Support triggering after dot of qualified name

src/Features/Core/Portable/Completion/Providers/ImportCompletionProvider/AbstractExtensionMethodImportCompletionProvider.cs

@@ -3,7 +3,6 @@
 #nullable enable
 
 using System.Collections.Generic;
-using System.Collections.Immutable;
 using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 using System.Threading;
@@ -45,16 +44,41 @@ protected override bool ShouldProvideCompletion(Document document, SyntaxContext
         private static bool TryGetReceiverTypeSymbol(SyntaxContext syntaxContext, ISyntaxFactsService syntaxFacts, [NotNullWhen(true)] out ITypeSymbol? receiverTypeSymbol)
         {
             var parentNode = syntaxContext.TargetToken.Parent;
-            var expressionNode = syntaxFacts.GetExpressionOfMemberAccessExpression(parentNode, allowImplicitTarget: true);
+            var expressionNode = syntaxFacts.GetLeftSideOfDot(parentNode, allowImplicitTarget: true);
 
-            if (expressionNode != null)
+            if (expressionNode == null)
             {
-                receiverTypeSymbol = syntaxContext.SemanticModel.GetTypeInfo(expressionNode).Type;
-                return receiverTypeSymbol != null;
+                receiverTypeSymbol = null;
+                return false;
             }
 
-            receiverTypeSymbol = null;
-            return false;
+            receiverTypeSymbol = syntaxContext.SemanticModel.GetTypeInfo(expressionNode).Type;
+
+            if (receiverTypeSymbol is IErrorTypeSymbol errorTypeSymbol)
+            {
+                receiverTypeSymbol = errorTypeSymbol.CandidateSymbols.Select(s => GetSymbolType(s)).FirstOrDefault(s => s != null);
+            }
+
+            return receiverTypeSymbol != null;
+        }
+
+        private static ITypeSymbol? GetSymbolType(ISymbol symbol)
+        {
+            switch (symbol)
+            {
+                case ILocalSymbol localSymbol:
+                    return localSymbol.Type;
+                case IFieldSymbol fieldSymbol:
+                    return fieldSymbol.Type;
+                case IPropertySymbol propertySymbol:
+                    return propertySymbol.Type;
+                case IParameterSymbol parameterSymbol:
+                    return parameterSymbol.Type;
+                case IAliasSymbol aliasSymbol:
+                    return aliasSymbol.Target as ITypeSymbol;
+            }
+
+            return symbol as ITypeSymbol;
         }
 
         private CompletionItem Convert(SerializableImportCompletionItem serializableItem)

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

@@ -1267,6 +1267,12 @@ public SyntaxNode GetRightSideOfDot(SyntaxNode node)
                 (node as MemberAccessExpressionSyntax)?.Name;
         }
 
+        public SyntaxNode GetLeftSideOfDot(SyntaxNode node, bool allowImplicitTarget)
+        {
+            return (node as QualifiedNameSyntax)?.Left ??
+                (node as MemberAccessExpressionSyntax)?.Expression;
+        }
+
         public bool IsLeftSideOfExplicitInterfaceSpecifier(SyntaxNode node)
             => (node as NameSyntax).IsLeftSideOfExplicitInterfaceSpecifier();
 

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

@@ -174,6 +174,7 @@ internal interface ISyntaxFactsService : ILanguageService
 
         bool IsLeftSideOfDot(SyntaxNode node);
         SyntaxNode GetRightSideOfDot(SyntaxNode node);
+        SyntaxNode GetLeftSideOfDot(SyntaxNode node, bool allowImplicitTarget = false);
 
         bool IsRightSideOfQualifiedName(SyntaxNode node);
         bool IsLeftSideOfExplicitInterfaceSpecifier(SyntaxNode node);

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

@@ -1312,6 +1312,11 @@ Namespace Microsoft.CodeAnalysis.VisualBasic
                       TryCast(node, MemberAccessExpressionSyntax)?.Name)
         End Function
 
+        Public Function GetLeftSideOfDot(node As SyntaxNode, Optional allowImplicitTarget As Boolean = False) As SyntaxNode Implements ISyntaxFactsService.GetLeftSideOfDot
+            Return If(TryCast(node, QualifiedNameSyntax)?.Left,
+                      TryCast(node, MemberAccessExpressionSyntax)?.GetExpressionOfMemberAccessExpression(allowImplicitTarget))
+        End Function
+
         Public Function IsLeftSideOfExplicitInterfaceSpecifier(node As SyntaxNode) As Boolean Implements ISyntaxFactsService.IsLeftSideOfExplicitInterfaceSpecifier
             Return IsLeftSideOfDot(node) AndAlso TryCast(node.Parent.Parent, ImplementsClauseSyntax) IsNot Nothing
         End Function