Adding self type breaks usages of an otherwise accessible protected method

[raquo]

Compiler version

3.3.7, 3.5.2, 3.8.0-RC1-bin-20251030-6b3569d-NIGHTLY

Minimized code

Must be in two separate files in different packages:

example/other/StyleProp.scala:

package example.other
import example.StyleBuilder

class StyleProp(val name: String) extends StyleBuilder {
  override protected def builder(value: String): String = value
}

example/Example.scala:

package example

import example.other.StyleProp

trait StyleBuilder {
  protected def builder(value: String): String
}

trait AutoStyle extends StyleBuilder { // this: StyleProp =>
  lazy val auto: String = builder("auto")
}

// Usage:
object AutoStyle {
  val width = new StyleProp("width") with AutoStyle
  assert(AutoStyle.width.auto == "auto")
}

Output

The code above compiles ok as-written, as expected.

However, if you uncomment the self-type constraintthis: StyleProp =>, then the code fails with:

[error] -- Error: .../src/main/scala/example/Example.scala:10:26
[error] 10 |  lazy val auto: String = builder("auto")
[error]    |                          ^^^^^^^
[error]    |illegal access to protected method builder in class StyleProp from trait AutoStyle

Changing class StyleProp(val name: String) to trait StyleProp(val name: String) avoids the error for some reason (in 3.3.7 and 3.7.1). However, note that after changing back from trait to class, you need to sbt clean to see the error again.

Also, perhaps a separate issue, but changing to trait StyleProp(val name: String) seems to crash the Scala.js part of the compiler in 3.8.0-RC1-bin-20251030-6b3569d-NIGHTLY:

Click to expand

[debug] [zinc] IncrementalCompile ----------- [debug] IncrementalCompile.incrementalCompile [debug] previous = Stamps for: 0 products, 0 sources, 0 libraries [debug] current source = Set(${BASE}/src/main/scala/example/Main.scala, ${BASE}/src/main/scala/example/HelloWorld.scala, ${BASE}/src/main/scala/example/Example.scala, ${BASE}/src/main/scala/example/other/StyleProp.scala, ${BASE}/src/main/scala/example/other/Direction.scala, ${BASE}/src/main/scala/example/other/EnumObj.scala) [debug] > initialChanges = InitialChanges(Changes(added = Set(${BASE}/src/main/scala/example/other/EnumObj.scala, ${BASE}/src/main/scala/example/HelloWorld.scala, ${BASE}/src/main/scala/example/other/Direction.scala, ${BASE}/src/main/scala/example/Example.scala, ${BASE}/src/main/scala/example/Main.scala, ${BASE}/src/main/scala/example/other/StyleProp.scala), removed = Set(), changed = Set(), unmodified = ...),Set(),Set(),API Changes: Set()) [debug] Full compilation, no sources in previous analysis. [debug] all 6 sources are invalidated [debug] Initial set of included nodes: [debug] Recompiling all sources: number of invalidated sources > 50.0 percent of all sources [debug] compilation cycle 1 [info] compiling 6 Scala sources to /Users/raquo/code/scala/test/target/scala-3.8.0-RC1-bin-20251030-6b3569d-NIGHTLY/classes ... [debug] Returning already retrieved and compiled bridge: /Users/raquo/Library/Caches/Coursier/v1/https/repo.scala-lang.org/artifactory/maven-nightlies/org/scala-lang/scala3-sbt-bridge/3.8.0-RC1-bin-20251030-6b3569d-NIGHTLY/scala3-sbt-bridge-3.8.0-RC1-bin-20251030-6b3569d-NIGHTLY.jar. [debug] [zinc] Running cached compiler 2ff617dd for Scala Compiler version 3.8.0-RC1-bin-20251030-6b3569d-NIGHTLY-git-6b3569d [debug] [zinc] The Scala compiler is invoked with: [debug] -scalajs [debug] -encoding [debug] utf-8 [debug] -deprecation [debug] -feature [debug] -classpath [debug] /Users/raquo/code/scala/test/target/scala-3.8.0-RC1-bin-20251030-6b3569d-NIGHTLY/classes:/Users/raquo/Library/Caches/Coursier/v1/https/repo.scala-lang.org/artifactory/maven-nightlies/org/scala-lang/scala3-library_sjs1_3/3.8.0-RC1-bin-20251030-6b3569d-NIGHTLY/scala3-library_sjs1_3-3.8.0-RC1-bin-20251030-6b3569d-NIGHTLY.jar:/Users/raquo/Library/Caches/Coursier/v1/https/repo1.maven.org/maven2/org/scala-js/scalajs-library_2.13/1.17.0/scalajs-library_2.13-1.17.0.jar:/Users/raquo/Library/Caches/Coursier/v1/https/repo1.maven.org/maven2/org/scala-js/scalajs-dom_sjs1_3/2.8.0/scalajs-dom_sjs1_3-2.8.0.jar:/Users/raquo/Library/Caches/Coursier/v1/https/repo1.maven.org/maven2/com/raquo/laminar_sjs1_3/17.2.0/laminar_sjs1_3-17.2.0.jar:/Users/raquo/Library/Caches/Coursier/v1/https/repo.scala-lang.org/artifactory/maven-nightlies/org/scala-js/scalajs-scalalib_2.13_sjs1_3/3.8.0-RC1-bin-20251030-6b3569d-NIGHTLY/scalajs-scalalib_2.13_sjs1_3-3.8.0-RC1-bin-20251030-6b3569d-NIGHTLY.jar:/Users/raquo/Library/Caches/Coursier/v1/https/repo.scala-lang.org/artifactory/maven-nightlies/org/scala-lang/scala-library/3.8.0-RC1-bin-20251030-6b3569d-NIGHTLY/scala-library-3.8.0-RC1-bin-20251030-6b3569d-NIGHTLY.jar:/Users/raquo/Library/Caches/Coursier/v1/https/repo1.maven.org/maven2/org/scala-js/scalajs-javalib/1.20.1/scalajs-javalib-1.20.1.jar:/Users/raquo/Library/Caches/Coursier/v1/https/repo1.maven.org/maven2/org/scala-js/scalajs-scalalib_2.13/2.13.13%2B1.17.0/scalajs-scalalib_2.13-2.13.13%2B1.17.0.jar:/Users/raquo/.ivy2/local/com.raquo/airstream_sjs1_3/17.2.0/jars/airstream_sjs1_3.jar:/Users/raquo/Library/Caches/Coursier/v1/https/repo1.maven.org/maven2/com/raquo/ew_sjs1_3/0.2.0/ew_sjs1_3-0.2.0.jar:/Users/raquo/Library/Caches/Coursier/v1/https/repo.scala-lang.org/artifactory/maven-nightlies/org/scala-lang/scala3-library_3/3.8.0-RC1-bin-20251030-6b3569d-NIGHTLY/scala3-library_3-3.8.0-RC1-bin-20251030-6b3569d-NIGHTLY.jar:/Users/raquo/Library/Caches/Coursier/v1/https/repo1.maven.org/maven2/app/tulz/tuplez-full-light_sjs1_3/0.4.0/tuplez-full-light_sjs1_3-0.4.0.jar [error] ## Exception when compiling 6 sources to /Users/raquo/code/scala/test/target/scala-3.8.0-RC1-bin-20251030-6b3569d-NIGHTLY/classes [error] dotty.tools.dotc.core.TypeError$$anon$1: package scala.scalajs.js does not have a member method async [error] [error] [error] dotty.tools.dotc.core.TypeError$$anon$1: package scala.scalajs.js does not have a member method async

Expectation

There is previous discussion of protected methods vs self-types in #18245, however I believe my example here is (related, but) a different problem: AutoStyle extends StyleBuilder, thus we should have access to StyleBuilder's protected def builder inside AutoStyle. The additional StyleProp self-type should not be taking away this capability that was granted by direct inheritance.

[som-snytt]

Can be in the same file; it's not separate or incremental compilation bug. "Works in Scala 2."

The lazy val initializer in a concrete class invokes the initializer in AutoStyle which calls builder, but that is AutoStyle.builder.

  #33 = InterfaceMethodref #2.#32         // example/AutoStyle.builder:(Ljava/lang/String;)Ljava/lang/String;

Scala 2 with the self type makes the adjustment:

         1: checkcast     #20                 // class example/other/StyleProp
         4: ldc           #21                 // String auto
         6: invokevirtual #25                 // Method example/other/StyleProp.builder:(Ljava/lang/String;)Ljava/lang/String;

[raquo]

Can be in the same file

Hm, not sure why I'm seeing different. Any single-file version I tried compiles fine, the problem is only if I put StyleProp in a different file from which it needs to be imported. Tested in sbt and scalac repl, with sbt clean, 3.3.7, 3.5.1, 3.7.1, various variations of this code, and it compiles with no error for me.

I tried to post this to scastie, but it doesn't seem to do anything when I click "run".

object SameFile {

  import package2.StyleBuilder

  object package1 {

    class StyleProp(val name: String) extends StyleBuilder {
      override protected def builder(value: String): String = value
    }
  }

  import package1.StyleProp

  object package2 {

    trait StyleBuilder {
      protected def builder(value: String): String
    }

    trait AutoStyle extends StyleBuilder { this: StyleProp =>
      lazy val auto: String = builder("auto")
    }
  }

  import package2.AutoStyle

  // Usage:
  val width = new StyleProp("width") with AutoStyle
  println(width.auto == "auto")
}

[som-snytt]

Here is what I tried, in a single source file. Some of linked issues highlight that Scala 2 & 3 diverge a bit on protected, especially with respect to packages.

package example.other {
  import example.StyleBuilder

  class StyleProp(val name: String) extends StyleBuilder {
    override protected def builder(value: String): String = value
  }
}

package example {

  import example.other.StyleProp

  trait StyleBuilder {
    protected def builder(value: String): String
  }

  trait AutoStyle extends StyleBuilder { // this: StyleProp =>
    lazy val auto: String = builder("auto")
  }

  // Usage:
  object AutoStyle {
    val width = new StyleProp("width") with AutoStyle
    assert(AutoStyle.width.auto == "auto")
  }

  trait AutoStyle2 extends StyleBuilder { this: StyleProp =>
    lazy val auto: String = builder("auto")
  }
  /*

  trait AutoStyle3 extends StyleBuilder { this: StyleProp with AutoStyle3 =>
    lazy val auto: String = builder("auto")
  }
  */

  class C extends StyleProp("c") with AutoStyle2
}

[raquo]

Thanks, your version fails to compile for me too. Good to know about using packages like this for providing a single file reproduction.