Wrong errors of type unsupported operand when type checking is done in list

Bug Report

(A clear and concise description of what the bug is.)

To Reproduce

a: None | float = 0.0
b: None | float = 0.0

c = None if None in (a, b) else a - b
print(c)

(Also the alternative approach of using a regular if...else... block fails)

Playground link

Expected Behavior

No error found by mypy. I cannot think of a way that this code fails. But maybe I'm just very code-blind right now.

Actual Behavior

coordinate_magic/test.py:4: error: Unsupported left operand type for - ("None")  [operator]
coordinate_magic/test.py:4: error: Unsupported operand types for - ("None" and "float")  [operator]
coordinate_magic/test.py:4: error: Unsupported operand types for - ("float" and "None")  [operator]
coordinate_magic/test.py:4: note: Both left and right operands are unions
Found 3 errors in 1 file (checked 1 source file)

Your Environment

• Mypy version used: mypy 1.8.0 (compiled: yes)
• Mypy command-line flags: None
• Mypy configuration options from mypy.ini (and other config files): pyproject.toml

  [tool.mypy]
  # Config for 'mypy'
  # See https://mypy.readthedocs.io/en/stable/config_file.html#using-a-pyproject-toml-file for more details
  python_version = "3.11"
  plugins = "numpy.typing.mypy_plugin"
  warn_return_any = true
  warn_unused_configs = true
  allow_redefinition = true        # Allows variables to be redefined with an arbitrary type, as long as the redefinition is in the same block and nesting level as the original definition.
  disallow_untyped_defs = true
  disallow_incomplete_defs = true  # Disallows defining functions with incomplete type annotations.
  check_untyped_defs = true        # Type-checks the interior of functions without type annotations.
  no_implicit_optional = true      # Changes the treatment of arguments with a default value of None by not implicitly making their type Optional.
  show_error_codes = true
  exclude = "gui/|doc/|coordinate.py"
  
  [[tool.mypy.overrides]]
  # mypy per-module options
  module = [
      "scipy",
      "scipy.*",
      "mpl_toolkits.mplot3d",
      "altair",       # Remove when issue https://github.com/altair-viz/altair/issues/2493 is solved
      "folium",       # Remove when issue https://github.com/python-visualization/folium/issues/1559 is solved
      "folium.plugins",
      "matplotlib",   # Remove when issue https://github.com/matplotlib/matplotlib/issues/20504 is solved
      "matplotlib.*",
      "traffic",
      "coloredlogs",  # Remove when issue https://github.com/xolox/python-coloredlogs/issues/93 is solved
      "pymap3d",      # Remove when discussion https://github.com/geospace-code/pymap3d/pull/76 is solved
  ]
  ignore_missing_imports = true
  

• Python version used: 3.11

The problem lies in the if None in (a, b) part.
As soon as code is changed to

a: None | float = 0.0
b: None | float = 0.0

c = None if a is None or b is None else a - b
print(c)

mypy does not report an error.

EDIT: Added a playground link with all examples

Reproduisez l’extrait de coordinate_magic/test.py avec mypy 1.8.0, puis comparez la forme None in (a, b) avec la forme is None. Examinez le chemin de vérification des types pour le narrowing basé sur l’appartenance ; le travail est terminé lorsque la soustraction valide ne produit plus d’erreurs unsupported-operand et que la régression est couverte par un test.