Dotty: Cannot implement setter method for `var extension`

[Sciss]

Minimized code

https://scastie.scala-lang.org/vtB4qDpEQWKhpRfRKpmfTQ

trait Config {
  def extension: String
}
trait ConfigBuilder extends Config {
  override var extension: String
}
class ConfigBuilderImpl extends ConfigBuilder {
  private var _ext = "cache"
  private var _extSet = false

  def extension: String = _ext

  def extension_=(value: String): Unit = {  // !
    _extSet = true
    _ext = value
  }
}

Output

illegal method name: extension_= may not start with `extension_`

Expectation

It seems counter-intuitive that you can define a var extension but not implement a setter method.

[bishabosha]

PR https://github.com/lampepfl/dotty/pull/9836 should be updated to deal with this and block var extension

[bishabosha]

or should we allow extension_= if it overrides a method with setter flag?

[b-studios]

I think it might end up more consistent if we disallow it altogether.

[bishabosha]

decision was made to forbid this

[nicolasstucki]

We could maybe try to use an _extension suffix instead of extension_ prefix for extension methods. At first glance it looks like it would have fewer conflicting names.

[griggt]

Would that require +_extension to be written as `+_extension` by the user in an explicit reference?

[Sciss]

is there a reason no special characters such as $ are used, as happens for example in specialization, default values, etc. ?

[bishabosha]

I would assume because the expanded name is intended to be called by the user in some circumstances, as opposed to those other constructs that should not be called explicitly

[TheElectronWill]

Isn't it a bit too "strong" to disallow var extension? Since extension is a relatively common name, newcomers might find surprising that val extension is allowed but if we use var and if the variable is non-local then it's forbidden.