Merge pull request #20509 from rchande/sighelpCrash

Fix crash in element access signature help

src/EditorFeatures/CSharpTest/SignatureHelp/ElementAccessExpressionSignatureHelpProviderTests.cs

@@ -898,6 +898,48 @@ public void Method(TestClass tc)
 ";
                 await TestAsync(markup, new[] { new SignatureHelpTestItem("int WithIndexer[int index]") }, usePreviousCharAsTrigger: true);
             }
+
+            [WorkItem(20507, "https://github.com/dotnet/roslyn/issues/20507")]
+            [Fact, Trait(Traits.Feature, Traits.Features.SignatureHelp)]
+            public async Task InConditionalIndexingFollowedByMemberAccess()
+            {
+                var markup = @"
+class Indexable
+{
+    public Indexable this[int x] { get => null; }
+
+    Indexable Count;
+
+    static void Main(string[] args)
+    {
+        Indexable x;
+        x?[$$].Count;
+    }
+}
+";
+                await TestAsync(markup, new[] { new SignatureHelpTestItem("Indexable Indexable[int x]") }, usePreviousCharAsTrigger: false);
+            }
+
+            [WorkItem(20507, "https://github.com/dotnet/roslyn/issues/20507")]
+            [Fact, Trait(Traits.Feature, Traits.Features.SignatureHelp)]
+            public async Task InConditionalIndexingFollowedByConditionalAcesss()
+            {
+                var markup = @"
+class Indexable
+{
+    public Indexable this[int x] { get => null; }
+
+    Indexable Count;
+
+    static void Main(string[] args)
+    {
+        Indexable x;
+        x?[$$].Count?.Count;
+    }
+}
+";
+                await TestAsync(markup, new[] { new SignatureHelpTestItem("Indexable Indexable[int x]") }, usePreviousCharAsTrigger: false);
+            }
         }
     }
 }

src/Features/CSharp/Portable/SignatureHelp/ElementAccessExpressionSignatureHelpProvider.cs

@@ -372,7 +372,11 @@ internal static bool TryGetSyntax(SyntaxNode root, int position, ISyntaxFactsSer
             {
                 if (CommonSignatureHelpUtilities.TryGetSyntax(root, position, syntaxFacts, triggerReason, IsTriggerToken, IsArgumentListToken, cancellationToken, out ElementBindingExpressionSyntax elementBindingExpression))
                 {
-                    identifier = ((ConditionalAccessExpressionSyntax)elementBindingExpression.Parent).Expression;
+                    // Find the first conditional access expression that starts left of our open bracket
+                    var conditionalAccess = elementBindingExpression.FirstAncestorOrSelf<ConditionalAccessExpressionSyntax>(
+                        c => c.SpanStart < elementBindingExpression.SpanStart);
+
+                    identifier = conditionalAccess.Expression;
                     openBrace = elementBindingExpression.ArgumentList.OpenBracketToken;
 
                     return true;