Confusing error due to mixup between WildcardType and TypeBounds

[Blaisorblade]

After recent fixes, the following code doesn't crash but gives an error message worth improving:

scala> class A1 extends _
1 |class A1 extends _
  |                 ^
  |                  is not a class type

That error comes from ctx.error(ex"$tp is not a class type", pos), but $tp is being pretty-printed to the empty string. This happens because $tp is TypeBounds(q"Nothing", q"Any") (where q"Nothing" is just informal quasiquote notation) instead of being WildcardType(TypeBounds(q"Nothing", q"Any")), and such type bounds are correctly pretty-printed to the empty string.

[smarter]

In that particular case, I think we should just emit an error about unbound wildcard in the parser, like for everything else in #4373

[odersky]

I actually think the error message is fine. The pointed to _ is indeed not a class type. Since this error is very unlikely to occur in the wild, I would not spend time and code lines to improve it.