Fix #12818

Unwrap arrays when determining whether to show the builder for lambda
expressions.

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

@@ -630,6 +630,20 @@ public async Task PartialInterfaceName()
             await VerifyBuilderAsync(markup);
         }
 
+        [WorkItem(12818, "https://github.com/dotnet/roslyn/issues/12818")]
+        [Fact, Trait(Traits.Feature, Traits.Features.Completion)]
+        public async Task UnwrapParamsArray()
+        {
+            var markup = @"
+using System;
+class C {
+    C(params Action<int>[] a) {
+        new C($$
+    }
+}";
+            await VerifyBuilderAsync(markup);
+        }
+
         private async Task VerifyNotBuilderAsync(string markup)
         {
             await VerifyWorkerAsync(markup, isBuilder: false);

src/Features/CSharp/Portable/Completion/SuggestionMode/CSharpSuggestionModeCompletionProvider.cs

@@ -159,7 +159,17 @@ private bool IsLambdaExpression(SemanticModel semanticModel, int position, Synta
             // open the builder if any overload takes a delegate at our argument position
             var inferredTypes = typeInferrer.InferTypes(semanticModel, position, cancellationToken: cancellationToken);
 
-            return inferredTypes.Any(type => type.GetDelegateType(semanticModel.Compilation).IsDelegateType());
+            return inferredTypes.Any(type => GetDelegateType(type, semanticModel.Compilation).IsDelegateType());
+        }
+
+        private ITypeSymbol GetDelegateType(ITypeSymbol type, Compilation compilation)
+        {
+            if (type.IsArrayType())
+            {
+                type = ((IArrayTypeSymbol)type).ElementType;
+            }
+
+            return type.GetDelegateType(compilation);
         }
     }
 }