Type member selection on argument of type Nothing fails

[japgolly]

Compiler version

3.0.0-RC1 with -Ykind-projector -Yerased-terms

Minimized code

final class CallbackTo[+A] {
  inline def map[B](f: A => B)(using erased ev: CallbackTo.MapGuard[B]): CallbackTo[ev.Out] = ???
}

object CallbackTo {

  type MapGuard[A] = { type Out = A }
  erased given MapGuard[A]: MapGuard[A] = ???

  def traverse[A, B](ta: List[A]): CallbackTo[List[B]] =
    val x: CallbackTo[List[A] => List[B]] = ???
    x.map(_(ta))
}

Output

12 |    x.map(_(ta))
   |    ^^^^^^^^^^^^
   |Out cannot be accessed as a member of (ev$proxy1 : Nothing) from module class CallbackTo$.
   | This location contains code that was inlined from x.scala:2
1 error found

Expectation

It should compile.

The problem occurs due to the combination of inline and erased because either of the following work as workarounds:

• Removing inline from .map
• Changing MapGuard to not be erased

[nicolasstucki]

Minimized

type Ev = { type Out }
inline def foo(ev: Ev): Option[ev.Out] = ???
def test: Unit = foo(???)

The issue is that when we inline an ??? argument the inliner takes it as if it was of type Nothing and forgets the type of the parameter. Then any member selection fails. Maybe the binder should have type Nothing & Ev where the precise type of the argument is Nothing and the type of the parameter is Ev. If we can keep this type without collapsing it to Nothing then we could have the info we need for the member selection.

This issue appeared with the erased given because we optimize it away early to ???.

[nicolasstucki]

This issue is the type selection version of issue #8612

[nicolasstucki]

Null has the same issue

[nicolasstucki]

An alternative might be to use the type of the parameter instead of a bottom type.

[liufengyun]

It seems to me your original proposal to use type ascriptions is a good and principled solution.

[odersky]

I agree, we should try to type ascribe in this case