Fix #3557: Add regression test

[odersky]

The test now compiles with errors, which is what we would expect.

[allanrenucci]

It is not a crash anymore however the error emitted is very confusing:

class C2 {
  def f(values: String*) = ()
  class C3 {
    @f.varargs
    def g = ???
  }
}

-- [E081] Syntax Error: tests/allan/Test.scala:21:5 ----------------------------
21 |    @f.varargs
   |     ^
   | missing parameter type for parameter values, expected = ?{ varargs: ? }

I would add the regression test but keep the issue open

[smarter]

I would add the regression test but keep the issue open

I think we should open a separate issue for that, the crash is fixed and that's what the original issue was about.