Give the TypeInferenceService the ability to communicate additional info about the inferred type.

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

@@ -644,6 +644,20 @@ class C {
             await VerifyBuilderAsync(markup);
         }
 
+        [WorkItem(12818, "https://github.com/dotnet/roslyn/issues/12818")]
+        [Fact, Trait(Traits.Feature, Traits.Features.Completion)]
+        public async Task DoNotUnwrapRegularArray()
+        {
+            var markup = @"
+using System;
+class C {
+    C(Action<int>[] a) {
+        new C($$
+    }
+}";
+            await VerifyNotBuilderAsync(markup);
+        }
+
         private async Task VerifyNotBuilderAsync(string markup)
         {
             await VerifyWorkerAsync(markup, isBuilder: false);

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

@@ -157,19 +157,20 @@ private bool IsLambdaExpression(SemanticModel semanticModel, int position, Synta
 
             // If we're an argument to a function with multiple overloads, 
             // open the builder if any overload takes a delegate at our argument position
-            var inferredTypes = typeInferrer.InferTypes(semanticModel, position, cancellationToken: cancellationToken);
+            var inferredTypes = typeInferrer.GetTypeInferenceInfo(semanticModel, position, cancellationToken: cancellationToken);
 
             return inferredTypes.Any(type => GetDelegateType(type, semanticModel.Compilation).IsDelegateType());
         }
 
-        private ITypeSymbol GetDelegateType(ITypeSymbol type, Compilation compilation)
+        private ITypeSymbol GetDelegateType(TypeInferenceInfo type, Compilation compilation)
         {
-            if (type.IsArrayType())
+            ITypeSymbol typeSymbol = type.InferredType;
+            if (type.IsParams && type.InferredType.IsArrayType())
             {
-                type = ((IArrayTypeSymbol)type).ElementType;
+                typeSymbol = ((IArrayTypeSymbol)type.InferredType).ElementType;
             }
 
-            return type.GetDelegateType(compilation);
+            return typeSymbol.GetDelegateType(compilation);
         }
     }
 }

src/Workspaces/Core/Portable/LanguageServices/TypeInferenceService/AbstractTypeInferenceService.cs

@@ -13,36 +13,37 @@ internal abstract class AbstractTypeInferenceService<TExpressionSyntax> : ITypeI
     {
         protected abstract class AbstractTypeInferrer
         {
+
             protected readonly CancellationToken CancellationToken;
             protected readonly SemanticModel SemanticModel;
-            protected readonly Func<ITypeSymbol, bool> IsUsableTypeFunc;
+            protected readonly Func<TypeInferenceInfo, bool> IsUsableTypeFunc;
 
             private readonly HashSet<TExpressionSyntax> _seenExpressionInferType = new HashSet<TExpressionSyntax>();
             private readonly HashSet<TExpressionSyntax> _seenExpressionGetType = new HashSet<TExpressionSyntax>();
 
-            private static readonly Func<ITypeSymbol, bool> s_isNotNull = t => t != null;
+            private static readonly Func<TypeInferenceInfo, bool> s_isNotNull = t => t.InferredType != null;
 
             protected AbstractTypeInferrer(SemanticModel semanticModel, CancellationToken cancellationToken)
             {
                 this.SemanticModel = semanticModel;
                 this.CancellationToken = cancellationToken;
-                this.IsUsableTypeFunc = t => t != null && !IsUnusableType(t);
+                this.IsUsableTypeFunc = t => t.InferredType != null && !IsUnusableType(t.InferredType);
             }
 
-            protected abstract IEnumerable<ITypeSymbol> InferTypesWorker_DoNotCallDirectly(int position);
-            protected abstract IEnumerable<ITypeSymbol> InferTypesWorker_DoNotCallDirectly(TExpressionSyntax expression);
-            protected abstract IEnumerable<ITypeSymbol> GetTypes_DoNotCallDirectly(TExpressionSyntax expression, bool objectAsDefault);
+            protected abstract IEnumerable<TypeInferenceInfo> InferTypesWorker_DoNotCallDirectly(int position);
+            protected abstract IEnumerable<TypeInferenceInfo> InferTypesWorker_DoNotCallDirectly(TExpressionSyntax expression);
+            protected abstract IEnumerable<TypeInferenceInfo> GetTypes_DoNotCallDirectly(TExpressionSyntax expression, bool objectAsDefault);
             protected abstract bool IsUnusableType(ITypeSymbol arg);
 
             protected Compilation Compilation => SemanticModel.Compilation;
 
-            public IEnumerable<ITypeSymbol> InferTypes(int position)
+            public IEnumerable<TypeInferenceInfo> InferTypes(int position)
             {
                 var types = InferTypesWorker_DoNotCallDirectly(position);
                 return Filter(types);
             }
 
-            public IEnumerable<ITypeSymbol> InferTypes(TExpressionSyntax expression, bool filterUnusable = true)
+            public IEnumerable<TypeInferenceInfo> InferTypes(TExpressionSyntax expression, bool filterUnusable = true)
             {
                 if (expression != null)
                 {
@@ -53,20 +54,20 @@ public IEnumerable<ITypeSymbol> InferTypes(TExpressionSyntax expression, bool fi
                     }
                 }
 
-                return SpecializedCollections.EmptyEnumerable<ITypeSymbol>();
+                return SpecializedCollections.EmptyEnumerable<TypeInferenceInfo>();
             }
 
-            protected IEnumerable<ITypeSymbol> GetTypes(TExpressionSyntax expression, bool objectAsDefault = false)
+            protected IEnumerable<TypeInferenceInfo> GetTypes(TExpressionSyntax expression, bool objectAsDefault = false)
             {
                 if (_seenExpressionGetType.Add(expression))
                 {
                     return GetTypes_DoNotCallDirectly(expression, objectAsDefault);
                 }
 
-                return SpecializedCollections.EmptyEnumerable<ITypeSymbol>();
+                return SpecializedCollections.EmptyEnumerable<TypeInferenceInfo>();
             }
 
-            private IEnumerable<ITypeSymbol> Filter(IEnumerable<ITypeSymbol> types, bool filterUnusable = true)
+            private IEnumerable<TypeInferenceInfo> Filter(IEnumerable<TypeInferenceInfo> types, bool filterUnusable = true)
             {
                 return types.Where(filterUnusable ? IsUsableTypeFunc : s_isNotNull)
                             .Distinct()
@@ -95,10 +96,20 @@ protected IEnumerable<ITypeSymbol> ExpandParamsParameter(IParameterSymbol parame
 
         public IEnumerable<ITypeSymbol> InferTypes(SemanticModel semanticModel, int position, CancellationToken cancellationToken)
         {
-            return CreateTypeInferrer(semanticModel, cancellationToken).InferTypes(position);
+            return CreateTypeInferrer(semanticModel, cancellationToken).InferTypes(position).Select(t => t.InferredType);
         }
 
         public IEnumerable<ITypeSymbol> InferTypes(SemanticModel semanticModel, SyntaxNode expression, CancellationToken cancellationToken)
+        {
+            return CreateTypeInferrer(semanticModel, cancellationToken).InferTypes(expression as TExpressionSyntax).Select(t => t.InferredType);
+        }
+
+        public IEnumerable<TypeInferenceInfo> GetTypeInferenceInfo(SemanticModel semanticModel, int position, CancellationToken cancellationToken)
+        {
+            return CreateTypeInferrer(semanticModel, cancellationToken).InferTypes(position);
+        }
+
+        public IEnumerable<TypeInferenceInfo> GetTypeInferenceInfo(SemanticModel semanticModel, SyntaxNode expression, CancellationToken cancellationToken)
         {
             return CreateTypeInferrer(semanticModel, cancellationToken).InferTypes(expression as TExpressionSyntax);
         }

src/Workspaces/Core/Portable/LanguageServices/TypeInferenceService/ITypeInferenceService.cs

@@ -24,5 +24,28 @@ internal interface ITypeInferenceService : ILanguageService
     {
         IEnumerable<ITypeSymbol> InferTypes(SemanticModel semanticModel, SyntaxNode expression, CancellationToken cancellationToken);
         IEnumerable<ITypeSymbol> InferTypes(SemanticModel semanticModel, int position, CancellationToken cancellationToken);
+
+        IEnumerable<TypeInferenceInfo> GetTypeInferenceInfo(SemanticModel semanticModel, int position, CancellationToken cancellationToken);
+
+        IEnumerable<TypeInferenceInfo> GetTypeInferenceInfo(SemanticModel semanticModel, SyntaxNode expression, CancellationToken cancellationToken);
+    }
+
+    internal struct TypeInferenceInfo
+    {
+        private bool isParams;
+
+        public TypeInferenceInfo(ITypeSymbol type) : this()
+        {
+            InferredType = type;
+        }
+
+        public TypeInferenceInfo(ITypeSymbol type, bool isParams) : this()
+        {
+            InferredType = type;
+            IsParams = isParams;
+        }
+
+        public ITypeSymbol InferredType { get; }
+        public bool IsParams { get; }
     }
 }