C#: Add unbound parameter for constructed generic extensions.

Author: michaelnebel

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

@@ -20,6 +20,8 @@ private NamedType(Context cx, INamedTypeSymbol init, bool constructUnderlyingTup
         public static NamedType Create(Context cx, INamedTypeSymbol type) =>
             NamedTypeFactory.Instance.CreateEntityFromSymbol(cx, type);
 
+        public NamedType OriginalDefinition => Create(Context, Symbol.OriginalDefinition);
+
         /// <summary>
         /// Creates a named type entity from a tuple type. Unlike <see cref="Create"/>, this
         /// will create an entity for the underlying `System.ValueTuple` struct.
@@ -97,7 +99,12 @@ public override void Populate(TextWriter trapFile)
                 // even when there is no parameter.
                 if (!string.IsNullOrEmpty(parameter.Name))
                 {
-                    Parameter.Create(Context, parameter, this);
+                    var originalType = OriginalDefinition;
+                    // In case the this is constructed generic, we also need to create the unbound parameter.
+                    var originalParameter = SymbolEqualityComparer.Default.Equals(Symbol, originalType.Symbol.ExtensionParameter) || originalType.Symbol.ExtensionParameter is null
+                        ? null
+                        : Parameter.Create(Context, originalType.Symbol.ExtensionParameter, originalType);
+                    Parameter.Create(Context, parameter, this, originalParameter);
                 }
 
                 // Use the parameter type as the receiver type.