Why is TryParse in C#7 syntax (empty out parameter) emitting a warning if you compile it?

Question

In C#7, you are allowed to do

            if (int.TryParse("123", out int result)) 
                Console.WriteLine($"Parsed: {result}");

or - if you don't use the result and just want to check if the parsing succeeds, discard the out value:

           if (int.TryParse("123", out _))
                Console.WriteLine("Syntax OK");                

That works fine usually, but in Visual Studio 2017 the second example, where the out parameter is empty, generates the warning

Warning AD0001: Analyzer 'Microsoft.CodeAnalysis.CSharp.Diagnostics.SimplifyTypeNames.CSharpSimplifyTypeNamesDiagnosticAnalyzer' threw an exception of type 'System.NullReferenceException' with message 'Object reference not set to an instance of an object.'.

The Visual Studio Versions where I could verify that it occurs is

Visual Studio Enterprise 2017 Version 15.1 (26403.7) Release
Visual Studio Enterprise 2017 Version 15.2 (26430.4) Release

Is this a bug, or is the usage of int.TryParse("123", out _) not officially supported? I could not find any hint so far.

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For completeness, here's the code of the console application showing the issue:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApp1
{
    class Program
    {
        static void Main(string[] args)
        {
            if (int.TryParse("123", out _))
                Console.WriteLine("Syntax OK");
        }
    }
}

Answer 1

I submitted a bug request (request #19180) to the development team, and they confirmed it is a bug. You can see the entire status here at GitHub dotnet/roslyn.

Pilchie commented 16 hours ago
I can repro that in 15.2, but not 15.3. Moving to compiler based on the stack, >Abut I'm pretty sure this is a dupe. @jcouv?

jcouv commented 16 hours ago
Yes, this is a duplicate (of #17229 and possibly another one too). It was fixed in dev15.3 (#17544) and we were unfortunately unable to pull the >fix into dev15.2. Thanks @Matt11 for filing the issue and sorry for the bug.

It seems to be already fixed and will be - as far as I understood - available in the next update. But there is no announced date when it will be included by Microsoft, so I submitted an issue through "Send Feedback/Report a Problem" in Visual Studio 2017.

Notes:

• The issue is not limited to TryParse. I verified that it also occurs if you write your own function, i.e. the following sample shows the warning AD0001 as well:
  

  static void Main(string[] args)
  {   
          bool myOutDemo(string str, out int result)
          {
                  result = (str??"").Length;
                  return result > 0;
          }
          // discard out parameter
          if (myOutDemo("123", out _)) Console.WriteLine("String not empty"); 
  }
  

• I noticed that there is now a VS Version 15.3 preview available, which should contain the fix mentioned in the GitHub comments. Check out the following link: Visual Studio 2017 Version 15.3 Preview. After installing it, I verified the issue again and can confirm it is fixed there.

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Thanks to all who participated in the discussion above! (question comments)