dict considered incompatible with dict type with supertype for key

[jwodder]

Bug Report

Consider the following code:

from pathlib import Path

def mkfiles(spec: dict[str | Path, str]) -> None:
    ...

spec: dict[str, str] = {
    "foo.txt": "Fooey",
    "bar.txt": "Bar-y",
}

mkfiles(spec)

Running mypy on this code with the default options produces the following error:

superkey01.py:11: error: Argument 1 to "mkfiles" has incompatible type "Dict[str, str]"; expected "Dict[Union[str, Path], str]"  [arg-type]

Changing dict to collections.abc.Mapping does not get rid of the error.

Expected Behavior

mypy should have accepted the code, as the key type str | Path is more general than str.

Your Environment

• Mypy version used: 1.2.0
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.11.3 on macOS 11.7.2

[JelleZijlstra]

https://mypy.readthedocs.io/en/stable/common_issues.html#invariance-vs-covariance