When gathering symbols for invocation expression signature help, consider the...

When gathering symbols for invocation expression signature help, consider the left expression explicitly, not just by name

src/EditorFeatures/CSharp/SignatureHelp/InvocationExpressionSignatureHelpProvider_MethodGroup.cs

@@ -43,7 +43,8 @@ internal partial class InvocationExpressionSignatureHelpProvider
                 }
 
                 var includeInstance = !throughExpression.IsKind(SyntaxKind.IdentifierName) ||
-                    semanticModel.LookupSymbols(throughExpression.SpanStart, name: throughSymbol.Name).Any(s => !(s is INamedTypeSymbol));
+                    semanticModel.LookupSymbols(throughExpression.SpanStart, name: throughSymbol.Name).Any(s => !(s is INamedTypeSymbol)) ||
+                    (!(throughSymbol is INamespaceOrTypeSymbol) && semanticModel.LookupSymbols(throughExpression.SpanStart, container: throughSymbol.ContainingType).Any(s => !(s is INamedTypeSymbol)));
 
                 var includeStatic = throughSymbol is INamedTypeSymbol ||
                     (throughExpression.IsKind(SyntaxKind.IdentifierName) &&

src/EditorFeatures/CSharpTest/SignatureHelp/InvocationExpressionSignatureHelpProviderTests.cs

@@ -1947,5 +1947,29 @@ void M()
 
             Test(markup, expectedOrderedItems);
         }
+
+        [WorkItem(4144, "https://github.com/dotnet/roslyn/issues/4144")]
+        [Fact, Trait(Traits.Feature, Traits.Features.SignatureHelp)]
+        public void TestSigHelpIsVisibleOnInaccessibleItem()
+        {
+            var markup = @"
+using System.Collections.Generic;
+
+class A
+{
+    List<int> args;
+}
+
+class B : A
+{
+    void M()
+    {
+        args.Add($$
+    }
+}
+";
+
+            Test(markup, new[] { new SignatureHelpTestItem("void List<int>.Add(int item)") });
+        }
     }
 }

src/EditorFeatures/VisualBasicTest/SignatureHelp/InvocationExpressionSignatureHelpProviderTests.vb

@@ -1849,5 +1849,27 @@ End Class
 "
             Test(markup, SpecializedCollections.SingletonEnumerable(New SignatureHelpTestItem("C.Next()", String.Empty)))
         End Sub
+
+        <WorkItem(4144, "https://github.com/dotnet/roslyn/issues/4144")>
+        <Fact, Trait(Traits.Feature, Traits.Features.SignatureHelp)>
+        Public Sub TestSigHelpIsVisibleOnInaccessibleItem()
+            Dim markup = "
+Imports System.Collections.Generic
+
+Class A
+    Dim args As List(Of Integer)
+End Class
+
+Class B
+    Inherits A
+
+    Sub M()
+        args.Add($$
+    End Sub
+End Class
+"
+
+            Test(markup, {New SignatureHelpTestItem("List(Of Integer).Add(item As Integer)")})
+        End Sub
     End Class
 End Namespace