Fix expression value of struct default argument values

Author: tamasvajk

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

@@ -190,7 +190,7 @@ private static bool ContainsPattern(SyntaxNode node) =>
             {
                 // = null, = default, = default(T), = new MyStruct()
                 // we're generating a default expression:
-                return Default.CreateGenerated(cx, parent, childIndex, location);
+                return Default.CreateGenerated(cx, parent, childIndex, location, parameter.Type.IsValueType ? null : ValueAsString(null));
             }
 
             // const literal:

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

@@ -15,7 +15,7 @@ protected override void PopulateExpression(TextWriter trapFile)
             TypeAccess.Create(Context, Syntax.Type, this, 0);
         }
 
-        public static Expression CreateGenerated(Context cx, IExpressionParentEntity parent, int childIndex, Extraction.Entities.Location location)
+        public static Expression CreateGenerated(Context cx, IExpressionParentEntity parent, int childIndex, Extraction.Entities.Location location, string? value)
         {
             var info = new ExpressionInfo(
                 cx,
@@ -25,7 +25,7 @@ public static Expression CreateGenerated(Context cx, IExpressionParentEntity par
                 parent,
                 childIndex,
                 true,
-                ValueAsString(null));
+                value);
 
             return new Expression(info);
         }

csharp/ql/test/library-tests/parameters/Parameters.expected

@@ -36,8 +36,8 @@ withDefaultValue
 | Parameters.dll:0:0:0:0 | M4 | Parameters.dll:0:0:0:0 | b | 1 | Parameters.dll:0:0:0:0 | default | null |
 | Parameters.dll:0:0:0:0 | M5 | Parameters.dll:0:0:0:0 | a | 0 | Parameters.dll:0:0:0:0 | 0 | 0 |
 | Parameters.dll:0:0:0:0 | M5 | Parameters.dll:0:0:0:0 | b | 1 | Parameters.dll:0:0:0:0 | default | null |
-| Parameters.dll:0:0:0:0 | M6 | Parameters.dll:0:0:0:0 | s2 | 1 | Parameters.dll:0:0:0:0 | default | null |
-| Parameters.dll:0:0:0:0 | M6 | Parameters.dll:0:0:0:0 | s3 | 2 | Parameters.dll:0:0:0:0 | default | null |
+| Parameters.dll:0:0:0:0 | M6 | Parameters.dll:0:0:0:0 | s2 | 1 | Parameters.dll:0:0:0:0 | default | - |
+| Parameters.dll:0:0:0:0 | M6 | Parameters.dll:0:0:0:0 | s3 | 2 | Parameters.dll:0:0:0:0 | default | - |
 | Parameters.dll:0:0:0:0 | M7 | Parameters.dll:0:0:0:0 | e2 | 1 | Parameters.dll:0:0:0:0 | (...) ... | 0 |
 | Parameters.dll:0:0:0:0 | M7 | Parameters.dll:0:0:0:0 | e3 | 2 | Parameters.dll:0:0:0:0 | (...) ... | 0 |
 | Parameters.dll:0:0:0:0 | M7 | Parameters.dll:0:0:0:0 | e4 | 3 | Parameters.dll:0:0:0:0 | (...) ... | 1 |