Only allow dictionary initializers in C# 7 and up.

src/EditorFeatures/CSharpTest/UseCollectionInitializer/UseCollectionInitializerTests.cs

@@ -77,6 +77,22 @@ void M()
 }");
         }
 
+        [Fact, Trait(Traits.Feature, Traits.Features.CodeActionsUseCollectionInitializer)]
+        public async Task TestIndexAccess1_NotInCSharp5()
+        {
+            await TestMissingAsync(
+@"
+using System.Collections.Generic;
+class C
+{
+    void M()
+    {
+        var c = [||]new List<int>();
+        c[1] = 2;
+    }
+}", parseOptions: CSharpParseOptions.Default.WithLanguageVersion(LanguageVersion.CSharp5));
+        }
+
         [Fact, Trait(Traits.Feature, Traits.Features.CodeActionsUseCollectionInitializer)]
         public async Task TestComplexIndexAccess1()
         {

src/Features/Core/Portable/UseCollectionInitializer/AbstractUseCollectionInitializerDiagnosticAnalyzer.cs

@@ -268,7 +268,7 @@ private void AddMatches(ArrayBuilder<TExpressionStatementSyntax> matches)
             out SyntaxNode instance)
         {
             instance = null;
-            if (!_syntaxFacts.SupportsIndexingInitializer)
+            if (!_syntaxFacts.SupportsIndexingInitializer(statement.SyntaxTree.Options))
             {
                 return false;
             }

src/Workspaces/CSharp/Portable/LanguageServices/CSharpSyntaxFactsService.cs

@@ -30,7 +30,8 @@ private CSharpSyntaxFactsService()
 
         public bool IsCaseSensitive => true;
 
-        public bool SupportsIndexingInitializer => true;
+        public bool SupportsIndexingInitializer(ParseOptions options) 
+            => ((CSharpParseOptions)options).LanguageVersion >= LanguageVersion.CSharp6;
 
         public bool IsAwaitKeyword(SyntaxToken token)
         {

src/Workspaces/Core/Portable/LanguageServices/SyntaxFactsService/ISyntaxFactsService.cs

@@ -12,7 +12,7 @@ namespace Microsoft.CodeAnalysis.LanguageServices
     internal interface ISyntaxFactsService : ILanguageService
     {
         bool IsCaseSensitive { get; }
-        bool SupportsIndexingInitializer { get; }
+        bool SupportsIndexingInitializer(ParseOptions options);
 
         bool IsAwaitKeyword(SyntaxToken token);
         bool IsIdentifier(SyntaxToken token);

src/Workspaces/VisualBasic/Portable/LanguageServices/VisualBasicSyntaxFactsService.vb

@@ -41,11 +41,9 @@ Namespace Microsoft.CodeAnalysis.VisualBasic
             End Get
         End Property
 
-        Public ReadOnly Property SupportsIndexingInitializer As Boolean Implements ISyntaxFactsService.SupportsIndexingInitializer
-            Get
-                Return False
-            End Get
-        End Property
+        Public Function SupportsIndexingInitializer(options As ParseOptions) As Boolean Implements ISyntaxFactsService.SupportsIndexingInitializer
+            Return False
+        End Function
 
         Public Function IsAwaitKeyword(token As SyntaxToken) As Boolean Implements ISyntaxFactsService.IsAwaitKeyword
             Return token.Kind = SyntaxKind.AwaitKeyword