Aliased Unit improperly boxed

[som-snytt]

Compiler version

3.7.4

Minimized code

➜  snips cat predef.scala
package p

object MyPredef {
  type Unit = scala.Unit
}

➜  snips cat no-unit-main.scala
//> using options -Yimports:java.lang,scala,p.MyPredef
///> using options -Yimports:java.lang,scala

trait Main {
  def main(args: Array[String]): Unit = Console.println("hello, world")
}

object Test extends Main

Output

➜  snips scala-cli compile --server=false -S 3.7.4 -d /tmp -Yno-imports predef.scala
WARNING: A terminally deprecated method in sun.misc.Unsafe has been called
WARNING: sun.misc.Unsafe::objectFieldOffset has been called by scala.runtime.LazyVals$ (file:/home/amarki/.cache/coursier/v1/https/repo1.maven.org/maven2/org/scala-lang/scala3-library_3/3.7.4/scala3-library_3-3.7.4.jar)
WARNING: Please consider reporting this to the maintainers of class scala.runtime.LazyVals$
WARNING: sun.misc.Unsafe::objectFieldOffset will be removed in a future release
➜  snips scala-cli run --server=false -S 3.7.4 -Vprint:eras -d /tmp -cp /tmp no-unit-main.scala
WARNING: A terminally deprecated method in sun.misc.Unsafe has been called
WARNING: sun.misc.Unsafe::objectFieldOffset has been called by scala.runtime.LazyVals$ (file:/home/amarki/.cache/coursier/v1/https/repo1.maven.org/maven2/org/scala-lang/scala3-library_3/3.7.4/scala3-library_3-3.7.4.jar)
WARNING: Please consider reporting this to the maintainers of class scala.runtime.LazyVals$
WARNING: sun.misc.Unsafe::objectFieldOffset will be removed in a future release
[[syntax trees at end of                   erasure]] // /home/amarki/snips/no-unit-main.scala
package <empty> {
  @SourceFile("no-unit-main.scala") trait Main() extends Object {
    def main(args: String[]): scala.runtime.BoxedUnit =
      {
        Console.println("hello, world")
        scala.runtime.BoxedUnit.UNIT
      }
  }
  final lazy module val Test: Test = new Test()
  @SourceFile("no-unit-main.scala") final module class Test() extends Object(),
    Main {
    private def writeReplace(): Object =
      new scala.runtime.ModuleSerializationProxy(classOf[Test])
  }
}

[error]  No main class found

Expectation

➜  snips scala-cli compile --server=false -S 2.13.17 -d /tmp -Yno-imports predef.scala
➜  snips scala-cli run --server=false -S 2.13.17 -Vprint:eras -d /tmp -cp /tmp no-unit-main.scala
[[syntax trees at end of                   erasure]] // no-unit-main.scala
package <empty> {
  abstract trait Main extends Object {
    def /*Main*/$init$(): Unit = {
      ()
    };
    def main(args: Array[String]): Unit = scala.Console.println("hello, world")
  };
  object Test extends Object with Main {
    def <init>(): Test.type = {
      Test.super.<init>();
      Test.super./*Main*/$init$();
      ()
    }
  }
}

hello, world

Noticed at https://users.scala-lang.org/t/inheriting-def-main/12139/6?u=som-snytt

[noti0na1]

object A:
  type U = Unit

object Test:
  def main(args: Array[String]): A.U =
    println("Test")

In eraseResult of TypeErasure, we only check if the symbol of a TypeRef is eq UnitClass. If we change to isRef(def.UnitClass), then type alias should work here.

I was looking at the Unit erasure logic recently, and I was also wondering why we didn't handle type alias...

[hamzaremmal]

@noti0na1 I was thinking of taking this issue, should be a quick fix, will you continue the work on it?

[sjrd]

It's a quick fix if we assume that we're allowed to fix it. It changes the erasure of working code, which is a big red flag!

[hamzaremmal]

It's a quick fix if we assume that we're allowed to fix it. It changes the erasure of working code, which is a big red flag!

It is erasure, we propose a fix and then we see if we merge it or if we keep it in the PR queue until someday.

[noti0na1]

Here is the fix which passes all compilation tests locally:

def eraseResult(tp: Type)(using Context): Type =
  if semiEraseVCs && isConstructor && !tp.isInstanceOf[MethodOrPoly] then
    erasureFn(sourceLanguage, semiEraseVCs = false, isConstructor, isSymbol, inSigName).eraseResult(tp)
  else
    if tp.isRef(defn.UnitClass) then defn.UnitType
    else tp match
      case tp: AppliedType =>
        val sym = tp.tycon.typeSymbol
        if (sym.isClass && !erasureDependsOnArgs(sym)) eraseResult(tp.tycon)
        else apply(tp)
      case _ =>
        apply(tp)

I know it's controversial so I didn't submit the pr ;)