Pickling typos involving tuples and recursive implicits computing their types

[anatoliykmetyuk]

To reproduce, compile with dotc -Ytest-pickler -color:never.

trait Duplicate[T <: Tuple] {
  type Res <: Tuple
  def apply(t: T): Res
}

object Duplicate {
  import Tuple.{ Head, Tail }
  type Aux[T <: Tuple, R] = Duplicate[T] { type Res = R }

  given [T <: NonEmptyTuple, TD <: Tuple] as Aux[T, (Head[T], Head[T]) *: TD]
  given (td: Aux[Tail[T], TD]) = new Duplicate[T] {
    type Res = (Head[T], Head[T]) *: TD
    def apply(t: T): Res = (t.head, t.head) *: td(t.tail)
  }

  given as Aux[Unit, Unit] = new Duplicate[Unit] {
    type Res = Unit
    def apply(t: Unit): Unit = ()
  }
}

def (x: T) duplicate[T <: Tuple, R] given (d: Duplicate.Aux[T, R]): R = d(x)

val x: (1, 2, 3) = (1, 2, 3)
val z: ((1, 1), (2, 2), (3, 3)) = x.duplicate

Crash

exception occurred while compiling ../pg/Main.scala
stale symbol; module class Main$package$#2546 in module class <empty>, defined in Period(1..9, run = 2), is referred to in run Period(1..1, run = 3)
class dotty.tools.dotc.reporting.diagnostic.messages$Error at ?: pickling difference for module class Main$package$ in ../pg/Main.scala, for details:

  diff before-pickling.txt after-pickling.txt while compiling ../pg/Main.scala
Exception in thread "main" class dotty.tools.dotc.reporting.diagnostic.messages$Error at ?: pickling difference for module class Main$package$ in ../pg/Main.scala, for details:

  diff before-pickling.txt after-pickling.txt
	at dotty.tools.dotc.reporting.Reporting.error(Reporter.scala:137)
	at dotty.tools.dotc.core.Contexts$Context.error(Contexts.scala:71)
	at dotty.tools.dotc.transform.Pickler.testSame(Pickler.scala:119)
	at dotty.tools.dotc.transform.Pickler.testUnpickler$$anonfun$4(Pickler.scala:111)
	at dotty.runtime.function.JProcedure1.apply(JProcedure1.java:15)
	at dotty.runtime.function.JProcedure1.apply(JProcedure1.java:10)
	at scala.collection.TraversableLike$WithFilter.$anonfun$foreach$1(TraversableLike.scala:792)
	at scala.collection.mutable.HashMap.$anonfun$foreach$1(HashMap.scala:149)
	at scala.collection.mutable.HashTable.foreachEntry(HashTable.scala:237)
	at scala.collection.mutable.HashTable.foreachEntry$(HashTable.scala:230)
	at scala.collection.mutable.HashMap.foreachEntry(HashMap.scala:44)
	at scala.collection.mutable.HashMap.foreach(HashMap.scala:149)
	at scala.collection.TraversableLike$WithFilter.foreach(TraversableLike.scala:791)
	at dotty.tools.dotc.transform.Pickler.testUnpickler(Pickler.scala:112)
	at dotty.tools.dotc.transform.Pickler.runOn(Pickler.scala:95)
	at dotty.tools.dotc.Run.runPhases$4$$anonfun$4(Run.scala:158)
	at dotty.runtime.function.JProcedure1.apply(JProcedure1.java:15)
	at dotty.runtime.function.JProcedure1.apply(JProcedure1.java:10)
	at scala.collection.IndexedSeqOptimized.foreach(IndexedSeqOptimized.scala:36)
	at scala.collection.IndexedSeqOptimized.foreach$(IndexedSeqOptimized.scala:33)
	at scala.collection.mutable.ArrayOps$ofRef.foreach(ArrayOps.scala:198)
	at dotty.tools.dotc.Run.runPhases$5(Run.scala:170)
	at dotty.tools.dotc.Run.compileUnits$$anonfun$1(Run.scala:178)
	at dotty.runtime.function.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:12)
	at dotty.tools.dotc.util.Stats$.maybeMonitored(Stats.scala:65)
	at dotty.tools.dotc.Run.compileUnits(Run.scala:185)
	at dotty.tools.dotc.Run.compileSources(Run.scala:120)
	at dotty.tools.dotc.Run.compile(Run.scala:104)
	at dotty.tools.dotc.Driver.doCompile(Driver.scala:35)
	at dotty.tools.dotc.Driver.process(Driver.scala:180)
	at dotty.tools.dotc.Driver.process(Driver.scala:149)
	at dotty.tools.dotc.Driver.process(Driver.scala:161)
	at dotty.tools.dotc.Driver.main(Driver.scala:188)
	at dotty.tools.dotc.Main.main(Main.scala)

Diff in question

So basically, a *: b *: Unit gets messed up to (a, b).

[anatoliykmetyuk]

Workaround: add misbehaving tests to /dotty/compiler/test/dotc/run-test-pickling.blacklist

[odersky]

So basically, a *: b *: Unit gets messed up to (a, b).

They mean the same thing. The printer desugars one to the other. The only question is why it did not do that before pickling. It could be that the type is aliased or that there is a type variable. But I think it is at present not very interesting to chase this down.