C#: Handle Nullable<T> default parameter values in assemblies

csharp/extractor/Semmle.Extraction.CSharp/Entities/Expression.cs

@@ -178,7 +178,13 @@ private static bool ContainsPattern(SyntaxNode node) =>
 
             var defaultValue = parameter.ExplicitDefaultValue;
 
-            if (parameter.Type is INamedTypeSymbol nt && nt.EnumUnderlyingType is not null)
+            var type = parameter.Type;
+            if (type.IsBoundNullable() && type is INamedTypeSymbol named)
+            {
+                type = named.TypeArguments[0];
+            }
+
+            if (type is INamedTypeSymbol nt && nt.EnumUnderlyingType is not null)
             {
                 // = (MyEnum)1, = MyEnum.Value1, = default(MyEnum), = new MyEnum()
                 // we're generating a (MyEnum)value cast expression:
@@ -194,7 +200,7 @@ private static bool ContainsPattern(SyntaxNode node) =>
                 return Default.CreateGenerated(cx, parent, childIndex, location, parameter.Type.IsReferenceType ? ValueAsString(null) : null);
             }
 
-            if (parameter.Type.SpecialType == SpecialType.System_Object)
+            if (parameter.Type.SpecialType is SpecialType.System_Object)
             {
                 // this can happen in VB.NET
                 cx.ExtractionError($"Extracting default argument value 'object {parameter.Name} = default' instead of 'object {parameter.Name} = {defaultValue}'. The latter is not supported in C#.",
@@ -205,7 +211,7 @@ private static bool ContainsPattern(SyntaxNode node) =>
             }
 
             // const literal:
-            return Literal.CreateGenerated(cx, parent, childIndex, parameter.Type, defaultValue, location);
+            return Literal.CreateGenerated(cx, parent, childIndex, type, defaultValue, location);
         }
 
         /// <summary>

csharp/extractor/Semmle.Extraction.CSharp/Entities/Expressions/Factory.cs

@@ -14,7 +14,7 @@ internal static Expression Create(ExpressionNodeInfo info)
             {
                 if (info.Node is null)
                 {
-                    info.Context.ModelError("Attempt to create a null expression");
+                    info.Context.ModelError(info.Location, "Attempt to create a null expression");
                     return new Unknown(info);
                 }
 

csharp/extractor/Semmle.Extraction.CSharp/Entities/Expressions/Literal.cs

@@ -32,10 +32,10 @@ private static ExprKind GetKind(ExpressionNodeInfo info)
             }
 
             var type = info.Type?.Symbol;
-            return GetExprKind(type, info.Node, info.Context);
+            return GetExprKind(type, info.Node, info.Location, info.Context);
         }
 
-        private static ExprKind GetExprKind(ITypeSymbol? type, ExpressionSyntax? expr, Context context)
+        private static ExprKind GetExprKind(ITypeSymbol? type, ExpressionSyntax? expr, Extraction.Entities.Location loc, Context context)
         {
             switch (type?.SpecialType)
             {
@@ -75,22 +75,24 @@ private static ExprKind GetExprKind(ITypeSymbol? type, ExpressionSyntax? expr, C
 
                 case null:
                 default:
+                    var kind = type?.SpecialType.ToString() ?? "null";
                     if (expr is not null)
-                        context.ModelError(expr, "Unhandled literal type");
+                        context.ModelError(expr, $"Unhandled literal type {kind}");
                     else
-                        context.ModelError("Unhandled literal type");
+                        context.ModelError(loc, $"Unhandled literal type {kind}");
                     return ExprKind.UNKNOWN;
             }
         }
 
         public static Expression CreateGenerated(Context cx, IExpressionParentEntity parent, int childIndex, ITypeSymbol type, object? value,
             Extraction.Entities.Location location)
         {
+            var kind = value is null ? ExprKind.NULL_LITERAL : GetExprKind(type, null, location, cx);
             var info = new ExpressionInfo(
                 cx,
                 AnnotatedTypeSymbol.CreateNotAnnotated(type),
                 location,
-                GetExprKind(type, null, cx),
+                kind,
                 parent,
                 childIndex,
                 true,

csharp/extractor/Semmle.Extraction/Context.cs

@@ -426,6 +426,16 @@ public void ModelError(ISymbol symbol, string msg)
             ReportError(new InternalError(symbol, msg));
         }
 
+        /// <summary>
+        /// Signal an error in the program model.
+        /// </summary>
+        /// <param name="loc">The location of the error.</param>
+        /// <param name="msg">The error message.</param>
+        public void ModelError(Entities.Location loc, string msg)
+        {
+            ReportError(new InternalError(loc.ReportingLocation, msg));
+        }
+
         /// <summary>
         /// Signal an error in the program model.
         /// </summary>

csharp/extractor/Semmle.Extraction/InternalError.cs

@@ -23,6 +23,13 @@ public InternalError(SyntaxNode node, string msg)
             Location = node.GetLocation();
         }
 
+        public InternalError(Location? loc, string msg)
+        {
+            Text = msg;
+            EntityText = "";
+            Location = loc;
+        }
+
         public InternalError(string msg)
         {
             Text = msg;