Wrong type inferred with Function1 and type parameter in result type

[panacekcz]

This test (loosely based on tests/pos/t2023.scala) normally compiles, but does not pass -Ycheck:all:

trait Or[A]
trait C

object ImplicitChainTest {
  def ipl[A](implicit from: A => Or[A]): C = null
  ipl
}

Output of dotc -Ycheck:all Test.scala:

checking Test.scala after phase frontend
exception while typing ImplicitChainTest.ipl[Or[_]]($conforms[Or[_]]) of class class dotty.tools.dotc.ast.Trees$Apply # 255
exception while typing final module class ImplicitChainTest() extends Object() { 
  this: ImplicitChainTest.type =>
 
  def ipl[A >: Nothing <: Any](implicit from: Function1[A, Or[A]]): C = null
  ImplicitChainTest.ipl[Or[_]]($conforms[Or[_]])
} of class class dotty.tools.dotc.ast.Trees$TypeDef # 257
exception while typing package <empty> {
  <trait> interface trait Or[A >: Nothing <: Any]() extends Object { 
     A
  }
  <trait> interface trait C() extends Object {}
  final lazy module val ImplicitChainTest: ImplicitChainTest = 
    new ImplicitChainTest()
  final module class ImplicitChainTest() extends Object() { 
    this: ImplicitChainTest.type =>
   
    def ipl[A >: Nothing <: Any](implicit from: Function1[A, Or[A]]): C = null
    ImplicitChainTest.ipl[Or[_]]($conforms[Or[_]])
  }
} of class class dotty.tools.dotc.ast.Trees$PackageDef # 258
*** error while checking Test.scala after phase frontend ***
exception occurred while compiling Test.scala
java.lang.AssertionError: assertion failed: found:    Or[_] <:< Or[_]
required: Or[_] => Or[Or[_]]


tree = $conforms[Or[_]] while compiling Test.scala
Exception in thread "main" java.lang.AssertionError: assertion failed: found:    Or[_] <:< Or[_]
required: Or[_] => Or[Or[_]]


tree = $conforms[Or[_]]
	at scala.Predef$.assert(Predef.scala:219)
	at dotty.tools.dotc.transform.TreeChecker$Checker.adapt(TreeChecker.scala:465)
	at dotty.tools.dotc.typer.ProtoTypes$FunProto.typedArg(ProtoTypes.scala:302)
	at dotty.tools.dotc.typer.Applications$ApplyToUntyped.typedArg(Applications.scala:711)
	at dotty.tools.dotc.typer.Applications$ApplyToUntyped.typedArg(Applications.scala:709)
	at dotty.tools.dotc.typer.Applications$Application.addTyped$1(Applications.scala:478)
	at dotty.tools.dotc.typer.Applications$Application.matchArgs(Applications.scala:520)
	at dotty.tools.dotc.typer.Applications$Application.init(Applications.scala:299)
	at dotty.tools.dotc.typer.Applications$TypedApply.<init>(Applications.scala:612)
	at dotty.tools.dotc.typer.Applications$ApplyToUntyped.<init>(Applications.scala:710)
	at dotty.tools.dotc.typer.Applications.simpleApply$1(Applications.scala:770)
	at dotty.tools.dotc.typer.Applications.$anonfun$typedApply$7(Applications.scala:795)
	at dotty.tools.dotc.typer.Typer.tryEither(Typer.scala:2053)
	at dotty.tools.dotc.typer.Applications.$anonfun$typedApply$1(Applications.scala:796)
	at dotty.tools.dotc.util.Stats$.track(Stats.scala:37)
	at dotty.tools.dotc.typer.Applications.realApply$1(Applications.scala:740)
	at dotty.tools.dotc.typer.Applications.typedApply(Applications.scala:841)
	at dotty.tools.dotc.typer.Applications.typedApply$(Applications.scala:738)
	at dotty.tools.dotc.typer.Typer.typedApply(Typer.scala:79)
	at dotty.tools.dotc.typer.Typer.typedUnnamed$1(Typer.scala:1885)
	at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:1939)
	at dotty.tools.dotc.typer.ReTyper.typedUnadapted(ReTyper.scala:110)
	at dotty.tools.dotc.transform.TreeChecker$Checker.typedUnadapted(TreeChecker.scala:269)
	at dotty.tools.dotc.typer.Typer.$anonfun$typed$2(Typer.scala:1970)
	at dotty.tools.dotc.reporting.trace$.apply(trace.scala:40)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:1966)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:1982)
	at dotty.tools.dotc.transform.TreeChecker$Checker.typed(TreeChecker.scala:257)
	at dotty.tools.dotc.typer.Typer.traverse$1(Typer.scala:2021)
	at dotty.tools.dotc.typer.Typer.typedStats(Typer.scala:2034)
	at dotty.tools.dotc.transform.TreeChecker$Checker.typedStats(TreeChecker.scala:432)
	at dotty.tools.dotc.typer.Typer.$anonfun$typedClassDef$1(Typer.scala:1597)
	at dotty.tools.dotc.util.Stats$.track(Stats.scala:37)
	at dotty.tools.dotc.typer.Typer.typedClassDef(Typer.scala:1519)
	at dotty.tools.dotc.transform.TreeChecker$Checker.typedClassDef(TreeChecker.scala:385)
	at dotty.tools.dotc.typer.Typer.typedNamed$1(Typer.scala:1875)
	at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:1938)
	at dotty.tools.dotc.typer.ReTyper.typedUnadapted(ReTyper.scala:110)
	at dotty.tools.dotc.transform.TreeChecker$Checker.typedUnadapted(TreeChecker.scala:269)
	at dotty.tools.dotc.typer.Typer.$anonfun$typed$2(Typer.scala:1970)
	at dotty.tools.dotc.reporting.trace$.apply(trace.scala:40)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:1966)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:1982)
	at dotty.tools.dotc.transform.TreeChecker$Checker.typed(TreeChecker.scala:257)
	at dotty.tools.dotc.typer.Typer.traverse$1(Typer.scala:2001)
	at dotty.tools.dotc.typer.Typer.typedStats(Typer.scala:2034)
	at dotty.tools.dotc.transform.TreeChecker$Checker.typedStats(TreeChecker.scala:432)
	at dotty.tools.dotc.typer.Typer.$anonfun$typedPackageDef$1(Typer.scala:1711)
	at dotty.tools.dotc.util.Stats$.track(Stats.scala:37)
	at dotty.tools.dotc.typer.Typer.typedPackageDef(Typer.scala:1704)
	at dotty.tools.dotc.typer.Typer.typedUnnamed$1(Typer.scala:1917)
	at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:1939)
	at dotty.tools.dotc.typer.ReTyper.typedUnadapted(ReTyper.scala:110)
	at dotty.tools.dotc.transform.TreeChecker$Checker.typedUnadapted(TreeChecker.scala:269)
	at dotty.tools.dotc.typer.Typer.$anonfun$typed$2(Typer.scala:1970)
	at dotty.tools.dotc.reporting.trace$.apply(trace.scala:40)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:1966)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:1982)
	at dotty.tools.dotc.transform.TreeChecker$Checker.typed(TreeChecker.scala:257)
	at dotty.tools.dotc.typer.Typer.typedExpr(Typer.scala:2045)
	at dotty.tools.dotc.transform.TreeChecker.check(TreeChecker.scala:122)
	at dotty.tools.dotc.transform.TreeChecker.run(TreeChecker.scala:94)
	at dotty.tools.dotc.core.Phases$Phase.$anonfun$runOn$1(Phases.scala:297)
	at scala.collection.immutable.List.map(List.scala:282)
	at dotty.tools.dotc.core.Phases$Phase.runOn(Phases.scala:295)
	at dotty.tools.dotc.core.Phases$Phase.runOn$(Phases.scala:294)
	at dotty.tools.dotc.transform.TreeChecker.runOn(TreeChecker.scala:38)
	at dotty.tools.dotc.Run.$anonfun$compileUnits$3(Run.scala:172)
	at scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:12)
	at dotty.tools.dotc.util.Stats$.trackTime(Stats.scala:49)
	at dotty.tools.dotc.Run.$anonfun$compileUnits$2(Run.scala:169)
	at dotty.tools.dotc.Run.$anonfun$compileUnits$2$adapted(Run.scala:167)
	at scala.collection.IndexedSeqOptimized.foreach(IndexedSeqOptimized.scala:32)
	at scala.collection.IndexedSeqOptimized.foreach$(IndexedSeqOptimized.scala:29)
	at scala.collection.mutable.ArrayOps$ofRef.foreach(ArrayOps.scala:194)
	at dotty.tools.dotc.Run.runPhases$1(Run.scala:167)
	at dotty.tools.dotc.Run.$anonfun$compileUnits$1(Run.scala:192)
	at scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:12)
	at dotty.tools.dotc.util.Stats$.maybeMonitored(Stats.scala:90)
	at dotty.tools.dotc.Run.compileUnits(Run.scala:147)
	at dotty.tools.dotc.Run.compileSources(Run.scala:134)
	at dotty.tools.dotc.Run.compile(Run.scala:118)
	at dotty.tools.dotc.Driver.doCompile(Driver.scala:30)
	at dotty.tools.dotc.Driver.process(Driver.scala:136)
	at dotty.tools.dotc.Driver.process(Driver.scala:105)
	at dotty.tools.dotc.Driver.process(Driver.scala:117)
	at dotty.tools.dotc.Driver.main(Driver.scala:144)
	at dotty.tools.dotc.Main.main(Main.scala)

[nicolasstucki]

This can be reproduced without implicits with

object Foo {
  type Or[A]
  def foo[A](from: A => Or[A]): Any = null
  def id[A]: A => A = identity
  foo(id)
}

where typer produces

package <empty> {
  final lazy module val Foo: Foo$ = new Foo$()
  final module class Foo$() extends Object(), _root_.scala.Serializable { 
    this: Foo.type =>
   
    type Or[A >: Nothing <: Any] >: Nothing <: Any
    def foo[A >: Nothing <: Any](from: Function1[A, Foo.Or[A]]): Any = null
    def id[A >: Nothing <: Any]: Function1[A, A] = 
      {
        def $anonfun(x: A): A = identity[A](x)
        closure($anonfun)
      }
    Foo.foo[Foo.Or[_]](Foo.id[Foo.Or[_]])
  }
}

this issue is that the inferred type Foo.Or[_] is not valid.

Though writing the inferred type explicitly does result in the correct typing error

6 |  foo(id[Or[_]])
  |      ^^^^^^^^^
  |      Found:    Foo.Or[_] => Foo.Or[_]
  |      Required: Nothing => Foo.Or[A]
  |      
  |      where:    A is a type variable

or

-- [E007] Type Mismatch Error: Foo.scala:6:13 ----------------------------------
6 |  foo[Or[_]](id)
  |             ^^
  |             Found:    Nothing => Nothing
  |             Required: Foo.Or[_] => Foo.Or[Foo.Or[_]]

This could be due to a missing constraint while inferring the type of A.

[odersky]

It infers a weird type, which looks nevertheless type correct.

package <empty> {
  final lazy module val Foo: Foo$ = new Foo$()
  final module class Foo$() extends Object(), _root_.scala.Serializable { 
    this: Foo.type =>
    type Or[A >: Nothing <: Any] >: Nothing <: Any
    def foo[A >: Nothing <: Any](from: Function1[A, Foo.Or[A]]): Any = null
    def id[A >: Nothing <: Any]: Function1[A, A] = 
      {
        def $anonfun(x: A): A = identity[A](x)
        closure($anonfun)
      }
    Foo.foo[Foo.Or[?]](Foo.id[Foo.Or[Foo.Or[?]]])
  }
}

This looks more like a problem with -Ycheck to me that it cannot typecheck this corner-case.