recompilation with commented change raises particular error(s)

[sjbiaga]

This behavior has been for quite a while: I don't know exactly whether it is a bug.

Compiler version

3.7.2-RC1

Minimized code

project/build.properties:

sbt.version=1.11.3

./build.sbt:

ThisBuild / scalaVersion := "3.7.2-RC1"

val scala2Opts = Seq("-feature", "-language:implicitConversions", "-deprecation", "-Ytasty-reader")
val scala3Opts = Seq("-feature", "-language:implicitConversions", "-indent", "-Xwiki-syntax", "-Xmax-inlines", "128", "-new-syntax")

lazy val root = (project in file("."))
  .settings(
    name := "test",
    organization := "test",
    organizationName := "test",
    version := "1.0",
    maxErrors := 5,
    scalaVersion := "3.7.2-RC1",
    javacOptions ++= Seq("-source", "16"),
    crossScalaVersions ++= Seq("2.13.16", "3.7.2-RC1"),
    scalacOptions ++= scala3Opts,
  )

./src/main/scala/Path.scala:

import UndoRedo.*


abstract trait Path[
  B <: Path[B, U, R],
  U <: Undo[B, U, R],
  R <: Redo[B, U, R]
]:
  val parent: Option[B]

  val undo: Option[U]
  val redo: Option[R]

  protected def apply(): B // fresh

./src/main/scala/UndoRedo.scala:

import UndoRedo.*


abstract trait UndoRedo[
  B <: Path[B, U, R],
  U <: Undo[B, U, R] & UndoRedo[B, U, R, U],
  R <: Redo[B, U, R] & UndoRedo[B, U, R, R],
  UR <: UndoRedo[B, U, R, UR]
]:

  val next: Option[UR]
  val path: B

  def apply(real: B): UR // clone

object UndoRedo:

  abstract trait Undo[
    B <: Path[B, U, R],
    U <: Undo[B, U, R],
    R <: Redo[B, U, R]
  ] extends UndoRedo[B, U, R, U]


  abstract trait Redo[
    B <: Path[B, U, R],
    U <: Undo[B, U, R],
    R <: Redo[B, U, R]
  ] extends UndoRedo[B, U, R, R]:

    val undo: U

Output

Just change the comment only in UndoRedo.scala from // clone to /* clone */ and issue compile in sbt shell again:

sbt:test> compile
[info] compiling 1 Scala source to /home/user/target/scala-3.7.2-RC1/classes ...
[error] -- [E057] Type Mismatch Error: /home/user/src/main/scala/UndoRedo.scala:22:30 
[error] 22 |  ] extends UndoRedo[B, U, R, U]
[error]    |                              ^
[error]    |    Type argument U does not conform to upper bound UndoRedo[B, U, R, U]
[error]    |
[error]    | longer explanation available when compiling with `-explain`
[error] -- [E057] Type Mismatch Error: /home/user/src/main/scala/UndoRedo.scala:29:27 
[error] 29 |  ] extends UndoRedo[B, U, R, R]:
[error]    |                           ^
[error]    |Type argument R does not conform to upper bound UndoRedo.Redo[B, U, R] & UndoRedo[B, U, R, R]
[error]    |
[error]    | longer explanation available when compiling with `-explain`
[error] -- [E057] Type Mismatch Error: /home/user/src/main/scala/UndoRedo.scala:29:30 
[error] 29 |  ] extends UndoRedo[B, U, R, R]:
[error]    |                              ^
[error]    |    Type argument R does not conform to upper bound UndoRedo[B, U, R, R]
[error]    |
[error]    | longer explanation available when compiling with `-explain`
[error] three errors found
[error] (Compile / compileIncremental) Compilation failed
[error] Total time: 0 s, completed 9 jul 2025, 20:36:17

The errors are the same: Type argument R does not conform to upper bound ... Change the comment back to // code and it's as nothing has happened.

Expectation

A modification of the file's digest should incrementally recompile ok.
If both *.scala source file's are modified, then recompilation succeeds.

[Gedochao]

Reproducible with much simplified build.sbt:

ThisBuild / scalaVersion := "3.7.2-RC1"

lazy val root = (project in file("."))
  .settings(scalaVersion := "3.7.2-RC1")

[Gedochao]

Also reproducible with Scala CLI (with Bloop).