C#: Shift arguments to extension property calls as the qualifier is an explicit argument to the getter/setter.

Author: michaelnebel

csharp/ql/lib/semmle/code/csharp/exprs/Call.qll

@@ -722,7 +722,21 @@ class IndexerCall extends AccessorCall, IndexerAccessExpr {
  * ```
  */
 class ExtensionPropertyCall extends PropertyCall {
-  ExtensionPropertyCall() { this.getProperty() instanceof ExtensionProperty }
+  private ExtensionProperty prop;
+
+  ExtensionPropertyCall() { this.getProperty() = prop }
+
+  override Expr getArgument(int i) {
+    if prop.isStatic()
+    then result = super.getArgument(i)
+    else (
+      // Shift arguments as the qualifier is an explicit argument in the getter/setter.
+      i = 0 and
+      result = this.getQualifier()
+      or
+      result = super.getArgument(i - 1)
+    )
+  }
 
   override string getAPrimaryQlClass() { result = "ExtensionPropertyCall" }
 }