Internal error when enabling reports

[quulah]

🐛 Bug Report

Mypy throws an internal error when running with a specific file, options and generating a coverage report.

I tried a few versions and the master, but all throw the same error.

It does however generate the report.

To Reproduce

These come from an actual project, but I managed to figure out the smallest possible reproducible example:

.
├── common
│   ├── __init__.py
│   └── encoder.py
└── empty.py

1 directory, 3 files

__init__.py:

from common import encoder

encoder.py:

import simplejson as json

class MyJSONEncoder(json.JSONEncoder):
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)  # type: ignore

empty.py is empty.

Then run mypy with either --txt-report or --cobertura-xml-report and the --disallow-untyped-calls and --check-untyped-defs flags.

Expected Behavior

Report should be generated without an error.

Actual Behavior

./common/encoder.py: 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: 0.782
Traceback (most recent call last):
  File "<snip>/python/3.8.0/bin/mypy", line 10, in <module>
    sys.exit(console_entry())
  File "mypy/build.py", line 1919, in wrap_context
  File "mypy/build.py", line 2147, in finish_passes
  File "mypy/build.py", line 801, in report_file
  File "mypy/report.py", line 83, in file
  File "mypy/report.py", line 476, in on_file
  File "mypy/nodes.py", line 294, in accept
  File "mypy/stats.py", line 89, in visit_mypy_file
  File "mypy/traverser.py", line 35, in visit_mypy_file
  File "mypy/nodes.py", line 939, in accept
  File "mypy/stats.py", line 158, in visit_class_def
  File "mypy/nodes.py", line 1004, in accept
  File "mypy/traverser.py", line 39, in visit_block
  File "mypy/nodes.py", line 676, in accept
  File "mypy/stats.py", line 139, in visit_func_def
  File "mypy/traverser.py", line 54, in visit_func_def
  File "mypy/traverser.py", line 51, in visit_func
  File "mypy/nodes.py", line 1004, in accept
  File "mypy/traverser.py", line 39, in visit_block
  File "mypy/nodes.py", line 1019, in accept
  File "mypy/stats.py", line 192, in visit_expression_stmt
  File "mypy/traverser.py", line 82, in visit_expression_stmt
  File "mypy/nodes.py", line 1544, in accept
  File "mypy/stats.py", line 225, in visit_call_expr
  File "mypy/stats.py", line 234, in record_call_target_precision
  File "mypy/stats.py", line 248, in record_callable_target_precision
  File "mypy/argmap.py", line 101, in map_formals_to_actuals
  File "mypy/argmap.py", line 39, in map_actuals_to_formals
  File "mypy/stats.py", line 253, in <lambda>
KeyError: <mypy.nodes.NameExpr object at 0x11286f430>
./common/encoder.py: : note: use --pdb to drop into pdb
Generated TXT report (via XSLT): <snip>/report/index.txt

Your Environment

• Mypy version used: 0.770, 0.782 and master
• Mypy command-line flags: --txt-report report --show-traceback --disallow-untyped-calls --check-untyped-defs
• Mypy configuration options from mypy.ini (and other config files):
• Python version used: 3.8.0
• Operating system and version:

[Michael0x2a]

This no longer reproduces on master. According to git-bisect, this bug was fixed by #9510.