--html-report cannot be used with namespace-packages, explicit-package-bases and -p: IsADirectoryError

[earshinov]

Example repo

To Reproduce

$ git clone https://github.com/earshinov/mypy-htmlreport-isadirectoryerror.git
$ cd mypy-htmlreport-isadirectoryerror

...and follow the instructions from README.

In short, the problem is an IsADirectoryError when trying to pass both -p and --html-report:

$ poetry run mypy -p folder --html-report=mypy-report --show-traceback
/mnt/c/tmp/mypy-bug/folder/subfolder: 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: 0.910
Traceback (most recent call last):
  File "/home/user/.cache/pypoetry/virtualenvs/mypy-bug-3PnpPrM--py3.8/bin/mypy", line 8, in <module>
    sys.exit(console_entry())
  File "mypy/build.py", line 1955, in wrap_context
  File "mypy/build.py", line 2205, in finish_passes
  File "mypy/build.py", line 818, in report_file
  File "mypy/report.py", line 83, in file
  File "mypy/report.py", line 482, in on_file
  File "mypy/report.py", line 131, in iterate_python_lines
  File "/usr/lib/python3.8/tokenize.py", line 392, in open
    buffer = _builtin_open(filename, 'rb')
IsADirectoryError: [Errno 21] Is a directory: 'folder/subfolder'
/mnt/c/tmp/mypy-bug/folder/subfolder: : note: use --pdb to drop into pdb
Generated HTML report (via XSLT): /mnt/c/tmp/mypy-bug/mypy-report/index.html

Expected Behavior

It should be possible to use -p and --html-report together.

Actual Behavior

-p and --html-report used together cause an IsADirectoryError.

Your Environment

• Windows 10 / WSL / Ubuntu 20.04
• Python 3.8.10
• mypy 0.910

Reproducible with mypy@master (a114826) as well.

mypy.ini:

[mypy]
explicit_package_bases = True
ignore_missing_imports = True
namespace_packages = True
# As recommended in PEP-484
# - https://github.com/python/typing/issues/275
# - https://www.python.org/dev/peps/pep-0484/#union-types
no_implicit_optional = True
show_column_numbers = True
show_error_codes = True
strict = True

[sobolevn]

Thanks a lot, @earshinov! Your reproduction really helped.
#11338