Fix ValueTracking for index parameters (dotnet#57727)

Index values weren't properly handled previously, and it was assumed all parameters existed in a method. In reality some exist in properties. Who knew!??!

No need to worry about the cast, symbol finder will do the right thing. Code does need to be updated to handle the fact that we want to track potentially two things with index: the argument passed in as the index in callsites, and potentially the item being indexed to. Update the FindReferencesProgress to handle references for index specifically.

Fixes dotnet#57100

src/EditorFeatures/Test/ValueTracking/CSharpValueTrackingTests.cs

@@ -959,5 +959,100 @@ public static int GetM()
 
             await ValidateChildrenEmptyAsync(workspace, items.Single());
         }
+
+        [Theory]
+        [CombinatorialData]
+        public async Task TestIndex(TestHost testHost)
+        {
+            var code =
+@"
+class Test
+{
+    public int this[string $$key] => 0;
+
+    public int M(Test localTest)
+    {
+        var assignedVariable = this[""test""];
+        System.Console.WriteLine(this[""test""]);
+        
+        return localTest[""test""];
+    }
+}";
+
+            //
+            //  |> public int this[string [|key|]] => 0; [Code.cs:4]
+            //    |> return [|localTest|][[|"test"|]]; [Code.cs:10]
+            //    |> System.Console.WriteLine(this[[|"test"|]]); [Code.cs:8]
+            //    |> var [|assignedVariable = this["test"]|]; [Code.cs:7]
+            using var workspace = CreateWorkspace(code, testHost);
+
+            var items = await ValidateItemsAsync(
+                workspace,
+                itemInfo: new[]
+                {
+                    (3, "string key") // |>public int this[[|string key|]] => 0; [Code.cs:4]
+                });
+
+            items = await ValidateChildrenAsync(
+                workspace,
+                items.Single(),
+                childInfo: new[]
+                {
+                    (10, "localTest"), // return [|localTest|]["test"]; [Code.cs:10] (This is included because it is part of a return statement, and follows same logic as other references for if it is tracked)
+                    (10, "\"test\""),  // return localTest[[|"test"|]]; [Code.cs:10]
+                    (8, "\"test\""),   // System.Console.WriteLine(this[[|"test"|]]); [Code.cs:8]
+                    (7, "\"test\""),   // var assignedVariable = this[[|"test"|]]; [Code.cs:7]
+                });
+
+            await ValidateChildrenEmptyAsync(workspace, items[0]);
+            await ValidateChildrenEmptyAsync(workspace, items[1]);
+            await ValidateChildrenEmptyAsync(workspace, items[2]);
+            await ValidateChildrenEmptyAsync(workspace, items[3]);
+        }
+
+        [Theory]
+        [CombinatorialData]
+        public async Task TestPropertyValue(TestHost testHost)
+        {
+            var code =
+@"
+class Test
+{
+    private int _i;
+    public int I 
+    {
+        get => _i;
+        set 
+        {
+            _i = $$value;
+        }
+    }
+
+    public int M(Test localTest)
+    {
+        localTest.I = 5;
+    }
+}";
+            //  _i = [|value|]; [Code.cs:9]
+            //    |> localTest.I = [|5|]; [Code.cs:15]
+            using var workspace = CreateWorkspace(code, testHost);
+
+            var items = await ValidateItemsAsync(
+                workspace,
+                itemInfo: new[]
+                {
+                    (9, "value") // _i = [|value|]; [Code.cs:9]
+                });
+
+            items = await ValidateChildrenAsync(
+                workspace,
+                items.Single(),
+                childInfo: new[]
+                {
+                    (15, "5") // localTest.I = [|5|]; [Code.cs:15]
+                });
+
+            await ValidateChildrenEmptyAsync(workspace, items.Single());
+        }
     }
 }

src/EditorFeatures/Test/ValueTracking/VisualBasicValueTrackingTests.cs

@@ -58,6 +58,58 @@ End Class
             ValidateItem(initialItems[1], 3);
         }
 
+        [Theory]
+        [CombinatorialData]
+        public async Task TestPropertyValue(TestHost testHost)
+        {
+            var code =
+@"
+Class C
+    Private _s As String
+    Public Property S() As String
+        Get
+            Return _s
+        End Get
+        Set(ByVal value As String)
+            _s = $$value
+        End Set
+    End Property
+
+    
+    Public Sub SetS(s As String)
+        Me.S = s
+    End Sub
+
+    Public Function GetS() As String
+        Return Me.S
+    End Function
+End Class
+";
+
+            //
+            // _s = value [Code.vb:8]
+            //  |> Me.S = s [Code.vb:14]
+            //
+            using var workspace = CreateWorkspace(code, testHost);
+
+            var items = await ValidateItemsAsync(
+                workspace,
+                itemInfo: new[]
+                {
+                    (8, "value") // _s = [|value|] [Code.vb:8]
+                });
+
+            var childItems = await ValidateChildrenAsync(
+                workspace,
+                items.Single(),
+                childInfo: new[]
+                {
+                    (14, "s") // Me.S = [|s|] [Code.vb:14]
+                });
+
+            await ValidateChildrenEmptyAsync(workspace, childItems.Single());
+        }
+
         [Theory]
         [CombinatorialData]
         public async Task TestField(TestHost testHost)

src/Features/Core/Portable/ValueTracking/ValueTracker.FindReferencesProgress.cs

@@ -59,7 +59,7 @@ public async ValueTask OnReferenceFoundAsync(SymbolGroup _, ISymbol symbol, Refe
                 else if (location.IsWrittenTo)
                 {
                     var syntaxFacts = location.Document.GetRequiredLanguageService<ISyntaxFactsService>();
-                    var node = location.Location.FindNode(CancellationToken.None);
+                    var node = location.Location.FindNode(cancellationToken);
 
                     // Assignments to a member using a "this." or "Me." result in the node being an
                     // identifier and the parent of the node being the member access expression. The member
@@ -86,6 +86,53 @@ public async ValueTask OnReferenceFoundAsync(SymbolGroup _, ISymbol symbol, Refe
                         await _operationCollector.VisitAsync(operation, cancellationToken).ConfigureAwait(false);
                     }
                 }
+                else if (symbol is IPropertySymbol { IsIndexer: true } propertySymbol)
+                {
+                    // If the location isn't written to but it's an index accessor we still want to follow
+                    // it as a source to be tracked for the property itself. Specifically, we want to check
+                    // invocation sites and track the arguments being used in the invocation and potentially
+                    // the expression being indexed to, if it is relavent (as determined by the OperationCollector).
+                    var node = location.Location.FindNode(cancellationToken);
+                    var syntaxFacts = location.Document.GetRequiredLanguageService<ISyntaxFactsService>();
+
+                    var elementAccess = node.FirstAncestorOrSelf<SyntaxNode>(syntaxFacts.IsElementAccessExpression);
+                    if (elementAccess is null)
+                    {
+                        return;
+                    }
+
+                    syntaxFacts.GetPartsOfElementAccessExpression(elementAccess, out var expression, out var argumentList);
+                    var semanticModel = await location.Document.GetRequiredSemanticModelAsync(cancellationToken).ConfigureAwait(false);
+
+                    if (argumentList is not null)
+                    {
+                        foreach (var argument in syntaxFacts.GetArgumentsOfArgumentList(argumentList))
+                        {
+                            var argumentOperation = semanticModel.GetOperation(argument, cancellationToken);
+                            if (argumentOperation is not null)
+                            {
+                                await _operationCollector.VisitAsync(argumentOperation, cancellationToken).ConfigureAwait(false);
+                            }
+                        }
+                    }
+
+                    if (expression is not null)
+                    {
+                        // We do not want to track "this" as part of an index operation, but we do 
+                        // want to track other variables that are accessed. Arguably they "contribute" even if
+                        // not specifically to the argument of the element access.
+                        if (syntaxFacts.IsThisExpression(expression))
+                        {
+                            return;
+                        }
+
+                        var expressionOperation = semanticModel.GetOperation(expression, cancellationToken);
+                        if (expressionOperation is not null)
+                        {
+                            await _operationCollector.VisitAsync(expressionOperation, cancellationToken).ConfigureAwait(false);
+                        }
+                    }
+                }
             }
 
             public ValueTask OnStartedAsync(CancellationToken _) => new();

src/Features/Core/Portable/ValueTracking/ValueTracker.cs

@@ -177,10 +177,10 @@ private static async Task TrackParameterSymbolAsync(
             OperationCollector collector,
             CancellationToken cancellationToken)
         {
-            var containingMethod = (IMethodSymbol)parameterSymbol.ContainingSymbol;
+            var containingSymbol = parameterSymbol.ContainingSymbol;
             var findReferenceProgressCollector = new FindReferencesProgress(collector);
             await SymbolFinder.FindReferencesAsync(
-                containingMethod,
+                containingSymbol,
                 collector.Solution,
                 findReferenceProgressCollector,
                 documents: null, FindReferencesSearchOptions.Default, cancellationToken).ConfigureAwait(false);