I just found it: https://groups.google.com/forum/#!topic/scala-user/OBEFMIYvO98
"Hi,
Are there any tools for debugging implicits? Other than getting the compiler to dump out gigabytes of logs? I want a 'debug implicits' console session were it lets me recursively explore what implicits it considered and let me say which I think it should have used, and it tells me why it couldn't use them. (...)"