C#: Extract the receiver type for extension types.

Author: michaelnebel

csharp/extractor/Semmle.Extraction.CSharp/Entities/Types/NamedType.cs

@@ -90,6 +90,20 @@ public override void Populate(TextWriter trapFile)
             {
                 trapFile.anonymous_types(this);
             }
+
+            if (Symbol.IsExtension && Symbol.ExtensionParameter is IParameterSymbol parameter)
+            {
+                // For some reason an extension type has a receiver parameter with an empty name
+                // even when there is no parameter.
+                if (!string.IsNullOrEmpty(parameter.Name))
+                {
+                    Parameter.Create(Context, parameter, this);
+                }
+
+                // Use the parameter type as the receiver type.
+                var receiverType = Type.Create(Context, parameter.Type).TypeRef;
+                trapFile.extension_receiver_type(this, receiverType);
+            }
         }
 
         private readonly Lazy<Type[]> typeArgumentsLazy;

csharp/extractor/Semmle.Extraction.CSharp/Trap/Tuples.cs

@@ -202,6 +202,9 @@ internal static void exprorstmt_name(this TextWriter trapFile, IEntity expr, str
         internal static void extend(this TextWriter trapFile, Type type, Type super) =>
             trapFile.WriteTuple("extend", type, super);
 
+        internal static void extension_receiver_type(this TextWriter trapFile, Type @extension, Type receiverType) =>
+            trapFile.WriteTuple("extension_receiver_type", extension, receiverType);
+
         internal static void anonymous_types(this TextWriter trapFile, Type type) =>
             trapFile.WriteTuple("anonymous_types", type);