Flow analysis failure on "obj is Type == false"

[fiinix00]

Version Used:

Multiple tries

1. Roslyn 2.0 (VS2017) (https://dotnetfiddle.net)

2. dotnet --version = 3.0.100-preview-009795
   Build Engine version 16.0.225-preview+g5ebeba52a1 for .NET Core

3. Branch master (28 Oct 2018)
   https://sharplab.io/#v2:EYLgtghgzgLgpgJwD4AEAMACFBGA3AWACgiUBmLAJgwEEiBvIjJrcnANiwBYMAhAVwDmACgCUjZg0LNpGAPbAAVnADGMOYowBeDACIAEnAA2h2ToLEpMpgEsAZkPkKM1qFmyZYCLdtsRDUODFLK0krKxwATiEdAHkNFwwAO1k1HDQdEXMwjABfcSsjAPyZUOzmSKEAElj41zStDDpPHIzcDAB6dowAUQRZLwBhAGU0bDYAVhAMAFUAuVsMPkToKGsBRLgAEwwTZT8MADcIBGsIYEM4DAByTyvi6Tzg3KIcoA

Steps to Reproduce:

Compile of this code

using System;

public class A
{
    public static void Bug()
    {
        object obj = "Hello";

        if(obj is string str == false)
        {
            Console.WriteLine("Obj is not string");
        }
        else
        {
            Console.WriteLine($"Obj is string = {str}"); // Eror CS0165: Use of unassigned local variable 'str'
        }
    }
}

Expected Behavior:

Code should compile, and print "Obj is string = Hello";

Actual Behavior:

Compiler failure, flow analysis fails

[alrz]

I believe this is by design. Probably it's best to move this to dotnet/csharplang.

[fiinix00]

Apparently there is a discussion on how to fix this kind of syntaxical chaos:
dotnet/csharplang#882

The odd thing is that this compiles correcly and 'str' is defined in the else scope instead.

if(!(obj is string str)) 
{ 
    str.Method(); //Use of unassigned local variable 'str' 
}
else 
{
    str.Method(); //Ok
}

Language reference
https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/is

• States that: "If expr is true and is is used with an if statement, varname is assigned and has local scope within the if statement only."
• Yet the example above goes against that

Stack overflow also has a interesting discussion on it.
Seems that more have come along this odd behavior:
https://stackoverflow.com/questions/47242221/c-sharp-7-compiler-error-pattern-matching

But yes, you are somewhat correct, it is a Roslyn yet a dotnet/csharplang problem at the same time.

• I would not say it is by design(?) as my counter example sais otherwise

[gafter]

The language-specified definite assignment rules for ! make the latter example work, but there are no special language-specified definite assignment rules for == false that would make the original example work as desired. Roslyn implements the language specification, and therefore this is by design.

[gafter]

Issue moved to dotnet/csharplang #2035 via ZenHub