Inferred type refers to a local TermRef

[liufengyun]

The following code compiles fine with Scalac, but fails in Dotty. If the commented version of ~++ is used, everything is fine.

trait Rule[-In, +A] extends (In => A) {
  def flatMap[In2 <: In, B](fa2ruleb: A => Rule[In2, Seq[B]]): Rule[In, Seq[B]] = ???
  def map[B](fa2b: A => B): Rule[In, B] = ???

  def ~++[In2, B >: A](next: => Rule[In2, Seq[B]]) = for (a <- this; b <- next) yield a :: b.toList
  // def ~++[In2, B >: A](next: => Rule[In2, Seq[B]]): Rule[In, Seq[B]] = for (a <- this; b <- next) yield a :: b.toList
}

class SeqRule {
  type S
  type A
  def * : Rule[S, List[A]] = ???
  def +(rule: Rule[S, A]) : Rule[S, Seq[A]] = rule ~++ *
}

The cause is that ~++ gets an "ill" type that refers to the local variable b (see below at bottom). The check is not handled by Typer#escapingRefs. I tried to modify TypeOps#simplify to simplify TypeRef(TermRef(NoPrefix,b),scala$collection$TraversableOnce$$A) to just ParamRef(B), but didn't succeed.

PolyType(
  List(In2, B),

  List(TypeBounds(TypeRef(ThisType(TypeRef(NoPrefix,scala)),Nothing), TypeRef(ThisType(TypeRef(NoPrefix,scala)),Any)), TypeBounds(TypeRef(TermRef(NoPrefix,rule),Rule$$A), TypeRef(ThisType(TypeRef(NoPrefix,scala)),Any))),

  MethodType(
    List(next),
    List(
      ExprType(
        RefinedType(
          RefinedType(
            TypeRef(ThisType(TypeRef(NoPrefix,<empty>)),Rule),
            Rule$$In,
            TypeAlias(ParamRef(In2), -1)
          ),

          Rule$$A,
          TypeAlias(
            RefinedType(
              TypeRef(ThisType(TypeRef(NoPrefix,collection)),Seq),
              scala$collection$Seq$$A,
              TypeAlias(ParamRef(B), 1)
            ),
          1))
      )
    ),

    RefinedType(
      RefinedType(
        TypeRef(ThisType(TypeRef(NoPrefix,<empty>)),Rule),
        Rule$$In,
        TypeAlias(
          TypeRef(TermRef(NoPrefix,rule),Rule$$In),
        -1)
      ),

      Rule$$A,

      TypeAlias(
        RefinedType(
          TypeRef(ThisType(TypeRef(NoPrefix,collection)),Seq),
          scala$collection$Seq$$A,
          TypeAlias(TypeRef(TermRef(NoPrefix,b),scala$collection$TraversableOnce$$A), 1) <=== b !!
        ),
      1)
   )
  )
)

The expanded version of ~++ looks like follows:

    def ~++[In2, B >: A](next: => Rule[In2, Seq[B]]):
      Rule[In, scala.collection.Seq[B]]
     =
      this.flatMap[Nothing^, B^](
        {
          def $anonfun(a: A): Rule[Nothing^, scala.collection.Seq[B^]] =
            next.map[scala.collection.immutable.List[B]^](
              {
                def $anonfun(b: scala.collection.Seq[B]):
                  scala.collection.immutable.List[B]
                 =
                  {
                    val $1$: A = a
                    b.toList.::[B^]($1$)
                  }
                closure($anonfun)
              }
            )
          closure($anonfun)
        }
      )
  }

Note: this test case is a minimized version from the library json4s.

[felixmulder]

What's the status of this? It seems to be blocking json4s for #2182? Or is that no longer relevant to said issue?

[liufengyun]

Note this issue is fixed by #3061 automatically, another merit of the native apply refactoring.