Compiler crash when a `given` instance of a trait with inline defs is not inline

[kubukoz]

Compiler version

3.0.0

Minimized code

trait Noop[T]:
  inline def noop(fa: T): T

final case class User(name: String, age: Int)

given Noop[User] = a => a

Output (click arrow to expand)

sbt:scala3-live> compile
[info] compiling 1 Scala source to /Users/jkoslowski/dev/scala3-live/target/scala-3.0.0/classes ...
exception while typing closure($anonfun:com.example.Noop[com.example.User]) of class class dotty.tools.dotc.ast.Trees$Closure # -1
exception while typing {
  def $anonfun(a: com.example.User): com.example.User = a
  closure($anonfun:com.example.Noop[com.example.User])
} of class class dotty.tools.dotc.ast.Trees$Block # -1
exception while typing final lazy given def given_Noop_User: com.example.Noop[com.example.User] = 
  {
    def $anonfun(a: com.example.User): com.example.User = a
    closure($anonfun:com.example.Noop[com.example.User])
  } of class class dotty.tools.dotc.ast.Trees$DefDef # -1
exception while typing @scala.annotation.internal.SourceFile(
  "src/main/scala/com/example/Examples.scala"
) final module class Examples$package$() extends Object() {
  private def writeReplace(): AnyRef = 
    new scala.runtime.ModuleSerializationProxy(
      classOf[com.example.Examples$package.type]
    )
  final lazy given def given_Noop_User: com.example.Noop[com.example.User] = 
    {
      def $anonfun(a: com.example.User): com.example.User = a
      closure($anonfun:com.example.Noop[com.example.User])
    }
} of class class dotty.tools.dotc.ast.Trees$TypeDef # -1
exception while typing package com.example {
  @scala.annotation.internal.SourceFile(
    "src/main/scala/com/example/Examples.scala"
  ) trait Noop[T]() extends Object {
    private type T
    private inline def noop(fa: T): T
  }
  @scala.annotation.internal.SourceFile(
    "src/main/scala/com/example/Examples.scala"
  ) final case class User(name: String, age: Int) extends Object(), Product, 
    Serializable
   {
    override def hashCode(): Int = 
      {
        var acc: Int = -889275714
        acc = scala.runtime.Statics$#mix(acc, this.productPrefix.hashCode())
        acc = 
          scala.runtime.Statics$#mix(acc, 
            scala.runtime.Statics$#anyHash(User.this.name)
          )
        acc = scala.runtime.Statics$#mix(acc, User.this.age)
        scala.runtime.Statics$#finalizeHash(acc, 2)
      }
    override def equals(x$0: Any): Boolean = 
      this.eq(x$0.$asInstanceOf$[Object]).||(
        matchResult1[Boolean]: 
          {
            case val x1: (x$0 : Object) = x$0
            if x1.$isInstanceOf$[com.example.User @unchecked] then 
              {
                case val x$0: com.example.User = 
                  x1.$asInstanceOf$[com.example.User @unchecked]
                return[matchResult1] 
                  this.age.==(x$0.age).&&(this.name.==(x$0.name))
              }
             else ()
            return[matchResult1] false
          }
      )
    override def toString(): String = scala.runtime.ScalaRunTime._toString(this)
    override def canEqual(that: Any): Boolean = 
      that.isInstanceOf[com.example.User @unchecked]
    override def productArity: Int = 2
    override def productPrefix: String = "User"
    override def productElement(n: Int): Any = 
      matchResult2[Object]: 
        {
          case val x3: (n : Int) = n
          if 0.==(x3) then return[matchResult2] this._1 else ()
          if 1.==(x3) then return[matchResult2] this._2 else ()
          throw new IndexOutOfBoundsException(n.toString())
        }
    override def productElementName(n: Int): String = 
      matchResult3[String]: 
        {
          case val x4: (n : Int) = n
          if 0.==(x4) then return[matchResult3] "name" else ()
          if 1.==(x4) then return[matchResult3] "age" else ()
          throw new IndexOutOfBoundsException(n.toString())
        }
    def name: String
    def age: Int
    def copy(name: String, age: Int): com.example.User = 
      new com.example.User(name, age)
    def copy$default$1: String @uncheckedVariance = User.this.name
    def copy$default$2: Int @uncheckedVariance = User.this.age
    def _1: String = this.name
    def _2: Int = this.age
  }
  final lazy module val User: com.example.User$ = new com.example.User$()
  @scala.annotation.internal.SourceFile(
    "src/main/scala/com/example/Examples.scala"
  ) final module class User$() extends AnyRef(), scala.deriving.Mirror.Product {
    private def writeReplace(): AnyRef = 
      new scala.runtime.ModuleSerializationProxy(classOf[com.example.User.type])
    def apply(name: String, age: Int): com.example.User = 
      new com.example.User(name, age)
    def unapply(x$1: com.example.User): com.example.User = x$1
    override def toString: String = "User"
    type MirroredMonoType = com.example.User
    def fromProduct(x$0: Product): com.example.User.MirroredMonoType = 
      new com.example.User(x$0.productElement(0).$asInstanceOf$[String], 
        x$0.productElement(1).$asInstanceOf$[Int]
      )
  }
  final lazy module val Examples$package: com.example.Examples$package$ = 
    new com.example.Examples$package$()
  @scala.annotation.internal.SourceFile(
    "src/main/scala/com/example/Examples.scala"
  ) final module class Examples$package$() extends Object() {
    private def writeReplace(): AnyRef = 
      new scala.runtime.ModuleSerializationProxy(
        classOf[com.example.Examples$package.type]
      )
    final lazy given def given_Noop_User: com.example.Noop[com.example.User] = 
      {
        def $anonfun(a: com.example.User): com.example.User = a
        closure($anonfun:com.example.Noop[com.example.User])
      }
  }
} of class class dotty.tools.dotc.ast.Trees$PackageDef # -1
[info] exception occurred while compiling /Users/jkoslowski/dev/scala3-live/src/main/scala/com/example/Examples.scala
scala.MatchError: List() (of class scala.collection.immutable.Nil$) while compiling /Users/jkoslowski/dev/scala3-live/src/main/scala/com/example/Examples.scala
[error] ## Exception when compiling 1 sources to /Users/jkoslowski/dev/scala3-live/target/scala-3.0.0/classes
[error] scala.MatchError: List() (of class scala.collection.immutable.Nil$)
[error] dotty.tools.dotc.transform.Erasure$Boxing$.adaptClosure(Erasure.scala:436)
[error] dotty.tools.dotc.transform.Erasure$Typer.typedClosure(Erasure.scala:1032)
[error] dotty.tools.dotc.typer.Typer.typedUnnamed$1(Typer.scala:2680)
[error] dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:2731)
[error] dotty.tools.dotc.typer.ReTyper.typedUnadapted(ReTyper.scala:118)
[error] dotty.tools.dotc.typer.Typer.typed(Typer.scala:2796)
[error] dotty.tools.dotc.typer.Typer.typed(Typer.scala:2800)
[error] dotty.tools.dotc.typer.Typer.typedExpr(Typer.scala:2916)
[error] dotty.tools.dotc.typer.Typer.typedBlock(Typer.scala:1018)
[error] dotty.tools.dotc.typer.Typer.typedUnnamed$1(Typer.scala:2677)
[error] dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:2731)
[error] dotty.tools.dotc.typer.ReTyper.typedUnadapted(ReTyper.scala:118)
[error] dotty.tools.dotc.typer.Typer.typed(Typer.scala:2796)
[error] dotty.tools.dotc.typer.Typer.typed(Typer.scala:2800)
[error] dotty.tools.dotc.typer.Typer.typedExpr(Typer.scala:2916)
[error] dotty.tools.dotc.typer.Typer.$anonfun$34(Typer.scala:2142)
[error] dotty.tools.dotc.typer.PrepareInlineable$.dropInlineIfError(PrepareInlineable.scala:225)
[error] dotty.tools.dotc.typer.Typer.typedDefDef(Typer.scala:2142)
[error] dotty.tools.dotc.transform.Erasure$Typer.typedDefDef(Erasure.scala:955)
[error] dotty.tools.dotc.typer.Typer.typedNamed$1(Typer.scala:2646)
[error] dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:2730)
[error] dotty.tools.dotc.typer.ReTyper.typedUnadapted(ReTyper.scala:118)
[error] dotty.tools.dotc.typer.Typer.typed(Typer.scala:2796)
[error] dotty.tools.dotc.typer.Typer.typed(Typer.scala:2800)
[error] dotty.tools.dotc.typer.Typer.traverse$1(Typer.scala:2822)
[error] dotty.tools.dotc.typer.Typer.typedStats(Typer.scala:2872)
[error] dotty.tools.dotc.transform.Erasure$Typer.typedStats(Erasure.scala:1068)
[error] dotty.tools.dotc.typer.Typer.typedClassDef(Typer.scala:2328)
[error] dotty.tools.dotc.transform.Erasure$Typer.typedClassDef(Erasure.scala:1057)
[error] dotty.tools.dotc.typer.Typer.typedTypeOrClassDef$2(Typer.scala:2657)
[error] dotty.tools.dotc.typer.Typer.typedNamed$1(Typer.scala:2661)
[error] dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:2730)
[error] dotty.tools.dotc.typer.ReTyper.typedUnadapted(ReTyper.scala:118)
[error] dotty.tools.dotc.typer.Typer.typed(Typer.scala:2796)
[error] dotty.tools.dotc.typer.Typer.typed(Typer.scala:2800)
[error] dotty.tools.dotc.typer.Typer.traverse$1(Typer.scala:2822)
[error] dotty.tools.dotc.typer.Typer.typedStats(Typer.scala:2872)
[error] dotty.tools.dotc.transform.Erasure$Typer.typedStats(Erasure.scala:1068)
[error] dotty.tools.dotc.typer.Typer.typedPackageDef(Typer.scala:2451)
[error] dotty.tools.dotc.typer.Typer.typedUnnamed$1(Typer.scala:2702)
[error] dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:2731)
[error] dotty.tools.dotc.typer.ReTyper.typedUnadapted(ReTyper.scala:118)
[error] dotty.tools.dotc.typer.Typer.typed(Typer.scala:2796)
[error] dotty.tools.dotc.typer.Typer.typed(Typer.scala:2800)
[error] dotty.tools.dotc.typer.Typer.typedExpr(Typer.scala:2916)
[error] dotty.tools.dotc.transform.Erasure.run(Erasure.scala:132)
[error] dotty.tools.dotc.core.Phases$Phase.runOn$$anonfun$1(Phases.scala:303)
[error] scala.collection.immutable.List.map(List.scala:246)
[error] dotty.tools.dotc.core.Phases$Phase.runOn(Phases.scala:304)
[error] dotty.tools.dotc.Run.runPhases$4$$anonfun$4(Run.scala:205)
[error] scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
[error] scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
[error] scala.collection.ArrayOps$.foreach$extension(ArrayOps.scala:1323)
[error] dotty.tools.dotc.Run.runPhases$5(Run.scala:215)
[error] dotty.tools.dotc.Run.compileUnits$$anonfun$1(Run.scala:223)
[error] scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.scala:18)
[error] dotty.tools.dotc.util.Stats$.maybeMonitored(Stats.scala:67)
[error] dotty.tools.dotc.Run.compileUnits(Run.scala:230)
[error] dotty.tools.dotc.Run.compileSources(Run.scala:166)
[error] dotty.tools.dotc.Run.compile(Run.scala:150)
[error] dotty.tools.dotc.Driver.doCompile(Driver.scala:39)
[error] dotty.tools.xsbt.CompilerBridgeDriver.run(CompilerBridgeDriver.java:88)
[error] dotty.tools.xsbt.CompilerBridge.run(CompilerBridge.java:22)
[error] sbt.internal.inc.AnalyzingCompiler.compile(AnalyzingCompiler.scala:91)
[error] sbt.internal.inc.MixedAnalyzingCompiler.$anonfun$compile$7(MixedAnalyzingCompiler.scala:186)
[error] scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:23)
[error] sbt.internal.inc.MixedAnalyzingCompiler.timed(MixedAnalyzingCompiler.scala:241)
[error] sbt.internal.inc.MixedAnalyzingCompiler.$anonfun$compile$4(MixedAnalyzingCompiler.scala:176)
[error] sbt.internal.inc.MixedAnalyzingCompiler.$anonfun$compile$4$adapted(MixedAnalyzingCompiler.scala:157)
[error] sbt.internal.inc.JarUtils$.withPreviousJar(JarUtils.scala:239)
[error] sbt.internal.inc.MixedAnalyzingCompiler.compileScala$1(MixedAnalyzingCompiler.scala:157)
[error] sbt.internal.inc.MixedAnalyzingCompiler.compile(MixedAnalyzingCompiler.scala:204)
[error] sbt.internal.inc.IncrementalCompilerImpl.$anonfun$compileInternal$1(IncrementalCompilerImpl.scala:528)
[error] sbt.internal.inc.IncrementalCompilerImpl.$anonfun$compileInternal$1$adapted(IncrementalCompilerImpl.scala:528)
[error] sbt.internal.inc.Incremental$.$anonfun$apply$5(Incremental.scala:174)
[error] sbt.internal.inc.Incremental$.$anonfun$apply$5$adapted(Incremental.scala:172)
[error] sbt.internal.inc.Incremental$$anon$2.run(Incremental.scala:457)
[error] sbt.internal.inc.IncrementalCommon$CycleState.next(IncrementalCommon.scala:116)
[error] sbt.internal.inc.IncrementalCommon$$anon$1.next(IncrementalCommon.scala:56)
[error] sbt.internal.inc.IncrementalCommon$$anon$1.next(IncrementalCommon.scala:52)
[error] sbt.internal.inc.IncrementalCommon.cycle(IncrementalCommon.scala:261)
[error] sbt.internal.inc.Incremental$.$anonfun$incrementalCompile$8(Incremental.scala:412)
[error] sbt.internal.inc.Incremental$.withClassfileManager(Incremental.scala:499)
[error] sbt.internal.inc.Incremental$.incrementalCompile(Incremental.scala:399)
[error] sbt.internal.inc.Incremental$.apply(Incremental.scala:166)
[error] sbt.internal.inc.IncrementalCompilerImpl.compileInternal(IncrementalCompilerImpl.scala:528)
[error] sbt.internal.inc.IncrementalCompilerImpl.$anonfun$compileIncrementally$1(IncrementalCompilerImpl.scala:482)
[error] sbt.internal.inc.IncrementalCompilerImpl.handleCompilationError(IncrementalCompilerImpl.scala:332)
[error] sbt.internal.inc.IncrementalCompilerImpl.compileIncrementally(IncrementalCompilerImpl.scala:420)
[error] sbt.internal.inc.IncrementalCompilerImpl.compile(IncrementalCompilerImpl.scala:137)
[error] sbt.Defaults$.compileIncrementalTaskImpl(Defaults.scala:2346)
[error] sbt.Defaults$.$anonfun$compileIncrementalTask$2(Defaults.scala:2303)
[error] sbt.internal.io.Retry$.apply(Retry.scala:46)
[error] sbt.internal.io.Retry$.apply(Retry.scala:28)
[error] sbt.internal.io.Retry$.apply(Retry.scala:23)
[error] sbt.internal.server.BspCompileTask$.compute(BspCompileTask.scala:31)
[error] sbt.Defaults$.$anonfun$compileIncrementalTask$1(Defaults.scala:2299)
[error] scala.Function1.$anonfun$compose$1(Function1.scala:49)
[error] sbt.internal.util.$tilde$greater.$anonfun$$u2219$1(TypeFunctions.scala:62)
[error] sbt.std.Transform$$anon$4.work(Transform.scala:68)
[error] sbt.Execute.$anonfun$submit$2(Execute.scala:282)
[error] sbt.internal.util.ErrorHandling$.wideConvert(ErrorHandling.scala:23)
[error] sbt.Execute.work(Execute.scala:291)
[error] sbt.Execute.$anonfun$submit$1(Execute.scala:282)
[error] sbt.ConcurrentRestrictions$$anon$4.$anonfun$submitValid$1(ConcurrentRestrictions.scala:265)
[error] sbt.CompletionService$$anon$2.call(CompletionService.scala:64)
[error] java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
[error] java.base/java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:515)
[error] java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
[error] java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
[error] java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
[error] java.base/java.lang.Thread.run(Thread.java:834)
[error]            
[error] stack trace is suppressed; run last Compile / compileIncremental for the full output
[error] (Compile / compileIncremental) scala.MatchError: List() (of class scala.collection.immutable.Nil$)
[error] Total time: 0 s, completed 21 May 2021, 16:09:51

Related: #12555

[kubukoz]

Also curious what happens if one uses SAM syntax to implement a trait with an inline method (like here) - I guess it's also marked inline automatically?

[LPTK]

I guess instance definition would desugar to something like:

implicit val given_Noop_User: Noop[User] = a => a

so even if the SAM type implementation used an inline def, that would still not be callable from the outside because of the type ascription.

What you'll probably want instead is:

given Noop[User] with
  inline def noop(fa: User): User = fa

[nicolasstucki]

There is another fundamental issue with the code. As we only provide given Noop[User] we will never be able to devirtualize the call at compile-time and will not be able to inline. The compiler will just say that we cannot inline Noop.noop.

We would need to make the given transparent inline for this to work.

transparent inline given Noop[User] = a => a

But devirtualization on SAM types does not seem to work.

[LPTK]

As we only provide given Noop[User] we will never be able to devirtualize the call at compile-time

Note that it works with given Noop[User] with ... (as in my message above).

[odersky]

I guess classes with only abstract inline defs should not be SAM types.