Roslyn: Pattern matching crashes if lambda captures pattern variable

[hi-octane]

Version Used:
Microsoft Visual Studio Enterprise 2017 RC
Version 15.0.26014.0 D15REL
Microsoft .NET Framework
Version 4.6.01055

Steps to Reproduce:
Run the following app:

using System.Collections.Generic;
using System.Linq;

namespace ConsoleApp2017
{
    class Program
    {
        class A
        {
        }

        static void Main(string[] args)
        {                           
            var dummy = new List<int>();

            switch((object)new A())
            {
                case A a:
                    {
                        dummy.Any(x => a != null);
                    }
                    break;
            }
        }
    }
}

Expected Behavior:
App just runs, no output.

Actual Behavior:
Null-reference on the switch statement, when trying to initialize not created instance of a hidden class <>c__DisplayClass27_0 CS$<>8__locals0;

Unhandled Exception: System.NullReferenceException: Object reference not set to an instance of an object.
at ConsoleApp2017.Program.Main(String[] args) in D:\Temp\ConsoleApp2017\ConsoleApp2017\Program.cs:line 16

[paulomorgado]

@gafter, why are these display classes even being used. I wasn't expecting any extra allocations when using pattern matching.

[CyrusNajmabadi]

There's a lambda that captures the pattern variable.

[paulomorgado]

Doh!

But it seems related to that.

In my case I was able to work around the problem by extracting the code in the cases to other methods.

[YaakovDavis]

Another workaround is simply assigning the pattern variable to another temp one. Change this:

 class C { public int A; }

    class Program
    {
        static void Main(string[] args)
        {
            switch (new C())
            {
                case C c:
                    Func<bool> f = () => c.A > 10;
                    break;
                default:
                    break;
            }
        }
    }

to this:

 class C { public int A; }

    class Program
    {
        static void Main(string[] args)
        {
            switch (new C())
            {
                case C c:
                    var rcWorkaround = c;
                    Func<bool> f = () => rcWorkaround.A > 10;
                    break;
                default:
                    break;
            }
        }
    }

[gafter]

@AlekseyTs Is working on making an LDM-approved change so that the extent of a case-block scoped expression variable will be the switch block. That should fix this issue as a side-effect. Leaving it open until we can verify that.

[paulomorgado]

@AlekseyTs Is working on making an LDM-approved change so that the extent of a case-block scoped expression variable will be the switch block. That should fix this issue as a side-effect. Leaving it open until we can verify that.

Say what?

[CyrusNajmabadi]

The extent. Not the scope. It gives the variable a place to live that doesn't have lifetime concerns as switch cases get entered/exited.

The variable will still be scoped such that it can only be legally referenced inside the appropriate switch-section.

[jcouv]

Tracked by VSO https://devdiv.visualstudio.com/DevDiv/_workitems?id=369824