Function default arguments false negative

[xmatthias]

Bug Report

Type-checking a method with a typed argument (typed to any regular type) with None as default yields no error.

I would expect this to be a type-error - as the real type of other in the below case is Optional[str].

To Reproduce

def random_function(value: str, other: str = None):
        pass

Playground:
https://mypy-play.net/?mypy=master&python=3.10&gist=c9a5da3dab27e76281498b25ca3a5e4d

Expected Behavior
Raising problematic default value, causing either a change in default, or a re-typing to Optional[str].

Actual Behavior

No error from mypy

Interestingly, pyright does properly detect this and raises the following error (this is how i discovered this bug).

error: Expression of type "None" cannot be assigned to parameter of type "str"
    Type "None" cannot be assigned to type "str" (reportGeneralTypeIssues)

Your Environment

• Mypy version used: 0.950
• Mypy command-line flags: /
• Mypy configuration options from mypy.ini (and other config files): /
• Python version used: 3.10

[erictraut]

The original PEP 484 indicated that when a parameter had a None default argument, it should be interpreted implicitly as though the declared type were unioned with None. The PEP was later revised, but it looks like mypy still defaults to this old behavior. If you use --strict mode, mypy does report this error.

[xmatthias]

--no-implicit-optional would explicitly enable this behavior (--strict has "other" side-effects too).

I'm personally more in favor of the implicit behavior (mypy default) - but struggled to find the "proper" flag for it while opening the issue.
I've now also discovered pyrights strictParameterNoneValue - which allows to align the behavior of the 2 tools (although they have diverging defaults).

[JelleZijlstra]

@erictraut is right. The specific flag for this behavior is --no-implicit-optional: https://mypy.readthedocs.io/en/stable/command_line.html#cmdoption-mypy-no-implicit-optional.

We already have issue #9091 about changing the default, so closing this as a duplicate.