Eq should extend AnyVal until Phantom types (now Erased terms) are ready

[Blaisorblade]

That way, at least you can't pass null as an instance. The suggestion is not mine, it comes from https://www.reddit.com/r/programming/comments/6elim6/announcing_dotty_012rc1_a_major_step_towards/dimg0nz/.

import dotty.DottyPredef.{eqAny => _, _}
class Foo
object Main {
  def bar[T](value: T)(implicit teq: Eq[T,T]) = value == value
  def main(args: Array[String]): Unit = bar(new Foo)(null) //<--
}

@nicolasstucki , can Eq be made a phantom type, now or later? I'm asking this at the design level for now.

[nicolasstucki]

@Blaisorblade it should be possible. https://github.com/lampepfl/dotty/blob/master/tests/pos/phantom-Eq.scala

[nicolasstucki]

We could think of making them phantom as soon as all phantom optimizations are in (currently in PRs).

[Blaisorblade]

Cool 👍

I wonder if == could also take Eq then, but Martin pointed out that calls to == are emitted by the pattern matcher, which comes after typing and apparently can't do implicit search. Not sure what to do there.

[smarter]

which comes after typing and apparently can't do implicit search.

I think it's a design decision to never do implicit search after typing, not a restriction of the implementation.

[odersky]

Hi Paolo, I am scanning the issues and thought maybe you want to take over some of the Eq stuff when you are here. So I am assigning issues as I see them.

[nicolasstucki]

Phantom types do not exist anymore.

With erased, now we can just do

def bar[T](value: T)(erased implicit teq: Eq[T,T]) = value == value