Fix override completion to preserve added imports

src/EditorFeatures/CSharpTest/Completion/CompletionProviders/OverrideCompletionProviderTests.cs

@@ -2979,5 +2979,68 @@ public override void M(in int x)
 
             Assert.Equal(after, actualCodeAfterCommit);
         }
+
+        [WorkItem(39909, "https://github.com/dotnet/roslyn/issues/39909")]
+        [WpfFact, Trait(Traits.Feature, Traits.Features.Completion)]
+        public async Task CommitAddsMissingImports()
+        {
+            var markupBeforeCommit = @"
+namespace NS1
+{
+    using NS2;
+
+    public class Foo
+    {
+        public virtual bool Bar(Baz baz) => true;
+    }
+}
+
+namespace NS2
+{
+    public class Baz {}
+}
+
+namespace NS3
+{
+    using NS1;
+
+    class D : Foo
+    {
+        override $$
+    }
+}";
+
+            var expectedCodeAfterCommit = @"
+namespace NS1
+{
+    using NS2;
+
+    public class Foo
+    {
+        public virtual bool Bar(Baz baz) => true;
+    }
+}
+
+namespace NS2
+{
+    public class Baz {}
+}
+
+namespace NS3
+{
+    using NS1;
+    using NS2;
+
+    class D : Foo
+    {
+        public override bool Bar(Baz baz)
+        {
+            return base.Bar(baz);$$
+        }
+    }
+}";
+
+            await VerifyCustomCommitProviderAsync(markupBeforeCommit, "Bar(NS2.Baz baz)", expectedCodeAfterCommit);
+        }
     }
 }

src/Features/Core/Portable/Completion/Providers/AbstractMemberInsertingCompletionProvider.cs

@@ -168,7 +168,7 @@ private TextSpan ComputeDestinationSpan(SyntaxNode insertionRoot)
         private async Task<string> GenerateInsertionTextAsync(
             Document memberContainingDocument, CancellationToken cancellationToken)
         {
-            memberContainingDocument = await Simplifier.ReduceAsync(memberContainingDocument, Simplifier.Annotation, null, cancellationToken).ConfigureAwait(false);
+            memberContainingDocument = await Simplifier.ReduceAsync(memberContainingDocument, Simplifier.Annotation, optionSet: null, cancellationToken).ConfigureAwait(false);
             memberContainingDocument = await Formatter.FormatAsync(memberContainingDocument, Formatter.Annotation, cancellationToken: cancellationToken).ConfigureAwait(false);
 
             var root = await memberContainingDocument.GetSyntaxRootAsync(cancellationToken).ConfigureAwait(false);
@@ -178,8 +178,11 @@ private async Task<string> GenerateInsertionTextAsync(
         private async Task<SyntaxNode> GetTreeWithAddedSyntaxNodeRemovedAsync(
             Document document, CancellationToken cancellationToken)
         {
-            var root = await document.GetSyntaxRootAsync(cancellationToken).ConfigureAwait(false);
+            // Added imports are annotated for simplification too. Therefore, we simplify the document
+            // before removing added member node to preserve those imports in the document.
+            document = await Simplifier.ReduceAsync(document, Simplifier.Annotation, optionSet: null, cancellationToken).ConfigureAwait(false);
 
+            var root = await document.GetSyntaxRootAsync(cancellationToken).ConfigureAwait(false);
             var members = root.GetAnnotatedNodesAndTokens(_annotation)
                               .AsImmutable()
                               .Select(m => m.AsNode());
@@ -188,7 +191,6 @@ private async Task<SyntaxNode> GetTreeWithAddedSyntaxNodeRemovedAsync(
 
             var dismemberedDocument = document.WithSyntaxRoot(root);
 
-            dismemberedDocument = await Simplifier.ReduceAsync(dismemberedDocument, Simplifier.Annotation, null, cancellationToken).ConfigureAwait(false);
             dismemberedDocument = await Formatter.FormatAsync(dismemberedDocument, Formatter.Annotation, cancellationToken: cancellationToken).ConfigureAwait(false);
             return await dismemberedDocument.GetSyntaxRootAsync(cancellationToken).ConfigureAwait(false);
         }