Fix expander to not crash when it encounters alias that binds to a missing namespace.

Fixes #1913

When expander encounters an alias identifier that binds to a unbound namespace, it would fall through a code path that would throw NotImplementedException. This change fixes the expander to 'handle' this case without crashing (by simply retruning the IdentifierNameSyntax for the namespace that the alias binds to).

I discovered a difference in behavior between VB and C# SemanticModel which means that although I added a corresponding change to VB's expander, my fix is never invoked. I have logged #2805 for the difference and added a skipped test.

src/EditorFeatures/Test2/Expansion/NameExpansionTests.vb

@@ -50,6 +50,193 @@ class C
             Test(input, expected)
         End Sub
 
+        <WorkItem(1913, "https://github.com/dotnet/roslyn/issues/1913")>
+        <Fact, Trait(Traits.Feature, Traits.Features.Expansion)>
+        Public Sub CSharp_SimpleIdentifierAliasExpansion_AliasBinds()
+            Dim input =
+<Workspace>
+    <Project Language="C#" CommonReferences="true">
+        <Document>
+using System;
+namespace NS
+{
+    using Short = LongNamespace;
+    class Test
+    {
+        public object Method1()
+        {
+            return (new {|Expand:Short|}.MyClass()).Prop;
+        }
+    }
+}
+namespace LongNamespace
+{
+    public class MyClass
+    {
+        public object Prop { get; }
+    }
+}
+        </Document>
+    </Project>
+</Workspace>
+
+            Dim expected =
+<code>
+using System;
+namespace NS
+{
+    using Short = LongNamespace;
+    class Test
+    {
+        public object Method1()
+        {
+            return (new global::LongNamespace.MyClass()).Prop;
+        }
+    }
+}
+namespace LongNamespace
+{
+    public class MyClass
+    {
+        public object Prop { get; }
+    }
+}
+</code>
+
+            Test(input, expected)
+        End Sub
+
+        <WorkItem(1913, "https://github.com/dotnet/roslyn/issues/1913")>
+        <Fact, Trait(Traits.Feature, Traits.Features.Expansion)>
+        Public Sub CSharp_SimpleIdentifierAliasExpansion_AliasDoesNotBind()
+            Dim input =
+<Workspace>
+    <Project Language="C#" CommonReferences="true">
+        <Document>
+using System;
+namespace NS
+{
+    using Short = LongNamespace;
+    class Test
+    {
+        public object Method1()
+        {
+            return (new {|Expand:Short|}.MyClass()).Prop;
+        }
+    }
+}
+        </Document>
+    </Project>
+</Workspace>
+
+            Dim expected =
+<code>
+using System;
+namespace NS
+{
+    using Short = LongNamespace;
+    class Test
+    {
+        public object Method1()
+        {
+            return (new LongNamespace.MyClass()).Prop;
+        }
+    }
+}
+</code>
+
+            Test(input, expected)
+        End Sub
+
+        <WorkItem(1913, "https://github.com/dotnet/roslyn/issues/1913")>
+        <Fact, Trait(Traits.Feature, Traits.Features.Expansion)>
+        Public Sub VisualBasic_SimpleIdentifierAliasExpansion_AliasBinds()
+            Dim input =
+<Workspace>
+    <Project Language="Visual Basic" CommonReferences="true">
+        <Document>
+Imports System
+Imports ShortName = LongNamespace
+Namespace NS
+    Class Test
+        Public Function Method1() As Object
+            Return (New {|Expand:ShortName|}.Class1()).Prop
+        End Function
+    End Class
+End Namespace
+Namespace LongNamespace
+    Public Class Class1
+        Public Readonly Property Prop As Object
+            Get
+                Return Nothing
+            End Get
+        End Property
+    End Class
+End Namespace
+        </Document>
+    </Project>
+</Workspace>
+
+            Dim expected =
+<code>
+Imports System
+Imports ShortName = LongNamespace
+Namespace NS
+    Class Test
+        Public Function Method1() As Object
+            Return (New Global.LongNamespace.Class1()).Prop
+        End Function
+    End Class
+End Namespace
+Namespace LongNamespace
+    Public Class Class1
+        Public Readonly Property Prop As Object
+            Get
+                Return Nothing
+            End Get
+        End Property
+    End Class
+End Namespace
+</code>
+
+            Test(input, expected)
+        End Sub
+
+        <WorkItem(1913, "https://github.com/dotnet/roslyn/issues/1913")>
+        <Fact(Skip:="https://github.com/dotnet/roslyn/issues/2805"), Trait(Traits.Feature, Traits.Features.Expansion)>
+        Public Sub VisualBasic_SimpleIdentifierAliasExpansion_AliasDoesNotBind()
+            Dim input =
+<Workspace>
+    <Project Language="Visual Basic" CommonReferences="true">
+        <Document>
+Imports System
+Imports ShortName = LongNamespace
+Namespace NS
+    Class Test
+        Public Function Method1() As Object
+            Return (New {|Expand:ShortName|}.Class1()).Prop
+        End Function
+    End Class
+End Namespace
+        </Document>
+    </Project>
+</Workspace>
+
+            Dim expected =
+<code>
+Imports System
+Imports ShortName = LongNamespace
+Namespace NS
+    Class Test
+        Public Function Method1() As Object
+            Return (New LongNamespace.Class1()).Prop
+        End Function
+    End Class
+End Namespace
+</code>
+
+            Test(input, expected)
+        End Sub
 #End Region
 
     End Class

src/Workspaces/CSharp/Portable/Simplification/CSharpSimplificationService.Expander.cs

@@ -10,14 +10,11 @@
 using Microsoft.CodeAnalysis.CSharp;
 using Microsoft.CodeAnalysis.CSharp.Extensions;
 using Microsoft.CodeAnalysis.CSharp.Extensions.ContextQuery;
-using Microsoft.CodeAnalysis.CSharp.Rename;
 using Microsoft.CodeAnalysis.CSharp.Symbols;
 using Microsoft.CodeAnalysis.CSharp.Syntax;
 using Microsoft.CodeAnalysis.CSharp.Utilities;
-using Microsoft.CodeAnalysis.Rename.ConflictEngine;
 using Microsoft.CodeAnalysis.Shared.Extensions;
 using Microsoft.CodeAnalysis.Simplification;
-using Roslyn.Utilities;
 
 namespace Microsoft.CodeAnalysis.CSharp.Simplification
 {
@@ -381,6 +378,30 @@ private ExpressionSyntax VisitSimpleName(SimpleNameSyntax rewrittenSimpleName, S
                             return replacement;
                         }
 
+                        if (replacement.IsKind(SyntaxKind.IdentifierName))
+                        {
+                            var identifierReplacement = (IdentifierNameSyntax)replacement;
+
+                            var newIdentifier = identifier.CopyAnnotationsTo(identifierReplacement.Identifier);
+
+                            if (_annotationForReplacedAliasIdentifier != null)
+                            {
+                                newIdentifier = newIdentifier.WithAdditionalAnnotations(_annotationForReplacedAliasIdentifier);
+                            }
+
+                            var aliasAnnotationInfo = AliasAnnotation.Create(aliasInfo.Name);
+
+                            newIdentifier = newIdentifier.WithAdditionalAnnotations(aliasAnnotationInfo);
+
+                            replacement = replacement.ReplaceToken(identifier, newIdentifier);
+
+                            replacement = newNode.CopyAnnotationsTo(replacement);
+
+                            replacement = AppendElasticTriviaIfNecessary(replacement, originalSimpleName);
+
+                            return replacement;
+                        }
+
                         throw new NotImplementedException();
                     }
                 }

src/Workspaces/VisualBasic/Portable/Simplification/VisualBasicSimplificationService.Expander.vb

@@ -3,9 +3,7 @@
 Imports System.Threading
 Imports Microsoft.CodeAnalysis
 Imports Microsoft.CodeAnalysis.Simplification
-Imports Microsoft.CodeAnalysis.VisualBasic.Symbols
 Imports Microsoft.CodeAnalysis.VisualBasic.Syntax
-Imports Microsoft.CodeAnalysis.VisualBasic.Rename
 Imports Microsoft.CodeAnalysis.VisualBasic.Utilities
 Imports Microsoft.CodeAnalysis.VisualBasic.Extensions.ContextQuery
 
@@ -468,6 +466,25 @@ Namespace Microsoft.CodeAnalysis.VisualBasic.Simplification
                             Return replacement
                         End If
 
+                        If replacement.IsKind(SyntaxKind.IdentifierName) Then
+                            Dim identifierReplacement = DirectCast(replacement, IdentifierNameSyntax)
+
+                            Dim newIdentifier = identifier.CopyAnnotationsTo(identifierReplacement.Identifier)
+
+                            If Me._annotationForReplacedAliasIdentifier IsNot Nothing Then
+                                newIdentifier = newIdentifier.WithAdditionalAnnotations(Me._annotationForReplacedAliasIdentifier)
+                            End If
+
+                            Dim aliasAnnotationInfo = AliasAnnotation.Create(aliasInfo.Name)
+                            newIdentifier = newIdentifier.WithAdditionalAnnotations(aliasAnnotationInfo)
+
+                            replacement = replacement.ReplaceToken(identifier, newIdentifier)
+
+                            replacement = newNode.CopyAnnotationsTo(replacement)
+
+                            Return replacement
+                        End If
+
                         Throw New NotImplementedException()
                     End If
                 End If