Honor refinements for modules that prepend other modules

This previously did not work, and the reason it did not work is
that:

1) Refining a module or class that prepends other modules places
   the refinements in the class itself and not the origin iclass.

2) Inclusion of a module that prepends other modules skips the
   module itself, including only iclasses for the prepended modules
   and the origin iclass.

Those two behaviors combined meant that the method table for the
refined methods for the included module never ends up in the
method lookup chain for the class including the module.

Fix this by not skipping the module itself when the module is
included.  This requires some code rearranging in
rb_include_class_new to make sure the correct method tables and
origin settings are used for the created iclass.

As origin iclasses shouldn't be exposed to Ruby, this also
requires skipping modules that have origin iclasses in
Module#ancestors (classes that have origin iclasses were already
skipped).

Fixes [Bug #16242]

class.c

@@ -825,10 +825,20 @@ VALUE
 rb_include_class_new(VALUE module, VALUE super)
 {
     VALUE klass = class_alloc(T_ICLASS, rb_cClass);
+    RCLASS_SET_ORIGIN(klass, klass);
+
+    RCLASS_M_TBL(OBJ_WB_UNPROTECT(klass)) =
+      RCLASS_M_TBL(OBJ_WB_UNPROTECT(module)); /* TODO: unprotected? */
 
     if (BUILTIN_TYPE(module) == T_ICLASS) {
+        if (module != RCLASS_ORIGIN(module)) {
+            RCLASS_SET_ORIGIN(klass, RCLASS_ORIGIN(module));
+        }
 	module = RBASIC(module)->klass;
     }
+    else if (module != RCLASS_ORIGIN(module)) {
+        RCLASS_SET_ORIGIN(klass, RCLASS_ORIGIN(module));
+    }
     if (!RCLASS_IV_TBL(module)) {
 	RCLASS_IV_TBL(module) = st_init_numtable();
     }
@@ -838,9 +848,6 @@ rb_include_class_new(VALUE module, VALUE super)
     RCLASS_IV_TBL(klass) = RCLASS_IV_TBL(module);
     RCLASS_CONST_TBL(klass) = RCLASS_CONST_TBL(module);
 
-    RCLASS_M_TBL(OBJ_WB_UNPROTECT(klass)) =
-      RCLASS_M_TBL(OBJ_WB_UNPROTECT(RCLASS_ORIGIN(module))); /* TODO: unprotected? */
-
     RCLASS_SET_SUPER(klass, super);
     if (RB_TYPE_P(module, T_ICLASS)) {
 	RBASIC_SET_CLASS(klass, RBASIC(module)->klass);
@@ -894,8 +901,6 @@ include_modules_at(const VALUE klass, VALUE c, VALUE module, int search_super)
 	int superclass_seen = FALSE;
 	struct rb_id_table *tbl;
 
-	if (RCLASS_ORIGIN(module) != module)
-	    goto skip;
 	if (klass_m_tbl && klass_m_tbl == RCLASS_M_TBL(module))
 	    return -1;
 	/* ignore if the module included already in superclasses */
@@ -1091,10 +1096,11 @@ rb_mod_ancestors(VALUE mod)
     VALUE p, ary = rb_ary_new();
 
     for (p = mod; p; p = RCLASS_SUPER(p)) {
+        if (p != RCLASS_ORIGIN(p)) continue;
 	if (BUILTIN_TYPE(p) == T_ICLASS) {
 	    rb_ary_push(ary, RBASIC(p)->klass);
 	}
-	else if (p == RCLASS_ORIGIN(p)) {
+        else {
 	    rb_ary_push(ary, p);
 	}
     }

test/ruby/test_refinement.rb

@@ -2322,6 +2322,34 @@ def test_refine_in_using
     assert_equal(:ok, RefineInUsing.test)
   end
 
+  class Bug16242
+    module OtherM
+    end
+
+    module M
+      prepend OtherM
+
+      refine M do
+        def refine_method
+          "refine_method"
+        end
+      end
+      using M
+
+      def hoge
+        refine_method
+      end
+    end
+
+    class X
+      include M
+    end
+  end
+
+  def test_refine_prepended_module
+    assert_equal("refine_method", Bug16242::X.new.hoge)
+  end
+
   private
 
   def eval_using(mod, s)