https://git.ruby-lang.org/ruby.git/commit/?id=8355a99883

From 8355a998839f17ff214a89062821a0a4287f6a54 Mon Sep 17 00:00:00 2001
From: Takashi Kokubun <takashikkbn@g...>
Date: Tue, 14 Apr 2020 23:24:50 -0700
Subject: Invalidate fastpath when calling attr_writer by super

We started to use fastpath on invokesuper when a method is not refinements
since 5c27681813, but we shouldn't have used fastpath for attr_writer either.

`cc->aux_.attr_index` is for an actual receiver class, while we store
its superclass in `cc->klass` and therefore there's no way to properly
invalidate attr_writer's inline cache when it's called by super.

[Bug #16785]

I suspect the same bug also exists in attr_reader. I'll address that in
another commit.

diff --git a/test/ruby/test_super.rb b/test/ruby/test_super.rb
index 3d5af3d..7cbe851 100644
--- a/test/ruby/test_super.rb
+++ b/test/ruby/test_super.rb
@@ -603,4 +603,34 @@ class TestSuper < Test::Unit::TestCase https://github.com/ruby/ruby/blob/trunk/test/ruby/test_super.rb#L603
     assert_equal :boo, subklass.new.baz
     assert_equal :boo2, subklass.new.boo
   end
+
+  def test_super_attr_writer # Bug #16785
+    writer_class = Class.new do
+      attr_writer :test
+    end
+    superwriter_class = Class.new(writer_class) do
+      def initialize
+        @test = 1 # index: 1
+      end
+
+      def test=(test)
+        super(test)
+      end
+    end
+    inherited_class = Class.new(superwriter_class) do
+      def initialize
+        @a = nil
+        @test = 2 # index: 2
+      end
+    end
+
+    superwriter = superwriter_class.new
+    superwriter.test = 3 # set ic->index of superwriter_class#test= to 1
+
+    inherited = inherited_class.new
+    inherited.test = 4 # it may set 4 to index=1 while it should be index=2
+
+    assert_equal 3, superwriter.instance_variable_get(:@test)
+    assert_equal 4, inherited.instance_variable_get(:@test)
+  end
 end
diff --git a/vm_insnhelper.c b/vm_insnhelper.c
index a4edd9a..c4a8dfa 100644
--- a/vm_insnhelper.c
+++ b/vm_insnhelper.c
@@ -3231,9 +3231,18 @@ vm_search_super_method(const rb_control_frame_t *reg_cfp, struct rb_call_data *c https://github.com/ruby/ruby/blob/trunk/vm_insnhelper.c#L3231
             const struct rb_callcache *cc = vm_cc_new(klass, cme, vm_call_super_method);
             RB_OBJ_WRITE(reg_cfp->iseq, &cd->cc, cc);
         }
-        else if (cached_cme->def->type == VM_METHOD_TYPE_REFINED) {
-            // vm_call_refined (search_refined_method) assumes cc->call is vm_call_super_method on invokesuper.
-            vm_cc_call_set(cd->cc, vm_call_super_method);
+        else {
+            switch (cached_cme->def->type) {
+              // vm_call_refined (search_refined_method) assumes cc->call is vm_call_super_method on invokesuper
+              case VM_METHOD_TYPE_REFINED:
+              // cc->klass is superclass of a class of receiver. Checking cc->klass is not enough to invalidate IVC for the receiver class.
+              case VM_METHOD_TYPE_ATTRSET:
+              // TODO: case VM_METHOD_TYPE_IVAR:
+                vm_cc_call_set(cd->cc, vm_call_super_method); // invalidate fastpath
+                break;
+              default:
+                break; // use fastpath
+            }
         }
     }
 }
-- 
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