C#: Synthesize the implicit extension parameters for generics.

Author: michaelnebel

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

@@ -14,22 +14,28 @@ internal abstract class Method : CachedSymbol<IMethodSymbol>, IExpressionParentE
         protected Method(Context cx, IMethodSymbol init)
             : base(cx, init) { }
 
-        protected void PopulateParameters()
-        {
-            var positionOffset = 0;
-            var originalMethod = OriginalDefinition;
+        private IParameter? SyntheticParameter { get; set; }
 
+        private int SynthesizeExtensionParameter()
+        {
             // Synthesize implicit parameter for extension methods declared using extension(...) syntax.
             if (Symbol.ContainingSymbol is INamedTypeSymbol type &&
-                type.IsExtension && !string.IsNullOrEmpty(type.ExtensionParameter?.Name) &&
-                !Symbol.IsStatic &&
-                Symbol.AssociatedExtensionImplementation is not null)
+                type.IsExtension && type.ExtensionParameter is IParameterSymbol parameter &&
+                !string.IsNullOrEmpty(parameter.Name) && !Symbol.IsStatic)
             {
-                // TODO: Check that this works for generics as well. We might need to also take
-                ImplicitExtensionParameter.Create(Context, this);
-                positionOffset++;
+                var originalSyntheticParam = OriginalDefinition.SyntheticParameter;
+                SyntheticParameter = ImplicitExtensionParameter.Create(Context, this, parameter, originalSyntheticParam);
+                return 1;
             }
 
+            return 0;
+        }
+
+        protected void PopulateParameters()
+        {
+            var originalMethod = OriginalDefinition;
+            var positionOffset = SynthesizeExtensionParameter();
+
             IEnumerable<IParameterSymbol> parameters = Symbol.Parameters;
             IEnumerable<IParameterSymbol> originalParameters = originalMethod.Symbol.Parameters;
 

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

@@ -271,11 +271,13 @@ internal class ImplicitExtensionParameter : FreshEntity, IParameter
     {
         private Method ExtensionMethod { get; }
         private IParameterSymbol ExtensionParameter { get; }
+        private IParameter Original { get; }
 
-        private ImplicitExtensionParameter(Context cx, Method method) : base(cx)
+        private ImplicitExtensionParameter(Context cx, Method method, IParameterSymbol parameter, IParameter? original) : base(cx)
         {
             ExtensionMethod = method;
-            ExtensionParameter = method.Symbol.AssociatedExtensionImplementation!.Parameters[0];
+            ExtensionParameter = parameter;
+            Original = original ?? this;
         }
 
         private static int Ordinal => 0;
@@ -326,7 +328,7 @@ protected override void Populate(TextWriter trapFile)
             PopulateRefKind(trapFile, ExtensionParameter.RefKind);
 
             var type = Type.Create(Context, ExtensionParameter.Type);
-            trapFile.@params(this, Name, type.TypeRef, Ordinal, ParamKind, ExtensionMethod, this);
+            trapFile.@params(this, Name, type.TypeRef, Ordinal, ParamKind, ExtensionMethod, Original);
 
             if (Context.OnlyScaffold)
             {
@@ -356,11 +358,11 @@ protected override void Populate(TextWriter trapFile)
             }
         }
 
-        public static ImplicitExtensionParameter Create(Context cx, Method method)
+        public static ImplicitExtensionParameter Create(Context cx, Method method, IParameterSymbol parameter, IParameter? original)
         {
-            var parameter = new ImplicitExtensionParameter(cx, method);
-            parameter.TryPopulate();
-            return parameter;
+            var p = new ImplicitExtensionParameter(cx, method, parameter, original);
+            p.TryPopulate();
+            return p;
         }
     }