):

[tool.mypy]
mypy_path = "$MYPY_CONFIG_FILE_DIR/src"
platform = "linux"
python_version = "3.14"

[[tool.mypy.overrides]]
ignore_missing_imports = true

• Python version used: Python 3.14.3
• Operating system and version: macOS 15.7.3 (24G419)

Start in mypy/checker.py at try_infer_partial_generic_type_from_assignment, where the traceback reaches del partial_types[var]. Run the standalone example with mypy --show-traceback to reproduce the KeyError. Done means the example no longer produces an INTERNAL ERROR or traceback.