Dotty typer crashes with scalajs Lambda SAM error

[evbo]

Compiler version

Scala 3.2.0 RC-2 and RC-1

Minimized code

https://scastie.scala-lang.org/W2mz3ejSTOOIOkuUzHC6Tg

//> using scala "3.2.0-RC2"
//> using platform "scala-js"

import scala.scalajs.js

sealed trait Foo[T]

given Foo[Int] with {}

class Bar[T: Foo](val getValue: () => T)

class FooWorks[T: Foo](val foo: Bar[T] => T = (f: Bar[T]) => f.getValue())

class FooSAM[T: Foo](val foo: js.Function1[Bar[T],T] = (f: Bar[T]) => f.getValue())

(note that scala3 types compile but scalajs type does not)

Output (click arrow to expand)

[error] ./test.scala:14:83: result type of lambda is an underspecified SAM type scalajs.js.Function1[Bar[<?>], <?>]
[error] class FooSAM[T: Foo](val foo: js.Function1[Bar[T],T] = (f: Bar[T]) => f.getValue())
[error]                                                                                   ^
exception occurred while typechecking /tmp/i15785/test.scala
exception occurred while compiling /tmp/i15785/test.scala
Error compiling project (Scala 3.2.0-RC2, Scala.js)
java.lang.Error: internal error: type of right-hand side scalajs.js.Function1[Bar[<?>], <?>] is not fully defined, pos = <260..329>
	at dotty.tools.dotc.typer.Inferencing$.fullyDefinedType(Inferencing.scala:54)
	at dotty.tools.dotc.typer.Namer.lhsType$1(Namer.scala:1893)
	at dotty.tools.dotc.typer.Namer.inferredResultType(Namer.scala:1904)
	at dotty.tools.dotc.typer.Namer.inferredType$1(Namer.scala:1657)
	at dotty.tools.dotc.typer.Namer.valOrDefDefSig(Namer.scala:1664)
	at dotty.tools.dotc.typer.Namer.defDefSig(Namer.scala:1749)
	at dotty.tools.dotc.typer.Namer$Completer.typeSig(Namer.scala:786)
	at dotty.tools.dotc.typer.Namer$Completer.completeInCreationContext(Namer.scala:921)
	at dotty.tools.dotc.typer.Namer$Completer.complete(Namer.scala:809)
	at dotty.tools.dotc.core.SymDenotations$SymDenotation.completeFrom(SymDenotations.scala:171)
	at dotty.tools.dotc.core.Denotations$Denotation.completeInfo$1(Denotations.scala:187)
	at dotty.tools.dotc.core.Denotations$Denotation.info(Denotations.scala:189)
	at dotty.tools.dotc.core.SymDenotations$SymDenotation.ensureCompleted(SymDenotations.scala:373)
	at dotty.tools.dotc.typer.Typer.retrieveSym(Typer.scala:2800)
	at dotty.tools.dotc.typer.Typer.typedNamed$1(Typer.scala:2825)
	at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:2921)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:2987)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:2991)
	at dotty.tools.dotc.typer.Typer.traverse$1(Typer.scala:3013)
	at dotty.tools.dotc.typer.Typer.typedStats(Typer.scala:3063)
	at dotty.tools.dotc.typer.Typer.typedClassDef(Typer.scala:2500)
	at dotty.tools.dotc.typer.Typer.typedTypeOrClassDef$1(Typer.scala:2847)
	at dotty.tools.dotc.typer.Typer.typedNamed$1(Typer.scala:2851)
	at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:2921)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:2987)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:2991)
	at dotty.tools.dotc.typer.Typer.traverse$1(Typer.scala:3013)
	at dotty.tools.dotc.typer.Typer.typedStats(Typer.scala:3063)
	at dotty.tools.dotc.typer.Typer.typedPackageDef(Typer.scala:2629)
	at dotty.tools.dotc.typer.Typer.typedUnnamed$1(Typer.scala:2892)
	at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:2922)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:2987)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:2991)
	at dotty.tools.dotc.typer.Typer.typedExpr(Typer.scala:3107)
	at dotty.tools.dotc.typer.TyperPhase.typeCheck$$anonfun$1(TyperPhase.scala:43)
	at dotty.tools.dotc.typer.TyperPhase.typeCheck$$anonfun$adapted$1(TyperPhase.scala:50)
	at scala.Function0.apply$mcV$sp(Function0.scala:39)
	at dotty.tools.dotc.core.Phases$Phase.monitor(Phases.scala:417)
	at dotty.tools.dotc.typer.TyperPhase.typeCheck(TyperPhase.scala:50)
	at dotty.tools.dotc.typer.TyperPhase.runOn$$anonfun$3(TyperPhase.scala:84)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
	at scala.collection.immutable.List.foreach(List.scala:333)
	at dotty.tools.dotc.typer.TyperPhase.runOn(TyperPhase.scala:84)
	at dotty.tools.dotc.Run.runPhases$1$$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.xsbt.CompilerBridgeDriver.run(CompilerBridgeDriver.java:88)
	at dotty.tools.xsbt.CompilerBridge.run(CompilerBridge.java:22)
	at sbt.internal.inc.AnalyzingCompiler.compile(AnalyzingCompiler.scala:91)
	at sbt.internal.inc.bloop.internal.BloopHighLevelCompiler.compileSources$1(BloopHighLevelCompiler.scala:133)
	at sbt.internal.inc.bloop.internal.BloopHighLevelCompiler.$anonfun$compile$7(BloopHighLevelCompiler.scala:151)
	at scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:23)
	at sbt.internal.inc.bloop.internal.BloopHighLevelCompiler.$anonfun$compile$1(BloopHighLevelCompiler.scala:71)
	at bloop.tracing.BraveTracer.traceInternal(BraveTracer.scala:65)
	at bloop.tracing.BraveTracer.trace(BraveTracer.scala:39)
	at sbt.internal.inc.bloop.internal.BloopHighLevelCompiler.timed$1(BloopHighLevelCompiler.scala:70)
	at sbt.internal.inc.bloop.internal.BloopHighLevelCompiler.$anonfun$compile$6(BloopHighLevelCompiler.scala:151)
	at scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:23)
	at monix.eval.internal.TaskRunLoop$.monix$eval$internal$TaskRunLoop$$loop$1(TaskRunLoop.scala:187)
	at monix.eval.internal.TaskRunLoop$RestartCallback$1.onSuccess(TaskRunLoop.scala:119)
	at monix.eval.Task$.$anonfun$forkedUnit$2(Task.scala:1463)
	at java.base/java.util.concurrent.ForkJoinTask$RunnableExecuteAction.exec(ForkJoinTask.java:1395)
	at java.base/java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:373)
	at java.base/java.util.concurrent.ForkJoinPool$WorkQueue.topLevelExec(ForkJoinPool.java:1182)
	at java.base/java.util.concurrent.ForkJoinPool.scan(ForkJoinPool.java:1655)
	at java.base/java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1622)
	at java.base/java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:165)
Error: Unexpected error when compiling project_87a0305fea_87a0305fea-a20649ef27: 'internal error: type of right-hand side scalajs.js.Function1[Bar[<?>], <?>] is not fully defined, pos = <260..329>'

Additional discussion on discord:
https://discord.com/channels/632150470000902164/635668814956068864/1002302314889084928

[sjrd]

I can reproduce this without Scala.js, by defining a SAM type as follows:

trait SAMFunction1[-T1, +R] {:
  def apply(x1: T1): R

and using it instead of js.Function1. So this is not Scala.js-related.

In fact, I can minimize it all the way to

trait SAMFunction1[-T1, +R]:
  def apply(x1: T1): R

def fooSAM[T](foo: SAMFunction1[T, T] = (f: T) => f): Unit = ()

No need for context bounds or givens, or for a class constructor. Just a parameter of a generic SAM type with a default value which is a lambda. (Replacing SAMFunction1[T, T] by SAMFunction1[Int, Int] fixes the crash, so the genericity of the SAM is relevant to the issue.)

[smarter]

The default argument is typed with an expected type of SAMFunction1[WildcardType, WildcardType], we can give it a more precise type manually with an explicit ascription:

trait SAMFunction1[-T1, +R]:
  def apply(x1: T1): R

def fooSAM[T](foo: SAMFunction1[T, T] = ((f: T) => f): SAMFunction1[T, T]): Unit = ()

Since SAM types with wildcards are hard to handle, it'd be simpler to always type them with the more precise expected type SAMFunction1[T, T]. I believe this is achievable by generalizing the check in https://github.com/lampepfl/dotty/blob/6e5be23c6baaa8b3b904bf32ad4b7712d89cedfc/compiler/src/dotty/tools/dotc/typer/Namer.scala#L1860-L1882
to also match on case SAMType(_) if !defn.isFunctionType(approxTp) (Function types are also matched by SAM types, but we do want to type them with wildcards so that def fooFun[T](foo: Function1[T, T] = (f: Int) => f): Unit = () can compile).