Update toolset compilers to 2.3.0-beta2-61716-09. Use LangVersion=7.1 (#19576)

* Update toolset compilers to 2.3.0-beta2-61716-09. Use LangVersion=7.1
* Revert LangVersion for VB. Follow-up issue #19610

build/Targets/Packages.props

@@ -45,8 +45,8 @@
     <MicrosoftInternalVisualStudioShellInterop140DesignTimeVersion>14.3.25407-alpha</MicrosoftInternalVisualStudioShellInterop140DesignTimeVersion>
     <MicrosoftMetadataVisualizerVersion>1.0.0-beta1-61531-03</MicrosoftMetadataVisualizerVersion>
     <MicrosoftMSXMLVersion>8.0.0.0-alpha</MicrosoftMSXMLVersion>
-    <MicrosoftNetCompilersVersion>2.0.1</MicrosoftNetCompilersVersion>
-    <MicrosoftNetCompilersnetcoreVersion>2.0.0-rc2-61102-09</MicrosoftNetCompilersnetcoreVersion>
+    <MicrosoftNetCompilersVersion>2.3.0-beta2-61716-09</MicrosoftNetCompilersVersion>
+    <MicrosoftNetCompilersnetcoreVersion>2.3.0-beta2-61716-09</MicrosoftNetCompilersnetcoreVersion>
     <MicrosoftNetRoslynDiagnosticsVersion>2.0.0-beta1-61628-06</MicrosoftNetRoslynDiagnosticsVersion>
     <MicrosoftNetCoreILAsmVersion>1.1.0</MicrosoftNetCoreILAsmVersion>
     <MicrosoftNETCoreCompilersVersion>2.3.0-dev-56735-00</MicrosoftNETCoreCompilersVersion>

build/Targets/Settings.props

@@ -166,6 +166,7 @@
     <When Condition="'$(MSBuildProjectExtension)' == '.csproj' OR '$(Language)' == 'C#' OR '$(ProjectLanguage)' == 'CSharp'">
       <PropertyGroup>
         <ProjectLanguage>CSharp</ProjectLanguage>
+        <LangVersion>7.1</LangVersion>
         <WarningLevel>4</WarningLevel>
         <ErrorReport>prompt</ErrorReport>
 

docs/contributing/Building, Debugging, and Testing on Windows.md

@@ -13,7 +13,7 @@ Using the command line Roslyn can be developed using the following pattern:
 
 1. [Visual Studio 2017](https://www.visualstudio.com/downloads/)
     - Ensure C#, VB, MSBuild and Visual Studio Extensibility are included in the selected work loads
-    - Ensure Visual Studio is on at least update 1 
+    - Ensure Visual Studio is on at least version 15.1
 1. Run Restore.cmd
 1. Open Roslyn.sln
 
@@ -25,6 +25,8 @@ do the following:
 - Click on the hamburger menu, click Modify 
 - Choose the workloads listed above and click Modify
 
+During the last few weeks of a development cycle for a quarterly release (versions 15.3, 15.6, etc.), it is possible for the Roslyn codebase to make use of new language features, which will not be suppored by the last released version. During that period, it is recommended to use the [preview version of Visual Studio](https://www.visualstudio.com/vs/preview/) or [Roslyn nightlies](https://github.com/dotnet/roslyn/issues/18783#issuecomment-299064434). Alternatively, you can just ignore some red squiggles produced by the IDE that do not correspond to build errors (on portions of the code using the latest language features).
+
 ## Running Tests
 
 There are a number of options for running the core Roslyn unit tests

src/Compilers/Core/Portable/AdditionalTextFile.cs

@@ -42,7 +42,7 @@ public AdditionalTextFile(CommandLineSourceFile sourceFile, CommonCompiler compi
         /// Returns a <see cref="SourceText"/> with the contents of this file, or <c>null</c> if
         /// there were errors reading the file.
         /// </summary>
-        public override SourceText GetText(CancellationToken cancellationToken = default(CancellationToken))
+        public override SourceText GetText(CancellationToken cancellationToken = default)
         {
             lock (_lockObject)
             {

src/Compilers/Core/Portable/FileSystemExtensions.cs

@@ -34,7 +34,7 @@ public static class FileSystemExtensions
             string xmlDocPath = null,
             string win32ResourcesPath = null,
             IEnumerable<ResourceDescription> manifestResources = null,
-            CancellationToken cancellationToken = default(CancellationToken))
+            CancellationToken cancellationToken = default)
         {
             if (compilation == null)
             {