C#: Fix special case of default argument value extraction

Author: tamasvajk

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

@@ -193,6 +193,16 @@ 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)
+            {
+                // 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#.",
+                    null, null, severity: Util.Logging.Severity.Warning);
+
+                // we're generating a default expression:
+                return Default.CreateGenerated(cx, parent, childIndex, location, ValueAsString(null));
+            }
+
             // const literal:
             return Literal.CreateGenerated(cx, parent, childIndex, parameter.Type, defaultValue, location);
         }

csharp/ql/test/library-tests/standalone/errorrecovery/DiagnosticsAndErrors.expected

@@ -1,2 +1,3 @@
 compilationMessages
 extractorMessages
+| file://:0:0:0:0 | Extracting default argument value 'object RecordNumber = default' instead of 'object RecordNumber = -1'. The latter is not supported in C#. |