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Match types fail `-Ycheck` with GADT constraints

See original GitHub issue

Compiler version

3.2.0-RC2 (main branch)

Minimized code

trait Foo
case class Bar[A, T <: Tuple]() extends Foo

object test {
  def bar[T <: Tuple](e: Foo) = e match {
    case _: Bar[a, t] =>
      // this works
      val xs0: a *: T = ???
      val t0: a = xs0.head

      // this breaks
      val xs1: a *: t = ???
      val t1: a = xs1.head
  }
}

Output

checking issues/pdgadt-tupof-mini.scala after phase typer
exception while typing val t1: a = xs1.head[(a *: t)] of class class dotty.tools.dotc.ast.Trees$ValDef # -1
exception while typing {
  val xs0: *:[a, T] = ???
  val t0: a = xs0.head[(a *: T)]
  val xs1: *:[a, t] = ???
  val t1: a = xs1.head[(a *: t)]
  ()
} of class class dotty.tools.dotc.ast.Trees$Block # -1
exception while typing e match 
  {
    case _:Bar[a @ _, t @ _] => 
      val xs0: *:[a, T] = ???
      val t0: a = xs0.head[(a *: T)]
      val xs1: *:[a, t] = ???
      val t1: a = xs1.head[(a *: t)]
      ()
  } of class class dotty.tools.dotc.ast.Trees$Match # -1
exception while typing def bar[T >: Nothing <: Tuple](e: Foo): Unit = 
  e match 
    {
      case _:Bar[a @ _, t @ _] => 
        val xs0: *:[a, T] = ???
        val t0: a = xs0.head[(a *: T)]
        val xs1: *:[a, t] = ???
        val t1: a = xs1.head[(a *: t)]
        ()
    } of class class dotty.tools.dotc.ast.Trees$DefDef # -1
exception while typing final module class test() extends Object() { this: test.type =>
  def bar[T >: Nothing <: Tuple](e: Foo): Unit = 
    e match 
      {
        case _:Bar[a @ _, t @ _] => 
          val xs0: *:[a, T] = ???
          val t0: a = xs0.head[(a *: T)]
          val xs1: *:[a, t] = ???
          val t1: a = xs1.head[(a *: t)]
          ()
      }
} of class class dotty.tools.dotc.ast.Trees$TypeDef # -1
exception while typing package <empty> {
  trait Foo() extends Object {}
  case class Bar[A >: Nothing <: Any, T >: Nothing <: Tuple]() extends Object(), Foo, _root_.scala.Product, _root_.scala.Serializable {
    A
    T <: Tuple
    def copy[A, T <: Tuple](): Bar[A, T] = new Bar[A, T]()
  }
  final lazy module val Bar: Bar = new Bar()
  final module class Bar() extends AnyRef() { this: Bar.type =>
    def apply[A, T <: Tuple](): Bar[A, T] = new Bar[A, T]()
    def unapply[A, T <: Tuple](x$1: Bar[A, T]): true.type = true
    override def toString: String = "Bar"
  }
  final lazy module val test: test = new test()
  final module class test() extends Object() { this: test.type =>
    def bar[T >: Nothing <: Tuple](e: Foo): Unit = 
      e match 
        {
          case _:Bar[a @ _, t @ _] => 
            val xs0: *:[a, T] = ???
            val t0: a = xs0.head[(a *: T)]
            val xs1: *:[a, t] = ???
            val t1: a = xs1.head[(a *: t)]
            ()
        }
  }
} of class class dotty.tools.dotc.ast.Trees$PackageDef # -1
exception occurred while compiling issues/pdgadt-tupof-mini.scala
*** error while checking issues/pdgadt-tupof-mini.scala after phase typer ***
java.lang.AssertionError: assertion failed: Found:    Tuple.Head[a *: t]
Required: a
found: ??
expected: type a with type a, flags = case, underlying = a, , Any, {...}
tree = xs1.head[(a *: t)] while compiling issues/pdgadt-tupof-mini.scala
Exception in thread "main" java.lang.AssertionError: assertion failed: Found:    Tuple.Head[a *: t]
Required: a
found: ??
expected: type a with type a, flags = case, underlying = a, , Any, {...}
tree = xs1.head[(a *: t)]
	at scala.runtime.Scala3RunTime$.assertFailed(Scala3RunTime.scala:8)
	at dotty.tools.dotc.transform.TreeChecker$Checker.adapt(TreeChecker.scala:636)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3029)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3033)
	at dotty.tools.dotc.transform.TreeChecker$Checker.typed(TreeChecker.scala:309)
	at dotty.tools.dotc.typer.Typer.typedExpr(Typer.scala:3149)
	at dotty.tools.dotc.typer.Typer.typedValDef(Typer.scala:2271)
	at dotty.tools.dotc.typer.Typer.typedNamed$1(Typer.scala:2874)
	at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:2963)
	at dotty.tools.dotc.typer.ReTyper.typedUnadapted(ReTyper.scala:126)
	at dotty.tools.dotc.transform.TreeChecker$Checker.typedUnadapted(TreeChecker.scala:325)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3029)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3033)
	at dotty.tools.dotc.transform.TreeChecker$Checker.typed(TreeChecker.scala:309)
	at dotty.tools.dotc.typer.Typer.traverse$1(Typer.scala:3055)
	at dotty.tools.dotc.typer.Typer.typedStats(Typer.scala:3105)
	at dotty.tools.dotc.transform.TreeChecker$Checker.typedStats(TreeChecker.scala:555)
	at dotty.tools.dotc.typer.Typer.typedBlockStats(Typer.scala:1058)
	at dotty.tools.dotc.typer.Typer.typedBlock(Typer.scala:1062)
	at dotty.tools.dotc.transform.TreeChecker$Checker.typedBlock$$anonfun$1$$anonfun$1(TreeChecker.scala:537)
	at dotty.tools.dotc.transform.TreeChecker$Checker.withDefinedSyms(TreeChecker.scala:191)
	at dotty.tools.dotc.transform.TreeChecker$Checker.typedBlock$$anonfun$1(TreeChecker.scala:537)
	at dotty.tools.dotc.transform.TreeChecker$Checker.withBlock(TreeChecker.scala:219)
	at dotty.tools.dotc.transform.TreeChecker$Checker.typedBlock(TreeChecker.scala:537)
	at dotty.tools.dotc.typer.Typer.typedUnnamed$1(Typer.scala:2909)
	at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:2964)
	at dotty.tools.dotc.typer.ReTyper.typedUnadapted(ReTyper.scala:126)
	at dotty.tools.dotc.transform.TreeChecker$Checker.typedUnadapted(TreeChecker.scala:325)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3029)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3033)
	at dotty.tools.dotc.transform.TreeChecker$Checker.typed(TreeChecker.scala:309)
	at dotty.tools.dotc.typer.Typer.typedExpr(Typer.scala:3149)
	at dotty.tools.dotc.typer.Typer.caseRest$1(Typer.scala:1752)
	at dotty.tools.dotc.typer.Typer.typedCase(Typer.scala:1768)
	at dotty.tools.dotc.transform.TreeChecker$Checker.typedCase$$anonfun$1(TreeChecker.scala:519)
	at dotty.tools.dotc.transform.TreeChecker$Checker.withPatSyms(TreeChecker.scala:209)
	at dotty.tools.dotc.transform.TreeChecker$Checker.typedCase(TreeChecker.scala:520)
	at dotty.tools.dotc.typer.Typer.typedCases$$anonfun$1(Typer.scala:1698)
	at dotty.tools.dotc.core.Decorators$ListDecorator$.loop$1(Decorators.scala:87)
	at dotty.tools.dotc.core.Decorators$ListDecorator$.mapconserve$extension(Decorators.scala:103)
	at dotty.tools.dotc.typer.Typer.typedCases(Typer.scala:1700)
	at dotty.tools.dotc.typer.Typer.$anonfun$28(Typer.scala:1690)
	at dotty.tools.dotc.typer.Applications.harmonic(Applications.scala:2226)
	at dotty.tools.dotc.typer.Applications.harmonic$(Applications.scala:327)
	at dotty.tools.dotc.typer.Typer.harmonic(Typer.scala:119)
	at dotty.tools.dotc.typer.Typer.typedMatchFinish(Typer.scala:1690)
	at dotty.tools.dotc.typer.Typer.typedMatch(Typer.scala:1624)
	at dotty.tools.dotc.typer.Typer.typedUnnamed$1(Typer.scala:2915)
	at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:2964)
	at dotty.tools.dotc.typer.ReTyper.typedUnadapted(ReTyper.scala:126)
	at dotty.tools.dotc.transform.TreeChecker$Checker.typedUnadapted(TreeChecker.scala:325)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3029)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3033)
	at dotty.tools.dotc.transform.TreeChecker$Checker.typed(TreeChecker.scala:309)
	at dotty.tools.dotc.typer.Typer.typedExpr(Typer.scala:3149)
	at dotty.tools.dotc.typer.Typer.$anonfun$49(Typer.scala:2335)
	at dotty.tools.dotc.inlines.PrepareInlineable$.dropInlineIfError(PrepareInlineable.scala:249)
	at dotty.tools.dotc.typer.Typer.typedDefDef(Typer.scala:2335)
	at dotty.tools.dotc.transform.TreeChecker$Checker.typedDefDef$$anonfun$1(TreeChecker.scala:512)
	at dotty.tools.dotc.transform.TreeChecker$Checker.withDefinedSyms(TreeChecker.scala:191)
	at dotty.tools.dotc.transform.TreeChecker$Checker.typedDefDef(TreeChecker.scala:515)
	at dotty.tools.dotc.typer.Typer.typedNamed$1(Typer.scala:2877)
	at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:2963)
	at dotty.tools.dotc.typer.ReTyper.typedUnadapted(ReTyper.scala:126)
	at dotty.tools.dotc.transform.TreeChecker$Checker.typedUnadapted(TreeChecker.scala:325)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3029)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3033)
	at dotty.tools.dotc.transform.TreeChecker$Checker.typed(TreeChecker.scala:309)
	at dotty.tools.dotc.typer.Typer.traverse$1(Typer.scala:3055)
	at dotty.tools.dotc.typer.Typer.typedStats(Typer.scala:3105)
	at dotty.tools.dotc.transform.TreeChecker$Checker.typedStats(TreeChecker.scala:555)
	at dotty.tools.dotc.typer.Typer.typedClassDef(Typer.scala:2540)
	at dotty.tools.dotc.transform.TreeChecker$Checker.typedClassDef(TreeChecker.scala:490)
	at dotty.tools.dotc.typer.Typer.typedTypeOrClassDef$1(Typer.scala:2889)
	at dotty.tools.dotc.typer.Typer.typedNamed$1(Typer.scala:2893)
	at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:2963)
	at dotty.tools.dotc.typer.ReTyper.typedUnadapted(ReTyper.scala:126)
	at dotty.tools.dotc.transform.TreeChecker$Checker.typedUnadapted(TreeChecker.scala:325)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3029)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3033)
	at dotty.tools.dotc.transform.TreeChecker$Checker.typed(TreeChecker.scala:309)
	at dotty.tools.dotc.typer.Typer.traverse$1(Typer.scala:3055)
	at dotty.tools.dotc.typer.Typer.typedStats(Typer.scala:3105)
	at dotty.tools.dotc.transform.TreeChecker$Checker.typedStats(TreeChecker.scala:555)
	at dotty.tools.dotc.typer.Typer.typedPackageDef(Typer.scala:2671)
	at dotty.tools.dotc.transform.TreeChecker$Checker.typedPackageDef(TreeChecker.scala:581)
	at dotty.tools.dotc.typer.Typer.typedUnnamed$1(Typer.scala:2934)
	at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:2964)
	at dotty.tools.dotc.typer.ReTyper.typedUnadapted(ReTyper.scala:126)
	at dotty.tools.dotc.transform.TreeChecker$Checker.typedUnadapted(TreeChecker.scala:325)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3029)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3033)
	at dotty.tools.dotc.transform.TreeChecker$Checker.typed(TreeChecker.scala:309)
	at dotty.tools.dotc.typer.Typer.typedExpr(Typer.scala:3149)
	at dotty.tools.dotc.transform.TreeChecker.check(TreeChecker.scala:143)
	at dotty.tools.dotc.transform.TreeChecker.run(TreeChecker.scala:110)
	at dotty.tools.dotc.core.Phases$Phase.runOn$$anonfun$1(Phases.scala:311)
	at scala.collection.immutable.List.map(List.scala:246)
	at dotty.tools.dotc.core.Phases$Phase.runOn(Phases.scala:312)
	at dotty.tools.dotc.Run.runPhases$1$$anonfun$1(Run.scala:234)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
	at scala.collection.ArrayOps$.foreach$extension(ArrayOps.scala:1328)
	at dotty.tools.dotc.Run.runPhases$1(Run.scala:245)
	at dotty.tools.dotc.Run.compileUnits$$anonfun$1(Run.scala:253)
	at dotty.tools.dotc.Run.compileUnits$$anonfun$adapted$1(Run.scala:262)
	at dotty.tools.dotc.util.Stats$.maybeMonitored(Stats.scala:68)
	at dotty.tools.dotc.Run.compileUnits(Run.scala:262)
	at dotty.tools.dotc.Run.compileSources(Run.scala:186)
	at dotty.tools.dotc.Run.compile(Run.scala:170)
	at dotty.tools.dotc.Driver.doCompile(Driver.scala:35)
	at dotty.tools.dotc.Driver.process(Driver.scala:195)
	at dotty.tools.dotc.Driver.process(Driver.scala:163)
	at dotty.tools.dotc.Driver.process(Driver.scala:175)
	at dotty.tools.dotc.Driver.main(Driver.scala:205)
	at dotty.tools.dotc.Main.main(Main.scala)
[error] Nonzero exit code returned from runner: 1
[error] (scala3-compiler / Compile / runMain) Nonzero exit code returned from runner: 1
[error] Total time: 2 s, completed Jul 24, 2022, 5:28:35 PM

Expectation

This code should pass the tree checker. As seen in the log, the tree checker fails at the expression xs1.head[(a *: t)], whose expected type is a but the found type is Tuple.Head[a *: t]. The Tuple.Head is a match type defined as:

type Head[X <: NonEmptyTuple] = X match {
  case x *: _ => x
}

The possible cause seems to be: to reduce Tuple.Head[a *: t] to a we need to show that t is a subtype of Tuple. However, the GADT bound t <: Tuple is not available when we run the checker phase. This is why the expression xs0.head[(a *: T)] whose type does not need GADT constraints to reduce works but xs1.head[(a *: t)] breaks.

Issue Analytics

  • State:closed
  • Created a year ago
  • Comments:6 (5 by maintainers)

github_iconTop GitHub Comments

1reaction
Linyxuscommented, Aug 11, 2022

Note that there are cases where we do have bounds as GADT constraints:

enum C[-T]:
  case C1() extends C[EmptyTuple]

def foo[A](e: C[A]) = e match {
  case C.C1() =>
    // GADT constr: A <: EmptyTuple
    val i: Int = (??? : Int *: A).head
}

Head[Int *: A] would also need A <: EmptyTuple to reduce (so this fails -Ycheck too), which is a GADT constraint.

1reaction
SethTisuecommented, Aug 10, 2022

Dale’s current working hypothesis is that patternSymbolBound is putting the bound in the GADT constraints but it should be going in the regular constraints. We’ll dig into it more tomorrow.

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