IDE0260 recommends switching to invalid pattern

[stephentoub]

Version Used:
Version 17.6.0 Preview 2.0 [33420.113.main]

Steps to Reproduce:

internal class Program
{
    static void Main() { }

    static B M(object o)
    {
        if ((o as A)?.Value is not B b)
        {
            throw new Exception();
        }

        return b;
    }
}

class A
{
    public B Value;
}

class B
{
}

Diagnostic Id:
IDE0260

Expected Behavior:
No warning.

Actual Behavior:
IDE0260 fires and recommends changing:

        if ((o as A)?.Value is not B b)

to:

        if (o is A { Value: not B b })

which is invalid and results in:

error CS8780: A variable may not be declared within a 'not' or 'or' pattern.

[CyrusNajmabadi]

Odd. The top expression defines a variable within a not pattern. @RikkiGibson do you know why the top is allowed but the bottom isn't. I keep looking at them and they seem functionally identical. It's unclear to me why the second form isn't ok.

[RikkiGibson]

My recollection is that 'is not A a' is a special case and it's generally not allowed in other contexts. Would need to dig up the and/or/not patterns proposal to recall why.