Add support for using alias = any_type.

src/Compilers/CSharp/Portable/Binder/Imports.cs

@@ -162,7 +162,7 @@ public static Imports FromSyntax(
                         if (usingAliases != null && usingAliases.ContainsKey(identifierValueText))
                         {
                             // Suppress diagnostics if we're already broken.
-                            if (!usingDirective.Name.IsMissing)
+                            if (!usingDirective.Type.IsMissing)
                             {
                                 // The using alias '{0}' appeared previously in this namespace
                                 diagnostics.Add(ErrorCode.ERR_DuplicateAlias, location, identifierValueText);
@@ -188,11 +188,12 @@ public static Imports FromSyntax(
 
                             // construct the alias sym with the binder for which we are building imports. That
                             // way the alias target can make use of extern alias definitions.
-                            usingAliases.Add(identifierValueText, new AliasAndUsingDirective(new AliasSymbol(usingsBinder, usingDirective.Name, usingDirective.Alias), usingDirective));
+                            usingAliases.Add(identifierValueText, new AliasAndUsingDirective(new AliasSymbol(usingsBinder, usingDirective.Type, usingDirective.Alias), usingDirective));
                         }
                     }
                     else
                     {
+                        Debug.Assert(usingDirective.Name != null);
                         if (usingDirective.Name.IsMissing)
                         {
                             //don't try to lookup namespaces inserted by parser error recovery
@@ -593,7 +594,7 @@ private void Validate()
                 if (@using.NamespaceOrType.IsType)
                 {
                     var typeSymbol = (TypeSymbol)@using.NamespaceOrType;
-                    var location = @using.UsingDirective?.Name.Location ?? NoLocation.Singleton;
+                    var location = @using.UsingDirective?.Type.Location ?? NoLocation.Singleton;
                     typeSymbol.CheckAllConstraints(_compilation, conversions, location, semanticDiagnostics);
                 }
             }

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -1,17 +1,17 @@
 <?xml version="1.0" encoding="utf-8"?>
 <root>
-  <!--
-    Microsoft ResX Schema
-
+  <!-- 
+    Microsoft ResX Schema 
+    
     Version 2.0
-
-    The primary goals of this format is to allow a simple XML format
-    that is mostly human readable. The generation and parsing of the
-    various data types are done through the TypeConverter classes
+    
+    The primary goals of this format is to allow a simple XML format 
+    that is mostly human readable. The generation and parsing of the 
+    various data types are done through the TypeConverter classes 
     associated with the data types.
-
+    
     Example:
-
+    
     ... ado.net/XML headers & schema ...
     <resheader name="resmimetype">text/microsoft-resx</resheader>
     <resheader name="version">2.0</resheader>
@@ -26,36 +26,36 @@
         <value>[base64 mime encoded string representing a byte array form of the .NET Framework object]</value>
         <comment>This is a comment</comment>
     </data>
-
-    There are any number of "resheader" rows that contain simple
+                
+    There are any number of "resheader" rows that contain simple 
     name/value pairs.
-
-    Each data row contains a name, and value. The row also contains a
-    type or mimetype. Type corresponds to a .NET class that support
-    text/value conversion through the TypeConverter architecture.
-    Classes that don't support this are serialized and stored with the
+    
+    Each data row contains a name, and value. The row also contains a 
+    type or mimetype. Type corresponds to a .NET class that support 
+    text/value conversion through the TypeConverter architecture. 
+    Classes that don't support this are serialized and stored with the 
     mimetype set.
-
-    The mimetype is used for serialized objects, and tells the
-    ResXResourceReader how to depersist the object. This is currently not
+    
+    The mimetype is used for serialized objects, and tells the 
+    ResXResourceReader how to depersist the object. This is currently not 
     extensible. For a given mimetype the value must be set accordingly:
-
-    Note - application/x-microsoft.net.object.binary.base64 is the format
-    that the ResXResourceWriter will generate, however the reader can
+    
+    Note - application/x-microsoft.net.object.binary.base64 is the format 
+    that the ResXResourceWriter will generate, however the reader can 
     read any of the formats listed below.
-
+    
     mimetype: application/x-microsoft.net.object.binary.base64
-    value   : The object must be serialized with
+    value   : The object must be serialized with 
             : System.Runtime.Serialization.Formatters.Binary.BinaryFormatter
             : and then encoded with base64 encoding.
-
+    
     mimetype: application/x-microsoft.net.object.soap.base64
-    value   : The object must be serialized with
+    value   : The object must be serialized with 
             : System.Runtime.Serialization.Formatters.Soap.SoapFormatter
             : and then encoded with base64 encoding.
 
     mimetype: application/x-microsoft.net.object.bytearray.base64
-    value   : The object must be serialized into a byte array
+    value   : The object must be serialized into a byte array 
             : using a System.ComponentModel.TypeConverter
             : and then encoded with base64 encoding.
     -->
@@ -6600,4 +6600,10 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
   <data name="WRN_DoNotCompareFunctionPointers_Title" xml:space="preserve">
     <value>Do not compare function pointer values</value>
   </data>
-</root>
+  <data name="IDS_FeatureUsingTypeAlias" xml:space="preserve">
+    <value>using type alias</value>
+  </data>
+  <data name="ERR_BadRefInUsingAlias" xml:space="preserve">
+    <value>Using alias cannot be a 'ref' type.</value>
+  </data>
+</root>

src/Compilers/CSharp/Portable/Compilation/SyntaxTreeSemanticModel.cs

@@ -1949,7 +1949,7 @@ public override IAliasSymbol GetDeclaredSymbol(
             else
             {
                 // Case: multiple aliases, not the first (see DevDiv #9368)
-                return new AliasSymbol(binder, declarationSyntax.Name, declarationSyntax.Alias).GetPublicSymbol();
+                return new AliasSymbol(binder, declarationSyntax.Type, declarationSyntax.Alias).GetPublicSymbol();
             }
         }
 

src/Compilers/CSharp/Portable/Errors/ErrorCode.cs

@@ -1931,6 +1931,12 @@ internal enum ErrorCode
 
         #endregion diagnostics introduced for C# 9.0
 
+        #region diagnostics introduced for C# preview
+
+        ERR_BadRefInUsingAlias = 9000,
+
+        #endregion 
+
         // Note: you will need to re-generate compiler code after adding warnings (eng\generate-compiler-code.cmd)
     }
 }

src/Compilers/CSharp/Portable/Errors/MessageID.cs

@@ -215,6 +215,8 @@ internal enum MessageID
         IDS_Return = MessageBase + 12790,
         IDS_FeatureVarianceSafetyForStaticInterfaceMembers = MessageBase + 12791,
         IDS_FeatureConstantInterpolatedStrings = MessageBase + 12792,
+
+        IDS_FeatureUsingTypeAlias = MessageBase + 12800,
     }
 
     // Message IDs may refer to strings that need to be localized.
@@ -348,6 +350,7 @@ internal static LanguageVersion RequiredVersion(this MessageID feature)
                 case MessageID.IDS_FeatureDefaultTypeParameterConstraint:
                     return LanguageVersion.CSharp9;
 
+                case MessageID.IDS_FeatureUsingTypeAlias: // syntax check
                 case MessageID.IDS_FeatureVarianceSafetyForStaticInterfaceMembers: //semantic check
                 case MessageID.IDS_FeatureConstantInterpolatedStrings: //semantic check
                     return LanguageVersion.Preview;