Dotty: Disallow nilary (empty parenthesis) of prefix unary methods

[soronpo]

Same as Scala 2 bug https://github.com/scala/bug/issues/12055

Minimized code

class Foo {
  def unary_~() : Foo = this

  // also bad
  def unary_+()(implicit pos: Long): Foo = this
}
val f = new Foo
val f2 = ~f //method unary_~ must be called with () argument

https://scastie.scala-lang.org/NKZQM3d8RrGYRYFoewJyQw

Expectation

Auto-application is disallowed and there a prefix def with a nilary(empty parenthesis) creates an error that cannot be avoided while maintaining a prefix position. To resolve this, we need to deprecate this possibility from the definition site.

[odersky]

It's not quite clear how this should be done. Do you want to disallow all parameters on methods named unary_n? what exactly is the pattern? I fear we will need quite a bit of spec language to cover this.

Also, is it really worth it? With a single unit test for your operator you'll find out that this will not work.

[nicolasstucki]

This is the same as #6192 which we closed without solving it.

[soronpo]

There was a PR submitted to Scala 2 to resolve this issue:
https://github.com/scala/scala/pull/9085

[soronpo]

It's not quite clear how this should be done. Do you want to disallow all parameters on methods named unary_n? what exactly is the pattern? I fear we will need quite a bit of spec language to cover this.

The PR in Scala 2 is sufficient enough. It removes a single empty-parent option from unary_x defs. We could go further and disallow all non-given parameter blocks from unary_x defs (given block support is required).

[odersky]

I note that #9085 comes without a spec, which makes it incomplete. I would be interested to see what a spec would look like. I suspect it would look rather out of place.

[sjrd]

A spec would be very similar to number 3. in https://dotty.epfl.ch/docs/reference/changed-features/operators.html#details-1. Something like:

The first parameter list of a unary operator method must not be empty. As a reminder from Section Prefix Operations, a unary operator is one whose name is "unary_op" for op in + - ! ~.

[odersky]

I think somebody else should take this on.