Compiler crash with extension methods and recursion

[drdozer]

I'm seeing dotty compiler errors with stack traces that appear and disappear depending on what lines are commented and uncommented. Lines that on the face of it should not be interacting. An example code-base is here:

Reproduction:

import scala.language.higherKinds

trait LogDSL[L]

object LogDSL {
  implicit object LogRecsLogDSL extends LogDSL[String]
  def runLog(ls: String): String = ???
}

trait Direction[D] {
  def north: D
}

object Direction {
  // needs both instances to trigger the bug
  implicit def logDirection[L](implicit L: LogDSL[L]): Direction[L] = ???
  implicit def RotateDirection[D](implicit D: Direction[D]): Direction[Rotate.Rotation[D]] = ???
}

trait Rotate[R] {
  def rotate(r: R): R
}

object Rotate {
  implicit def logRotate[L](implicit L: LogDSL[L]): Rotate[L] = ???

  opaque type Rotation[T] = Int => T
}

object Main {
  // the instances have to be acquired through implicit resolution to cause the crash
  def north[D](implicit D: Direction[D]): D = ???
  def rotate[R](r: R)(implicit RR: Rotate[R]): R = ???

  def main(args: Array[String]): Unit = {
    // commenting out either the first or the second of these lines removes the compiler crash
    // removing the wrapping println call abolishes the crash
    println(LogDSL.runLog(rotate(north)))
    println(LogDSL.runLog(rotate(north)))
  }
}

[info] Compiling 1 Scala source to /home/nmrp3/devel/turingatemyhamster/thirty-min/target/scala-0.13/classes ...
[error] -- Error: /home/nmrp3/devel/turingatemyhamster/thirty-min/src/main/scala/Main.scala:38:11 
[error] 38 |    println(LogDSL.runLog(rotate(north)))
[error]    |           ^
[error]    |Recursion limit exceeded.
[error]    |Maybe there is an illegal cyclic reference?
[error]    |If that's not the case, you could also try to increase the stacksize using the -Xss JVM option.
[error]    |A recurring operation is (inner to outer):
[error]    |
[error]    |  subtype String <:< D
[error]    |  subtype LogDSL[String] <:< LogDSL[D]
[error]    |  subtype LogDSL.LogRecsLogDSL.type <:< LogDSL[D]
[info] exception occurred while typechecking /home/nmrp3/devel/turingatemyhamster/thirty-min/src/main/scala/Main.scala
[info] exception occurred while compiling /home/nmrp3/devel/turingatemyhamster/thirty-min/src/main/scala/Main.scala
dotty.tools.dotc.ast.Trees$UnAssignedTypeException: type of Apply(Select(Ident(LogDSL),runLog),List(Apply(Ident(rotate),List(Ident(north))))) is not assigned while compiling /home/nmrp3/devel/turingatemyhamster/thirty-min/src/main/scala/Main.scala
[error] ## Exception when compiling 1 sources to /home/nmrp3/devel/turingatemyhamster/thirty-min/target/scala-0.13/classes
[error] type of Apply(Select(Ident(LogDSL),runLog),List(Apply(Ident(rotate),List(Ident(north))))) is not assigned
[error] dotty.tools.dotc.ast.Trees$Tree.tpe(Trees.scala:85)
[error] dotty.tools.dotc.ast.TypedTreeInfo.exprPurity(TreeInfo.scala:402)
[error] dotty.tools.dotc.ast.TypedTreeInfo.exprPurity$$anonfun$1(TreeInfo.scala:405)
[error] dotty.tools.dotc.ast.TypedTreeInfo.exprPurity$$anonfun$adapted$1(TreeInfo.scala:405)
[error] scala.collection.immutable.List.map(List.scala:286)
[error] dotty.tools.dotc.ast.TypedTreeInfo.exprPurity(TreeInfo.scala:405)
[error] dotty.tools.dotc.ast.TypedTreeInfo.isPureExpr(TreeInfo.scala:425)
[error] dotty.tools.dotc.typer.Typer.checkStatementPurity(Typer.scala:2817)
...

https://gist.github.com/drdozer/8e2e345a3e834f9d771e03621404e875

I think it's something to do with ambiguous implicit instances. It appears to only happen in conjunction with a "recursion limit exceeded" type inference problem.

[biboudis]

Thanks a lot for reporting @drdozer!

Try posting a minimized snippet of the issue in code format and the output in shell code format. It will be tremendously helpful.

I am changing the subject and labelling as a typer error.

[drdozer]

@biboudis I'm slimming it down. But it's really hit-and-miss what combination of statements trigger it. Almost as if a corrupted type dictionary is getting built on one line and may or may not get the corrupted entry used on the next line.

I've updated the gist with the smallest example I can build to trigger the problem.
https://gist.github.com/drdozer/8e2e345a3e834f9d771e03621404e875

The gist code is below also:

import scala.language.higherKinds

trait LogDSL[L]

object LogDSL {
  implicit object LogRecsLogDSL extends LogDSL[String]
  def runLog(ls: String): String = ???
}

trait Direction[D] {
  def north: D
}

object Direction {
  // needs both instances to trigger the bug
  implicit def logDirection[L](implicit L: LogDSL[L]): Direction[L] = ???
  implicit def RotateDirection[D](implicit D: Direction[D]): Direction[Rotate.Rotation[D]] = ???
}

trait Rotate[R] {
  def rotate(r: R): R
}

object Rotate {
  implicit def logRotate[L](implicit L: LogDSL[L]): Rotate[L] = ???

  opaque type Rotation[T] = Int => T
}

object Main {
  // the instances have to be acquired through implicit resolution to cause the crash
  def north[D](implicit D: Direction[D]): D = ???
  def rotate[R](r: R)(implicit RR: Rotate[R]): R = ???

  def main(args: Array[String]): Unit = {
    // commenting out either the first or the second of these lines removes the compiler crash
    // removing the wrapping println call abolishes the crash
    println(LogDSL.runLog(rotate(north)))
    println(LogDSL.runLog(rotate(north)))
  }
}

[biboudis]

I updated the first post, moving your example there for self-containment. Your minimization is very helpful! Thanks a lot.

[anatoliykmetyuk]

This compiles fine for me. @drdozer does it work for you with the current version of Dotty?

[biboudis]

If this works on master we can close it 🎉
(if it doesn't exist as a test, we can add it)

[drdozer]

Yes, I think it works now. Thanks.