Roslyn doesn't understand tuple expression syntax in switches when analyzing nullability

[TessenR]

Version Used:

commit 27654b066e600a350706a13ebd67d1da542c1a7b
Merge: 436d17816f6 0c9f90368a5
Author: msftbot[bot] <48340428+msftbot[bot]@users.noreply.github.com>
Date:   Thu May 7 05:33:48 2020 +0000

    Merge pull request #44029 from dotnet/merges/release/dev16.7-preview1-to-master

    Merge release/dev16.7-preview1 to master

Steps to Reproduce:

Compile the following code:

using System;

public class C
{
  public void Test1(string? x)
  {
    switch (x)
    {
      case string _:
        Console.WriteLine(x.Length); // no warnings
        break;
    }
  }

  public void Test2(string? x)
  {
    switch (x, 1)
    {
      case (string _, _):
        Console.WriteLine(x.Length); // False CS8602: Dereference of a possibly null reference.
        break;
    }
  }
}

public class D
{
  public void Test1(string x)
  {
    switch (x)
    {
      case null:
        Console.WriteLine(x.Length); // CS8602: Dereference of a possibly null reference.
        break;
    }
  }

  public void Test2(string x)
  {
    switch (x, 1)
    {
      case (null, _):
        Console.WriteLine(x.Length); // missing warning
        break;
    }
  }
}

Expected Behavior:
No warnings in both methods in class C
Warnings in both methods in class D

Actual Behavior:
Roslyn doesn't update matched variables' nullability when a switche's governing expression is a tuple expression

[RikkiGibson]

Fixed by #51001. /cc @jcouv