Compiler Crashes when Upper-Bounding a Class' Type Parameter with caps.Capability under -Xprint:cc

[bracevac]

Compiler version

3.6.0-RC1-bin-20240924-3f4fee2-NIGHTLY-git-3f4fee2

Minimized code

import language.experimental.captureChecking
import caps.Capability

trait File extends Capability

class Resource[T <: Capability](gen: T):
  def use[U](f: T => U): U = 
    f(gen)

@main def run = 
  val myFile: File = ???
  val r = Resource(myFile)
  ()

Edit: minimized further

Output (click arrow to expand)

[error] ## Exception when compiling 1 sources to /Users/oliver/playground/caps/caps/target/scala-3.6.0-RC1-bin-20240924-3f4fee2-NIGHTLY/classes
[error] dotty.tools.dotc.core.TypeError$$anon$1: invalid new prefix [T <: box caps.Capability^] =>> Test.Resource[box T^] cannot replace Test.Resource in type (Test.Resource#<init> : [T <: box caps.Capability^](gen: T^): Test.Resource[T])

[bracevac]

@odersky

[odersky]

I could not reproduce this. #21659 contains the example as a test case (21646.scala). This one compiles with errors but does not crash.

[bracevac]

@odersky The exception appears when BOTH the -Xprint:cc compiler option is enabled AND having the upper bound T <: Capability in class Resource.

Say this is in Caps.scala:

//> using scala 3.6.0-RC1-bin-20240918-be10596-NIGHTLY
import language.experimental.captureChecking
import caps.Capability

trait File extends Capability

class Resource[T <: Capability](gen: T):
  def use[U](f: T => U): U = 
    f(gen)

@main def run = 
  val myFile: File = ???
  val r = Resource(myFile)
  ()

and we run

scala compile Caps.scala -O -Xprint:cc

(resp. with scala-cli pre 3.5) then compiler will crash with an exception. It will also happen when compiling via a traditional sbt project using the nightly compiler build.

On the other hand, if we remove the bound <: Capability from T, then the compiler exception will not occur even if -Xprint:cc is enabled.

[bracevac]

I can also confirm that this happens with the latest nightly //> using scala 3.nightly

[bracevac]

The problem appears to be in the show method of class Recheck respectively the TreeMap used in addRecheckedTypes.