Strange behaviour of pattern matching

[vabka]

Version Used: 3.1.100-preview1-014459 (but reproduced in earlier versions)

Steps to Reproduce:

using System;
static class Demo 
{
    static void NotOkCase(object obj)
    {
        if((obj is string s) == false) 
        {
             Console.WriteLine("obj is not a string");                
        }
        else
        {
            //uh-oh use of unassigned variable
            //Console.WriteLine(s);
        }
    }
    
    static void OkCase(object obj)
    {
        if(!(obj is string s)) 
        {
            Console.WriteLine("obj is not a string");
        }
        else 
        {
            Console.WriteLine(s);
        }
    }
}

Expected Behavior:
I can use 's' in NotOkCase.

Actual Behavior:
I can't.

[CyrusNajmabadi]

This is how the language works. There is no understanding about how == bool-val works wrt to definite-assignment.

If you want this to work, you'll have to file a language request over at csharplang.

[gafter]

You need to use ! to negate the expression, which is specified to preserve definite assignment states. == false does not do the same.