Null annotate the RQName code

This annotation revealed a number of places where if some part of RQName
generation failed, we'd end up creating RQNode objects with null fields;
trying to convert that to the text would end up crashing. This adds
proper error handling when issues were discovered by analysis.

src/Features/Core/Portable/RQName/Nodes/RQArrayOrPointerType.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 namespace Microsoft.CodeAnalysis.Features.RQName.Nodes
 {
     internal abstract class RQArrayOrPointerType : RQType

src/Features/Core/Portable/RQName/Nodes/RQArrayType.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using Microsoft.CodeAnalysis.Features.RQName.SimpleTree;
 
 namespace Microsoft.CodeAnalysis.Features.RQName.Nodes

src/Features/Core/Portable/RQName/Nodes/RQConstructedType.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using System.Collections.Generic;
 using System.Collections.ObjectModel;
 using System.Linq;

src/Features/Core/Portable/RQName/Nodes/RQConstructor.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using System.Collections.Generic;
 
 namespace Microsoft.CodeAnalysis.Features.RQName.Nodes

src/Features/Core/Portable/RQName/Nodes/RQErrorType.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using Microsoft.CodeAnalysis.Features.RQName.SimpleTree;
 
 namespace Microsoft.CodeAnalysis.Features.RQName.Nodes

src/Features/Core/Portable/RQName/Nodes/RQEvent.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 namespace Microsoft.CodeAnalysis.Features.RQName.Nodes
 {
     internal class RQEvent : RQMethodPropertyOrEvent

src/Features/Core/Portable/RQName/Nodes/RQExplicitInterfaceMemberName.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using Microsoft.CodeAnalysis.Features.RQName.SimpleTree;
 
 namespace Microsoft.CodeAnalysis.Features.RQName.Nodes

src/Features/Core/Portable/RQName/Nodes/RQIndexer.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using System.Collections.Generic;
 
 namespace Microsoft.CodeAnalysis.Features.RQName.Nodes

src/Features/Core/Portable/RQName/Nodes/RQMember.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using System.Collections.Generic;
 using Microsoft.CodeAnalysis.Features.RQName.SimpleTree;
 

src/Features/Core/Portable/RQName/Nodes/RQMemberParameterIndex.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using System.Collections.Generic;
 using Microsoft.CodeAnalysis.Features.RQName.SimpleTree;
 

src/Features/Core/Portable/RQName/Nodes/RQMemberParameterIndexFromPartialImplementation.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using System.Collections.Generic;
 using Microsoft.CodeAnalysis.Features.RQName.SimpleTree;
 

src/Features/Core/Portable/RQName/Nodes/RQMemberParameterIndexFromPartialSignature.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using System.Collections.Generic;
 using Microsoft.CodeAnalysis.Features.RQName.SimpleTree;
 

src/Features/Core/Portable/RQName/Nodes/RQMemberVariable.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using System.Collections.Generic;
 using Microsoft.CodeAnalysis.Features.RQName.SimpleTree;
 

src/Features/Core/Portable/RQName/Nodes/RQMethod.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using System.Collections.Generic;
 
 namespace Microsoft.CodeAnalysis.Features.RQName.Nodes

src/Features/Core/Portable/RQName/Nodes/RQMethodBase.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using System.Collections.Generic;
 
 namespace Microsoft.CodeAnalysis.Features.RQName.Nodes

src/Features/Core/Portable/RQName/Nodes/RQMethodOrProperty.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using System.Collections.Generic;
 using System.Collections.ObjectModel;
 using System.Linq;

src/Features/Core/Portable/RQName/Nodes/RQMethodPropertyOrEvent.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using System.Collections.Generic;
 using Microsoft.CodeAnalysis.Features.RQName.SimpleTree;
 

src/Features/Core/Portable/RQName/Nodes/RQMethodPropertyOrEventName.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using Microsoft.CodeAnalysis.Features.RQName.SimpleTree;
 
 namespace Microsoft.CodeAnalysis.Features.RQName.Nodes

src/Features/Core/Portable/RQName/Nodes/RQNamespace.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using System.Collections.Generic;
 
 namespace Microsoft.CodeAnalysis.Features.RQName.Nodes

src/Features/Core/Portable/RQName/Nodes/RQNode.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using System.Collections.Generic;
 using Microsoft.CodeAnalysis.Features.RQName.SimpleTree;
 

src/Features/Core/Portable/RQName/Nodes/RQNormalParameter.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using Microsoft.CodeAnalysis.Features.RQName.SimpleTree;
 
 namespace Microsoft.CodeAnalysis.Features.RQName.Nodes

src/Features/Core/Portable/RQName/Nodes/RQNullType.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using Microsoft.CodeAnalysis.Features.RQName.SimpleTree;
 
 namespace Microsoft.CodeAnalysis.Features.RQName.Nodes

src/Features/Core/Portable/RQName/Nodes/RQOrdinaryMethodPropertyOrEventName.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using Microsoft.CodeAnalysis.Features.RQName.SimpleTree;
 
 namespace Microsoft.CodeAnalysis.Features.RQName.Nodes

src/Features/Core/Portable/RQName/Nodes/RQOutParameter.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using Microsoft.CodeAnalysis.Features.RQName.SimpleTree;
 
 namespace Microsoft.CodeAnalysis.Features.RQName.Nodes

src/Features/Core/Portable/RQName/Nodes/RQParameter.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using Microsoft.CodeAnalysis.Features.RQName.SimpleTree;
 
 namespace Microsoft.CodeAnalysis.Features.RQName.Nodes
@@ -11,7 +13,6 @@ internal abstract class RQParameter
         public readonly RQType Type;
         public RQParameter(RQType type)
         {
-            System.Diagnostics.Debug.Assert(type != null);
             Type = type;
         }
 

src/Features/Core/Portable/RQName/Nodes/RQPointerType.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using Microsoft.CodeAnalysis.Features.RQName.SimpleTree;
 
 namespace Microsoft.CodeAnalysis.Features.RQName.Nodes

src/Features/Core/Portable/RQName/Nodes/RQProperty.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using System.Collections.Generic;
 
 namespace Microsoft.CodeAnalysis.Features.RQName.Nodes

src/Features/Core/Portable/RQName/Nodes/RQPropertyBase.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using System.Collections.Generic;
 
 namespace Microsoft.CodeAnalysis.Features.RQName.Nodes

src/Features/Core/Portable/RQName/Nodes/RQRefParameter.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using Microsoft.CodeAnalysis.Features.RQName.SimpleTree;
 
 namespace Microsoft.CodeAnalysis.Features.RQName.Nodes

src/Features/Core/Portable/RQName/Nodes/RQType.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using System;
 using Microsoft.CodeAnalysis.Features.RQName.SimpleTree;
 

src/Features/Core/Portable/RQName/Nodes/RQTypeOrNamespace.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using System.Collections.Generic;
 using System.Collections.ObjectModel;
 using System.Linq;

src/Features/Core/Portable/RQName/Nodes/RQTypeVariableType.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using Microsoft.CodeAnalysis.Features.RQName.SimpleTree;
 
 namespace Microsoft.CodeAnalysis.Features.RQName.Nodes

src/Features/Core/Portable/RQName/Nodes/RQUnconstructedType.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using System.Collections.Generic;
 using System.Collections.ObjectModel;
 using System.Linq;

src/Features/Core/Portable/RQName/Nodes/RQVoidType.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using Microsoft.CodeAnalysis.Features.RQName.SimpleTree;
 
 namespace Microsoft.CodeAnalysis.Features.RQName.Nodes

src/Features/Core/Portable/RQName/Nodes/ResolvedRQNode.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 namespace Microsoft.CodeAnalysis.Features.RQName.Nodes
 {
     internal abstract class RQNode<ResolvedType> : UnresolvedRQNode

src/Features/Core/Portable/RQName/ParenthesesTreeWriter.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using System.Text;
 using Microsoft.CodeAnalysis.Features.RQName.SimpleTree;
 
@@ -11,11 +13,6 @@ internal static class ParenthesesTreeWriter
     {
         public static string ToParenthesesFormat(SimpleTreeNode tree)
         {
-            if (tree == null)
-            {
-                return null;
-            }
-
             var sb = new StringBuilder();
             WriteNode(tree, sb);
             return sb.ToString();

src/Features/Core/Portable/RQName/RQNameInternal.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 namespace Microsoft.CodeAnalysis.Features.RQName
 {
     /// <summary>
@@ -13,10 +15,16 @@ internal static class RQNameInternal
         /// Returns an RQName for the given symbol, or <see langword="null"/> if the symbol cannot be represented by an RQName.
         /// </summary>
         /// <param name="symbol">The symbol to build an RQName for.</param>
-        public static string From(ISymbol symbol)
+        public static string? From(ISymbol symbol)
         {
             var node = RQNodeBuilder.Build(symbol);
-            return ParenthesesTreeWriter.ToParenthesesFormat(node?.ToSimpleTree());
+
+            if (node == null)
+            {
+                return null;
+            }
+
+            return ParenthesesTreeWriter.ToParenthesesFormat(node.ToSimpleTree());
         }
     }
 }

src/Features/Core/Portable/RQName/RQNameStrings.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 namespace Microsoft.CodeAnalysis.Features.RQName
 {
     internal sealed class RQNameStrings

src/Features/Core/Portable/RQName/RQNodeBuilder.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using System.Collections.Generic;
 using System.Collections.Immutable;
 using System.Linq;
@@ -16,15 +18,15 @@ internal static class RQNodeBuilder
         /// Builds the RQName for a given symbol.
         /// </summary>
         /// <returns>The node if it could be created, otherwise null</returns>
-        public static UnresolvedRQNode Build(ISymbol symbol)
-            => symbol.Kind switch
+        public static UnresolvedRQNode? Build(ISymbol symbol)
+            => symbol switch
             {
-                SymbolKind.Namespace => BuildNamespace(symbol as INamespaceSymbol),
-                SymbolKind.NamedType => BuildNamedType(symbol as INamedTypeSymbol),
-                SymbolKind.Method => BuildMethod(symbol as IMethodSymbol),
-                SymbolKind.Field => BuildField(symbol as IFieldSymbol),
-                SymbolKind.Event => BuildEvent(symbol as IEventSymbol),
-                SymbolKind.Property => BuildProperty(symbol as IPropertySymbol),
+                INamespaceSymbol namespaceSymbol => BuildNamespace(namespaceSymbol),
+                INamedTypeSymbol namedTypeSymbol => BuildUnconstructedNamedType(namedTypeSymbol),
+                IMethodSymbol methodSymbol => BuildMethod(methodSymbol),
+                IFieldSymbol fieldSymbol => BuildField(fieldSymbol),
+                IEventSymbol eventSymbol => BuildEvent(eventSymbol),
+                IPropertySymbol propertySymbol => BuildProperty(propertySymbol),
                 _ => null,
             };
 
@@ -50,7 +52,7 @@ private static IList<string> GetNameParts(INamespaceSymbol @namespace)
             return parts;
         }
 
-        private static RQUnconstructedType BuildNamedType(INamedTypeSymbol type)
+        private static RQUnconstructedType? BuildUnconstructedNamedType(INamedTypeSymbol type)
         {
             // Anything that is a valid RQUnconstructed types is ALWAYS safe for public APIs
 
@@ -84,9 +86,9 @@ private static RQUnconstructedType BuildNamedType(INamedTypeSymbol type)
             return new RQUnconstructedType(namespaceNames, typeInfos);
         }
 
-        private static RQMember BuildField(IFieldSymbol symbol)
+        private static RQMember? BuildField(IFieldSymbol symbol)
         {
-            var containingType = BuildNamedType(symbol.ContainingType);
+            var containingType = BuildUnconstructedNamedType(symbol.ContainingType);
 
             if (containingType == null)
             {
@@ -96,7 +98,7 @@ private static RQMember BuildField(IFieldSymbol symbol)
             return new RQMemberVariable(containingType, symbol.Name);
         }
 
-        private static RQProperty BuildProperty(IPropertySymbol symbol)
+        private static RQProperty? BuildProperty(IPropertySymbol symbol)
         {
             RQMethodPropertyOrEventName name = symbol.IsIndexer ?
                 RQOrdinaryMethodPropertyOrEventName.CreateOrdinaryIndexerName() :
@@ -109,12 +111,17 @@ private static RQProperty BuildProperty(IPropertySymbol symbol)
                     return null;
                 }
 
-                name = new RQExplicitInterfaceMemberName(
-                    BuildType(symbol.ExplicitInterfaceImplementations.Single().ContainingType as ITypeSymbol),
-                    (RQOrdinaryMethodPropertyOrEventName)name);
+                var interfaceType = BuildType(symbol.ExplicitInterfaceImplementations.Single().ContainingType);
+
+                if (interfaceType != null)
+                {
+                    name = new RQExplicitInterfaceMemberName(
+                        interfaceType,
+                        (RQOrdinaryMethodPropertyOrEventName)name);
+                }
             }
 
-            var containingType = BuildNamedType(symbol.ContainingType);
+            var containingType = BuildUnconstructedNamedType(symbol.ContainingType);
 
             if (containingType == null)
             {
@@ -123,10 +130,15 @@ private static RQProperty BuildProperty(IPropertySymbol symbol)
 
             var parameterList = BuildParameterList(symbol.Parameters);
 
+            if (parameterList == null)
+            {
+                return null;
+            }
+
             return new RQProperty(containingType, name, typeParameterCount: 0, parameters: parameterList);
         }
 
-        private static IList<RQParameter> BuildParameterList(ImmutableArray<IParameterSymbol> parameters)
+        private static IList<RQParameter>? BuildParameterList(ImmutableArray<IParameterSymbol> parameters)
         {
             var parameterList = new List<RQParameter>();
 
@@ -134,6 +146,11 @@ private static IList<RQParameter> BuildParameterList(ImmutableArray<IParameterSy
             {
                 var parameterType = BuildType(parameter.Type);
 
+                if (parameterType == null)
+                {
+                    return null;
+                }
+
                 if (parameter.RefKind == RefKind.Out)
                 {
                     parameterList.Add(new RQOutParameter(parameterType));
@@ -151,9 +168,9 @@ private static IList<RQParameter> BuildParameterList(ImmutableArray<IParameterSy
             return parameterList;
         }
 
-        private static RQEvent BuildEvent(IEventSymbol symbol)
+        private static RQEvent? BuildEvent(IEventSymbol symbol)
         {
-            var containingType = BuildNamedType(symbol.ContainingType);
+            var containingType = BuildUnconstructedNamedType(symbol.ContainingType);
 
             if (containingType == null)
             {
@@ -169,13 +186,18 @@ private static RQEvent BuildEvent(IEventSymbol symbol)
                     return null;
                 }
 
-                name = new RQExplicitInterfaceMemberName(BuildType(symbol.ExplicitInterfaceImplementations.Single().ContainingType as ITypeSymbol), (RQOrdinaryMethodPropertyOrEventName)name);
+                var interfaceType = BuildType(symbol.ExplicitInterfaceImplementations.Single().ContainingType);
+
+                if (interfaceType != null)
+                {
+                    name = new RQExplicitInterfaceMemberName(interfaceType, (RQOrdinaryMethodPropertyOrEventName)name);
+                }
             }
 
             return new RQEvent(containingType, name);
         }
 
-        private static RQMethod BuildMethod(IMethodSymbol symbol)
+        private static RQMethod? BuildMethod(IMethodSymbol symbol)
         {
             if (symbol.MethodKind == MethodKind.UserDefinedOperator ||
                 symbol.MethodKind == MethodKind.BuiltinOperator ||
@@ -209,10 +231,15 @@ private static RQMethod BuildMethod(IMethodSymbol symbol)
                     return null;
                 }
 
-                name = new RQExplicitInterfaceMemberName(BuildType(symbol.ExplicitInterfaceImplementations.Single().ContainingType as ITypeSymbol), (RQOrdinaryMethodPropertyOrEventName)name);
+                var interfaceType = BuildType(symbol.ExplicitInterfaceImplementations.Single().ContainingType);
+
+                if (interfaceType != null)
+                {
+                    name = new RQExplicitInterfaceMemberName(interfaceType, (RQOrdinaryMethodPropertyOrEventName)name);
+                }
             }
 
-            var containingType = BuildNamedType(symbol.ContainingType);
+            var containingType = BuildUnconstructedNamedType(symbol.ContainingType);
 
             if (containingType == null)
             {
@@ -222,10 +249,15 @@ private static RQMethod BuildMethod(IMethodSymbol symbol)
             var typeParamCount = symbol.TypeParameters.Length;
             var parameterList = BuildParameterList(symbol.Parameters);
 
+            if (parameterList == null)
+            {
+                return null;
+            }
+
             return new RQMethod(containingType, name, typeParamCount, parameterList);
         }
 
-        private static RQType BuildType(ITypeSymbol symbol)
+        private static RQType? BuildType(ITypeSymbol symbol)
         {
             if (symbol.IsAnonymousType)
             {
@@ -236,13 +268,25 @@ private static RQType BuildType(ITypeSymbol symbol)
             {
                 return RQVoidType.Singleton;
             }
-            else if (symbol.TypeKind == TypeKind.Pointer)
+            else if (symbol is IPointerTypeSymbol pointerType)
             {
-                return new RQPointerType(BuildType((symbol as IPointerTypeSymbol).PointedAtType));
+                var pointedAtType = BuildType(pointerType.PointedAtType);
+                if (pointedAtType == null)
+                {
+                    return null;
+                }
+
+                return new RQPointerType(pointedAtType);
             }
-            else if (symbol.TypeKind == TypeKind.Array)
+            else if (symbol is IArrayTypeSymbol arrayType)
             {
-                return new RQArrayType((symbol as IArrayTypeSymbol).Rank, BuildType((symbol as IArrayTypeSymbol).ElementType));
+                var elementType = BuildType(arrayType.ElementType);
+                if (elementType == null)
+                {
+                    return null;
+                }
+
+                return new RQArrayType(arrayType.Rank, elementType);
             }
             else if (symbol.TypeKind == TypeKind.TypeParameter)
             {
@@ -259,10 +303,8 @@ private static RQType BuildType(ITypeSymbol symbol)
                 // not, we just erase dynamic to object here.
                 return RQType.ObjectType;
             }
-            else if (symbol.Kind == SymbolKind.NamedType || symbol.Kind == SymbolKind.ErrorType)
+            else if (symbol is INamedTypeSymbol namedTypeSymbol)
             {
-                var namedTypeSymbol = symbol as INamedTypeSymbol;
-
                 var definingType = namedTypeSymbol.ConstructedFrom ?? namedTypeSymbol;
 
                 var typeChain = new List<INamedTypeSymbol>();
@@ -283,11 +325,17 @@ private static RQType BuildType(ITypeSymbol symbol)
                 {
                     foreach (var typeArgument in entry.TypeArguments)
                     {
-                        typeArgumentList.Add(BuildType(typeArgument));
+                        var rqType = BuildType(typeArgument);
+                        if (rqType == null)
+                        {
+                            return null;
+                        }
+
+                        typeArgumentList.Add(rqType);
                     }
                 }
 
-                var containingType = BuildNamedType(definingType);
+                var containingType = BuildUnconstructedNamedType(definingType);
 
                 if (containingType == null)
                 {

src/Features/Core/Portable/RQName/SimpleTree/SimpleGroupNode.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using System.Collections.Generic;
 using System.Linq;
 

src/Features/Core/Portable/RQName/SimpleTree/SimpleLeafNode.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 namespace Microsoft.CodeAnalysis.Features.RQName.SimpleTree
 {
     internal class SimpleLeafNode : SimpleTreeNode

src/Features/Core/Portable/RQName/SimpleTree/SimpleTreeNode.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 namespace Microsoft.CodeAnalysis.Features.RQName.SimpleTree
 {
     internal abstract class SimpleTreeNode

src/VisualStudio/Core/Def/Implementation/RQName/RQName.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using Microsoft.CodeAnalysis;
 using Microsoft.CodeAnalysis.Features.RQName;
 using Microsoft.VisualStudio.Shell.Interop;
@@ -20,7 +22,7 @@ public static class RQName
         /// <param name="symbol">The symbol to build an RQName for.</param>
         /// <returns>A string suitable to pass as the pszRQName argument to methods in <see cref="IVsRefactorNotify"/>
         /// and <see cref="IVsSymbolicNavigationNotify"/>.</returns>
-        public static string From(ISymbol symbol)
+        public static string? From(ISymbol symbol)
             => RQNameInternal.From(symbol);
     }
 }