Merge pull request #5151 from brettfo/element-access-sig-help-with-qualified-name

when generating sig help, use SymbolInfo if TypeInfo resolution fails

src/EditorFeatures/CSharp/SignatureHelp/ElementAccessExpressionSignatureHelpProvider.cs

@@ -208,7 +208,10 @@ private bool TryGetIndexers(int position, SemanticModel semanticModel, Expressio
 
             if (expressionType is IErrorTypeSymbol)
             {
-                expressionType = (expressionType as IErrorTypeSymbol).CandidateSymbols.FirstOrDefault().GetSymbolType();
+                // If `expression` is a QualifiedNameSyntax then GetTypeInfo().Type won't have any CandidateSymbols, so
+                // we should then fall back to getting the actual symbol for the expression.
+                expressionType = (expressionType as IErrorTypeSymbol).CandidateSymbols.FirstOrDefault().GetSymbolType()
+                    ?? semanticModel.GetSymbolInfo(expression).GetAnySymbol().GetSymbolType();
             }
 
             indexers = semanticModel.LookupSymbols(position, expressionType, WellKnownMemberNames.Indexer).OfType<IPropertySymbol>();

src/EditorFeatures/CSharpTest/SignatureHelp/ElementAccessExpressionSignatureHelpProviderTests.cs

@@ -833,6 +833,30 @@ public void InvokedWithNoToken()
 
                 Test(markup);
             }
+
+            [WorkItem(2482, "https://github.com/dotnet/roslyn/issues/2482")]
+            [Fact, Trait(Traits.Feature, Traits.Features.SignatureHelp)]
+            public void WhereExpressionLooksLikeArrayTypeSyntaxOfQualifiedName()
+            {
+                var markup = @"
+class WithIndexer
+{
+    public int this[int index] { get { return 0; } }
+}
+
+class TestClass
+{
+    public WithIndexer Item { get; set; }
+
+    public void Method(TestClass tc)
+    {
+        // `tc.Item[]` parses as ArrayTypeSyntax with an ElementType of QualifiedNameSyntax
+        tc.Item[$$]
+    }
+}
+";
+                Test(markup, new[] { new SignatureHelpTestItem("int WithIndexer[int index]") }, usePreviousCharAsTrigger: true);
+            }
         }
     }
 }