(🎁) Warn when NoneType is used in a type

[KotlinIsland]

NoneType is a mangled up mess in the type realm and should never be used in a typing context.

With the special case of None being a secret alias to NoneType I think that many, many people are going to try to use NoneType as an annotation. Adding a warning for the 99% of the time that it's invalidly used would be a great one.

I understand that there are always edge cases, and disabling those warnings with a type comment would be the way to go IMO.

from types import NoneType

def foo(a: NoneType):  # sus alert, NoneType is a mess in the type realm, you should use `None` or `type[None]` instead.
    print(isinstance(a, NoneType))

[JukkaL]

Some more context from the duplicate issue #13139 below.

Hints:

• Fix this in mypy.typeanal
• Currently types.NoneType is treated as an mypy.types.Instance type but None is mapped to mypy.types.NoneType
• If we'd create an Instance with fullname equal to types.NoneType, generate an error

Example illustrating the problem:

from types import NoneType

def f(x: NoneType) -> None:
    pass

f(None)   # Incompatible type None, expected NoneType

[KotlinIsland]

@JukkaL I have partially improved this case in basedmypy.

basedmypy:

from types import NoneType

def foo(n: NoneType):  # error: Variable "types.NoneType" is not valid as a type
    print("hi")

nt: type[None] = NoneType  # no error

just by simply making NoneType an alias instead of a class def. It's not perfect, but I think it's a big improvement.

KotlinIsland/basedmypy@f424a34