Error in `-Ycheck:typer` when refining String on StringOps defs

[timotheeandres]

Compiler version

3.3.0-RC4

Minimized code

Compile this code with the -Ycheck:typer flag.

import reflect.Selectable.reflectiveSelectable

trait A[T](x: T{ def *(y: Int): T }):
  def f: T = x * 2

class B extends A("Hello")

Edit: further minimization (thanks to @nicolasstucki):

import reflect.Selectable.reflectiveSelectable
def f[T](x: T{ def *(y: Int): T }): T = x * 2
def test = f[scala.collection.StringOps | String]("Hello")

Output

checking a.scala after phase typer

  exception while retyping new A[scala.collection.StringOps | String](augmentString("Hello")) of class Apply # -1

  An unhandled exception was thrown in the compiler.
  Please file a crash report here:
  https://github.com/lampepfl/dotty/issues/new/choose

     while compiling: a.scala
        during phase: typer
                mode: Mode(ImplicitsEnabled)
     library version: version 2.13.10
    compiler version: version 3.3.1-RC1-bin-SNAPSHOT-nonbootstrapped-git-b3643ee
            settings: -Ycheck List(typer) -classpath C:\Users\Tim\AppData\Local\Coursier\Cache\v1\https\repo1.maven.org\maven2\org\scala-lang\scala-library\2.13.10\scala-library-2.13.10.jar;D:\Bureau\dotty\library\..\out\bootstrap\scala3-library-bootstrapped\scala-3.3.1-RC1-bin-SNAPSHOT-nonbootstrapped\scala3-library_3-3.3.1-RC1-bin-SNAPSHOT.jar

                tree: PackageDef(object <empty>)
       tree position: a.scala:1:0
           tree type: <empty>.type
              symbol: package <empty>
   symbol definition: final lazy module object <empty>: <empty> (a Symbol)
      symbol package: <root>
       symbol owners:
           call site: constructor B in class B in module class <empty>

  == Source file context for tree position ==

-- Error: a.scala:1:0 --------------------------------------------------------------------------------------------------
1 |import reflect.Selectable.reflectiveSelectable
  |^

2 |trait A[T](x: T{ def *(y: Int): T }):
3 |  def f: T = x * 2
4 |class B extends A("Hello")
*** error while checking a.scala after phase typer ***
Exception in thread "main" java.lang.AssertionError: assertion failed: Types differ
Original type : (x: (scala.collection.StringOps | String){def *(y: Int): scala.collection.StringOps | String}):
  A[scala.collection.StringOps | String]
After checking: (x: (scala.collection.StringOps | String){def *(y: Int): scala.collection.StringOps | String}):
  A[scala.collection.StringOps | String]
Original tree : new A[scala.collection.StringOps | String]
After checking: new A[scala.collection.StringOps | String]
Why different :
             Subtype trace:
   A[scala.collection.StringOps | String]  <:  (x: (scala.collection.StringOps | String){def *(y: Int): scala.collection.StringOps | String}):
    ==> (scala.collection.StringOps | String){def *(y: Int): scala.collection.StringOps | String}  <:  (scala.collection.StringOps | String){def *(y: Int): scala.collection.StringOps | String} in frozen constraint
      ==> (scala.collection.StringOps | String){def *(y: Int): scala.collection.StringOps | String}  <:  scala.collection.StringOps | String in frozen constraint
        ==> (scala.collection.StringOps | String){def *(y: Int): scala.collection.StringOps | String}  <:  scala.collection.StringOps | String in frozen constraint
          ==> (scala.collection.StringOps | String){def *(y: Int): scala.collection.StringOps | String}  <:  scala.collection.StringOps in frozen constraint
            ==> lub(scala.collection.StringOps, <notype>, canConstrain=false, isSoft=true)
            <== lub(scala.collection.StringOps, <notype>, canConstrain=false, isSoft=true) = <notype>
          <== (scala.collection.StringOps | String){def *(y: Int): scala.collection.StringOps | String}  <:  scala.collection.StringOps in frozen constraint = false
          ==> (scala.collection.StringOps | String){def *(y: Int): scala.collection.StringOps | String}  <:  String in frozen constraint
            ==> lub(<notype>, String, canConstrain=false, isSoft=true)
            <== lub(<notype>, String, canConstrain=false, isSoft=true) = <notype>
          <== (scala.collection.StringOps | String){def *(y: Int): scala.collection.StringOps | String}  <:  String in frozen constraint = false
        <== (scala.collection.StringOps | String){def *(y: Int): scala.collection.StringOps | String}  <:  scala.collection.StringOps | String in frozen constraint = false
      <== (scala.collection.StringOps | String){def *(y: Int): scala.collection.StringOps | String}  <:  scala.collection.StringOps | String in frozen constraint = false
    <== (scala.collection.StringOps | String){def *(y: Int): scala.collection.StringOps | String}  <:  (scala.collection.StringOps | String){def *(y: Int): scala.collection.StringOps | String} in frozen constraint = false
   A[scala.collection.StringOps | String] = false: (scala.collection.StringOps | String){def *(y: Int): scala.collection.StringOps | String}):
        at scala.runtime.Scala3RunTime$.assertFailed(Scala3RunTime.scala:8)
        at dotty.tools.dotc.transform.TreeChecker$Checker.typedUnadapted(TreeChecker.scala:408)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3175)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3179)
        at dotty.tools.dotc.transform.TreeChecker$Checker.typed(TreeChecker.scala:378)
        at dotty.tools.dotc.typer.Typer.typedExpr(Typer.scala:3291)
        at dotty.tools.dotc.typer.Applications.realApply$1(Applications.scala:941)
        at dotty.tools.dotc.typer.Applications.typedApply(Applications.scala:1101)
        at dotty.tools.dotc.typer.Applications.typedApply$(Applications.scala:352)
        at dotty.tools.dotc.transform.TreeChecker$Checker.typedApply(TreeChecker.scala:501)
        at dotty.tools.dotc.typer.Typer.typedUnnamed$1(Typer.scala:3040)
        at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:3103)
        at dotty.tools.dotc.typer.ReTyper.typedUnadapted(ReTyper.scala:126)
        at dotty.tools.dotc.transform.TreeChecker$Checker.typedUnadapted(TreeChecker.scala:395)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3175)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3179)
        at dotty.tools.dotc.transform.TreeChecker$Checker.typed(TreeChecker.scala:378)
        at dotty.tools.dotc.typer.Typer.typedExpr(Typer.scala:3291)
        at dotty.tools.dotc.typer.Typer.typedParent$1(Typer.scala:2560)
        at dotty.tools.dotc.typer.Typer.$anonfun$58(Typer.scala:2644)
        at dotty.tools.dotc.core.Decorators$.loop$1(Decorators.scala:94)
        at dotty.tools.dotc.core.Decorators$.mapconserve(Decorators.scala:110)
        at dotty.tools.dotc.typer.Typer.typedClassDef(Typer.scala:2644)
        at dotty.tools.dotc.transform.TreeChecker$Checker.typedClassDef(TreeChecker.scala:567)
        at dotty.tools.dotc.typer.Typer.typedTypeOrClassDef$1(Typer.scala:3028)
        at dotty.tools.dotc.typer.Typer.typedNamed$1(Typer.scala:3032)
        at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:3102)
        at dotty.tools.dotc.typer.ReTyper.typedUnadapted(ReTyper.scala:126)
        at dotty.tools.dotc.transform.TreeChecker$Checker.typedUnadapted(TreeChecker.scala:395)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3175)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3179)
        at dotty.tools.dotc.transform.TreeChecker$Checker.typed(TreeChecker.scala:378)
        at dotty.tools.dotc.typer.Typer.traverse$1(Typer.scala:3201)
        at dotty.tools.dotc.typer.Typer.typedStats(Typer.scala:3247)
        at dotty.tools.dotc.transform.TreeChecker$Checker.typedStats(TreeChecker.scala:632)
        at dotty.tools.dotc.typer.Typer.typedPackageDef(Typer.scala:2804)
        at dotty.tools.dotc.transform.TreeChecker$Checker.typedPackageDef(TreeChecker.scala:658)
        at dotty.tools.dotc.typer.Typer.typedUnnamed$1(Typer.scala:3073)
        at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:3103)
        at dotty.tools.dotc.typer.ReTyper.typedUnadapted(ReTyper.scala:126)
        at dotty.tools.dotc.transform.TreeChecker$Checker.typedUnadapted(TreeChecker.scala:395)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3175)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3179)
        at dotty.tools.dotc.transform.TreeChecker$Checker.typed(TreeChecker.scala:378)
        at dotty.tools.dotc.typer.Typer.typedExpr(Typer.scala:3291)
        at dotty.tools.dotc.transform.TreeChecker.check(TreeChecker.scala:126)
        at dotty.tools.dotc.transform.TreeChecker.run(TreeChecker.scala:106)
        at dotty.tools.dotc.core.Phases$Phase.runOn$$anonfun$1(Phases.scala:324)
        at scala.collection.immutable.List.map(List.scala:246)
        at dotty.tools.dotc.core.Phases$Phase.runOn(Phases.scala:328)
        at dotty.tools.dotc.Run.runPhases$1$$anonfun$1(Run.scala:246)
        at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
        at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
        at scala.collection.ArrayOps$.foreach$extension(ArrayOps.scala:1321)
        at dotty.tools.dotc.Run.runPhases$1(Run.scala:262)
        at dotty.tools.dotc.Run.compileUnits$$anonfun$1(Run.scala:270)
        at dotty.tools.dotc.Run.compileUnits$$anonfun$adapted$1(Run.scala:279)
        at dotty.tools.dotc.util.Stats$.maybeMonitored(Stats.scala:67)
        at dotty.tools.dotc.Run.compileUnits(Run.scala:279)
        at dotty.tools.dotc.Run.compileSources(Run.scala:194)
        at dotty.tools.dotc.Run.compile(Run.scala:179)
        at dotty.tools.dotc.Driver.doCompile(Driver.scala:37)
        at dotty.tools.dotc.Driver.process(Driver.scala:197)
        at dotty.tools.dotc.Driver.process(Driver.scala:165)
        at dotty.tools.dotc.Driver.process(Driver.scala:177)
        at dotty.tools.dotc.Driver.main(Driver.scala:207)
        at dotty.tools.dotc.Main.main(Main.scala)

Expectation

The code compiles both with and without the -Ycheck:typer flag.

More information

This seems to be related to the widening of String to scala.collection.StringOps | String, as this issue does not arise with a homemade class: the following code passes the check.

import reflect.Selectable.reflectiveSelectable

class C:
  def *(y: Int): C = this

trait A[T](x: T{ def *(y: Int): T }):
  def f: T = x * 2

class B extends A(new C())

[nicolasstucki]

Minimization

import reflect.Selectable.reflectiveSelectable
def f[T](x: T{ def *(y: Int): T }): T = x * 2
def test = f[scala.collection.StringOps | String]("Hello")

[nicolasstucki]

Maybe it is because class StringOps extends AnyVal.

[scala-center-bot]

This issue was picked for the Issue Spree No. 31 of 30 May 2023 which takes place in 2 days. @Linyxus, @hamzaremmal, @TheElectronWill will be working on it. If you have any insight into the issue or guidance on how to fix it, please leave it here.

[mbovel]

Relevant documentation:

• Structural types
• Selectable
• reflectiveSelectable
• StringOps