Better error messages for missing type of recursive definitions

[odersky]

Fixes #2001.

@ennru Do you want to review this one?

Btw, thanks for all the good work on error messages!

[ennru]

My pleasure.
I'd like to see @tailrec on errorMsg.
Indentation of the part after your changes suggests the if (cycleSym.is(Implicit, is part of the errorMsg method.
For error reporting with explanation, we'd like to return Message instead of String, but I can fix that once you merged your change.

[odersky]

Fixed the indentation. Regarding @tailrec, I think it's better to leave it out, even though the method happens to be tail recursive. But it is neither performance sensitive nor likely to traverse very deeply, so there's no need to require tail recursion here.

[ennru]

@odersky I'm converting the error messages for recursive things to the case class scheme and want to add a unit test for "cyclic reference involving implicit". Can you help me to come up with a smallish example?

[odersky]

Hi Enno, there's one in neg/implicitDefs.