Unreachable "else block" code isn't reported as such, when "is var" is used

[DavidArno]

Version Used: VS2017 RC

Steps to Reproduce:

class Program
{
    static void Main(string[] args)
    {
        var s = "test";
        if (s is var x)
        {
            Console.WriteLine($"x = {x}");
        }
        else
        {
            Console.WriteLine("s isn't a var!"); // unreachable code
        }
    }
}

Expected Behavior:
As s is var x will always be true, an "Unreachable code detected" warning should be reported for the contents of the else block.

Actual Behavior:
No warning is reported.

[gafter]

As currently specified, an unreachable code warning is only produced when the if expression is the constant true, not when it is a non-constant expression that is always true. However, this is a reasonable feature request.

[gafter]

@MadsTorgersen Does this belong on the LDM agenda?

[alrz]

I think there was a proposal for disallowing e is var because that's pointless! Declaration expressions would be a better solution when you want to introduce a variable in the expression context and don't care about its type.

[gafter]

The LDM met yesterday and confirmed that the current behavior is intended. We do not make any reachability accommodation for an is-pattern-matching operation that is irrefutable.