Make SingletonClassDeclarationTree post-order

ql/src/codeql_ruby/controlflow/internal/ControlFlowGraphImpl.qll

@@ -1176,13 +1176,27 @@ module Trees {
     }
   }
 
-  private class SingletonClassTree extends BodyStmtPreOrderTree, SingletonClass {
+  private class SingletonClassTree extends BodyStmtPostOrderTree, SingletonClass {
+    final override predicate first(AstNode first) {
+      this.firstInner(first)
+      or
+      not exists(this.getAChild(_)) and
+      first = this
+    }
+
+    final override predicate succ(AstNode pred, AstNode succ, Completion c) {
+      BodyStmtPostOrderTree.super.succ(pred, succ, c)
+      or
+      succ = this and
+      this.lastInner(pred, c)
+    }
+
     /** Gets the `i`th child in the body of this block. */
     final override AstNode getBodyChild(int i, boolean rescuable) {
       (
         result = this.getValue() and i = 0 and rescuable = false
         or
-        result = BodyStmtPreOrderTree.super.getBodyChild(i - 1, rescuable)
+        result = BodyStmtPostOrderTree.super.getBodyChild(i - 1, rescuable)
       )
     }
   }

ql/test/library-tests/controlflow/graph/Cfg.expected

@@ -1599,10 +1599,10 @@ cfg.rb:
 #-----|  -> call to itself
 
 #  141| Constant
-#-----|  -> class << ...
+#-----|  -> Silly
 
 #  143| class << ...
-#-----|  -> Silly
+#-----|  -> silly
 
 #  143| call to itself
 #-----|  -> setter=
@@ -1617,7 +1617,7 @@ cfg.rb:
 #-----|  -> self
 
 #  145| print
-#-----|  -> silly
+#-----|  -> class << ...
 
 #  145| exit print