Improve the error message when calling unary with parentheses

[cacapouh]

Improve the error message to something like 'Do not put parentheses on unary methods', for the compilation error mentioned in the following issue.

#17325

[som-snytt]

I would expect that I can "elevate" warnings to errors and see the underlying problem.

But:

scalac -d /tmp -feature -Werror i17325.scala

says

-- [E100] Syntax Error: i17325.scala:12:4 ------------------------------------------------------------------------------
12 |    ~(0.5) // compilation error
   |    ^^^^^^
   |    method unary_~ in class DoubleOps must be called with () argument
   |
   | longer explanation available when compiling with `-explain`
-- Error: i17325.scala:7:15 --------------------------------------------------------------------------------------------
7 |  implicit def doubleOps(d: Double): DoubleOps = new DoubleOps(
  |               ^
  |               Definition of implicit conversion method doubleOps should be enabled
  |               by adding the import clause 'import scala.language.implicitConversions'
  |               or by setting the compiler option -language:implicitConversions.
  |               See the Scala docs for value scala.language.implicitConversions for a discussion
  |               why the feature should be explicitly enabled.
2 errors found

The expected warning is not boosted to error.

That is, for the text

@main
def main() = {
  class DoubleOps(d: Double) {
    def unary_~(): Double = scala.math.ceil(d)
  }
  implicit def doubleOps(d: Double): DoubleOps = new DoubleOps(
    d
  )

  println {
    //~(0.5) // compilation error
    "hi"
  }
}

and

➜  scalac -d /tmp -feature -Werror -language:implicitConversions i17325.scala
-- Error: i17325.scala:5:8 ---------------------------------------------------------------------------------------------
5 |    def unary_~(): Double = scala.math.ceil(d)
  |        ^
  |        unary_<op> method cannot take empty parameter list.
  |
  |        Possible fix: remove the `()` arguments.
1 error found

The "meta" issue is that I should be able to ask the compiler to tell me everything it knows about my code. That might include warnings or also open tickets.

[som-snytt]

Actually Scala 2 is the converse because "empty apply" is only a warning. So you see both warnings, or you see one error.

➜  scalac -d /tmp -Xlint ti17325.scala
ti17325.scala:6: warning: unary prefix operator definition with empty parameter list is deprecated: instead, remove () to declare as `def unary_~ : Double = scala.math.ceil(d)`
    def unary_~(): Double = scala.math.ceil(d)
        ^
ti17325.scala:12: warning: Auto-application to `()` is deprecated. Supply the empty argument list `()` explicitly to invoke method unary_~,
or remove the empty argument list from its definition (Java-defined methods are exempt).
In Scala 3, an unapplied method like this will be eta-expanded into a function.
      ~(0.5) // compilation error
      ^
2 warnings
➜  scalac -d /tmp -Xlint -Xsource:3 ti17325.scala
ti17325.scala:6: error: unary prefix operator definition with empty parameter list is unsupported: instead, remove () to declare as `def unary_~ : Double = scala.math.ceil(d)`
    def unary_~(): Double = scala.math.ceil(d)
        ^
1 error