Raise ArgumentError if trying to instance_exec proc created from method

Fixes [Bug #18069]

test/ruby/test_object.rb

@@ -816,6 +816,25 @@ def test_instance_exec
     end
   end
 
+  def test_instance_exec_method_to_proc
+    a = Class.new do
+      def a
+        1
+      end
+    end
+
+    b = Class.new do
+      def b(x)
+        a + x
+      end
+    end
+
+    bug18069 = '[ruby-dev:51089]'
+    assert_raise(ArgumentError, bug18069) do
+      a.new.instance_exec(1, &b.new.method(:b).to_proc)
+    end
+  end
+
   def test_extend
     assert_raise(ArgumentError) do
       1.extend

vm_eval.c

@@ -1977,9 +1977,7 @@ yield_under(VALUE under, VALUE self, int argc, const VALUE *argv, int kw_splat)
 	    new_block_handler = VM_BH_FROM_ISEQ_BLOCK(&new_captured);
 	    break;
 	  case block_handler_type_ifunc:
-	    captured = VM_BH_TO_CAPT_BLOCK(block_handler);
-	    new_captured = *captured;
-	    new_block_handler = VM_BH_FROM_IFUNC_BLOCK(&new_captured);
+            rb_raise(rb_eArgError, "cannot change self on proc created from method");
 	    break;
 	  case block_handler_type_proc:
 	    is_lambda = rb_proc_lambda_p(block_handler) != Qfalse;