Changes to the type checker with respect to UndecidableInstances

Hi, I found a difference between GHC 7.6.3 and 7.8.3 with respect to UndecidableInstances. https://gist.github.com/maoe/57a4346eb36aee159916 https://gist.github.com/maoe/57a4346eb36aee159916 7.6.3 requires UndecidableInstances to compile this snippet whereas 7.8.3 doesn't. What has changed in the type checker? Mitsutoshi