spurious ambiguous overload with Tasty, GADTs and cake pattern

[odersky]

Minimized code

The following is a minimization of the cps-async project which starts failing after #9063.

import scala.quoted._

trait App[F[_],CT]:
  this: Base[F,CT] =>

  import qctx.tasty._

  trait AA

  def f(cpsCtx: FC[F]): AA =
    g(cpsCtx)

  def g(cpsCtx: FC[F]): AA =
    val paramSym: Symbol = ???
    println(paramSym.tree)   // println necessary for failure
    val t: Term = ???
    t match {                // match necessary for failure
      case Typed(_, _) => ???  // both cases necessary for failure
      case Lambda(_, _) => ???
    }
    f(cpsCtx)                 // necessary for failure
    val cpsBody = runRoot()   // necessary for failure
    g(cpsCtx)                 // failure
      // 26 |    g(cpsCtx)
      //    |    ^
      //    |Ambiguous overload. The overloaded alternatives of method g in trait App with types
      //    | (cpsCtx: FC[F]): Base.this.AA
      //    | (cpsCtx: FC[F]): App.this.AA
      //    |both match arguments ((cpsCtx : FC[F]))
      //    |
      //    |Note: this happens because two or more alternatives have the same erasure,
      //    |      so they cannot be distinguished by overloading resolution


class FC[F[_]]()

trait Base[F[_]:Type,CT:Type]  // Both :Type context bounds are necessary for failure
extends Cps with Root[F, CT] with App[F, CT]:
  implicit val qctx: QuoteContext

trait Root[F[_], CT]:
  this: Base[F, CT] =>
  def runRoot(): CpsTree = ???

trait Cps:
  sealed abstract class CpsTree

Somewhat depressingly, I could not make it smaller than that. There are lots of elements that need to appear in exactly the order given, or the error will go away.

Output

-- [E051] Reference Error: cps-async-failure.scala:24:4 ------------------------
24 |    g(cpsCtx)                 // failure
   |    ^
   |Ambiguous overload. The overloaded alternatives of method g in trait App with types
   | (cpsCtx: FC[F]): Base.this.AA
   | (cpsCtx: FC[F]): App.this.AA
   |both match arguments ((cpsCtx : FC[F]))
   |
   |Note: this happens because two or more alternatives have the same erasure,
   |      so they cannot be distinguished by overloading resolution

Expectation

Should compile. There is only one method g.

Previously, the error was hidden by the somewhat arbitrary disambiguation in mergeDenots. But it was always there, and manifested itself in the project with a wrong exhaustivity warning.

It might be a cache invalidation problem. But probably not one of the caches under control from Config. I tried to turn these caches off, but the error still persisted.

[nicolasstucki]

I tried the minimization with dotc -Ycheck:all on #9063 and it compiled. How should I run this to get the failure?

[nicolasstucki]

Might have been fixed by the last commit f47f51b

[odersky]

No it was a commit before that: Something about black magic to make it compile. I have now pushed a version without the black magic that exhibits the error. The test itself is in

tests/pending/pos/cps-async-failure.scala

[odersky]

The black magic just picked one of two conflicting denotations. It was wrong half of the time, but happened to be right for this particular problem in cps-async. But then there were other problems, so I had to disable cps-async in the end.

[odersky]

I have pushed a new #9063 that exhibits the failure.

[odersky]

The test case is in tests/pending/pos/cps-async-failure.scala

[odersky]

I can't make it compile anymore with latest Tasty, so let's close.