Virtual AttributeType property and signature generic types

src/System.Private.CoreLib/shared/System/Reflection/SignatureConstructedGenericType.cs

@@ -10,11 +10,25 @@ namespace System.Reflection
 {
     internal sealed class SignatureConstructedGenericType : SignatureType
     {
-        internal SignatureConstructedGenericType(Type genericTypeDefinition, Type[] genericTypeArguments)
+        // The exception-visible name "typeArguments" is chosen to match the parameter name to Type.MakeGenericType() since that's the
+        // intended user of this constructor.
+        internal SignatureConstructedGenericType(Type genericTypeDefinition, Type[] typeArguments)
         {
-            Debug.Assert(genericTypeDefinition != null && genericTypeArguments != null);
+            if (genericTypeDefinition == null)
+                throw new ArgumentNullException(nameof(genericTypeDefinition));
+
+            if (typeArguments == null)
+                throw new ArgumentNullException(nameof(typeArguments));
+
+            typeArguments = (Type[])(typeArguments.Clone());
+            for (int i = 0; i < typeArguments.Length; i++)
+            {
+                if (typeArguments[i] == null)
+                    throw new ArgumentNullException(nameof(typeArguments));
+            }
+
             _genericTypeDefinition = genericTypeDefinition;
-            _genericTypeArguments = (Type[])(genericTypeArguments.Clone());
+            _genericTypeArguments = typeArguments;
         }
 
         public sealed override bool IsTypeDefinition => false;

src/System.Private.CoreLib/shared/System/Type.cs

@@ -346,6 +346,8 @@ public virtual Array GetEnumValues()
         public virtual Type MakeGenericType(params Type[] typeArguments) { throw new NotSupportedException(SR.NotSupported_SubclassOverride); }
         public virtual Type MakePointerType() { throw new NotSupportedException(); }
 
+        public static Type MakeGenericSignatureType(Type genericTypeDefinition, params Type[] typeArguments) => new SignatureConstructedGenericType(genericTypeDefinition, typeArguments);
+
         public static Type MakeGenericMethodParameter(int position)
         {
             if (position < 0)

src/System.Private.CoreLib/src/System/Reflection/CustomAttribute.cs

@@ -500,7 +500,7 @@ public override bool Equals(object obj)
         #endregion
 
         #region Public Members
-        public Type AttributeType { get { return Constructor.DeclaringType; } }
+        public virtual Type AttributeType { get { return Constructor.DeclaringType; } }
 
         public virtual ConstructorInfo Constructor { get { return m_ctor; } }