Fix scala#14240: Fix off-by-1 when emitting Scala.js IR Positions.

scalac positions are 1-based, while IR positions are 0-based. There
is therefore a `- 1` in the nsc plugin, that was ported over to
dotc without questioning. It turns out that dotc uses 0-based
positions as well (as documented in `SourceFile`), and so we should
not make any adaptation there.

compiler/src/dotty/tools/backend/sjs/JSPositions.scala

@@ -39,11 +39,11 @@ class JSPositions()(using Context) {
   private def sourceAndSpan2irPos(source: SourceFile, span: Span): ir.Position = {
     if (!span.exists) ir.Position.NoPosition
     else {
-      // dotty positions are 1-based but IR positions are 0-based
+      // dotty positions and IR positions are both 0-based
       val irSource = span2irPosCache.toIRSource(source)
       val point = span.point
-      val line = source.offsetToLine(point) - 1
-      val column = source.column(point) - 1
+      val line = source.offsetToLine(point)
+      val column = source.column(point)
       ir.Position(irSource, line, column)
     }
   }

sbt-test/scalajs/basic/build.sbt

@@ -1,3 +1,5 @@
+lazy val testIRPositions = taskKey[Unit]("test IR positions (#14240)")
+
 enablePlugins(ScalaJSPlugin)
 
 scalaVersion := sys.props("plugin.scalaVersion")
@@ -6,3 +8,46 @@ scalaVersion := sys.props("plugin.scalaVersion")
 libraryDependencies += ("org.scala-js" %%% "scalajs-dom" % "1.1.0").cross(CrossVersion.for3Use2_13)
 
 scalaJSUseMainModuleInitializer := true
+
+// #14240 Make sure that generated IR positions are 0-based
+testIRPositions := {
+  import scala.concurrent.{Future, _}
+  import scala.concurrent.ExecutionContext.Implicits.global
+  import scala.concurrent.duration._
+  import scala.util.{Failure, Success}
+
+  import org.scalajs.ir.Names._
+  import org.scalajs.ir.Position
+  import org.scalajs.ir.Trees._
+  import org.scalajs.linker.interface.unstable.IRFileImpl
+
+  val ir = (Compile / scalaJSIR).value
+  val classNameToTest = ClassName("test.Main$")
+
+  val classDefFuture = {
+    // This logic is copied from the implementation of `scalajsp` in sbt-scalajs
+    Future.traverse(ir.data) { irFile =>
+      val ir = IRFileImpl.fromIRFile(irFile)
+      ir.entryPointsInfo.map { i =>
+        if (i.className == classNameToTest) Success(Some(ir))
+        else Success(None)
+      }.recover { case t => Failure(t) }
+    }.flatMap { irs =>
+      irs.collectFirst {
+        case Success(Some(f)) => f.tree
+      }.getOrElse {
+        val t = new MessageOnlyException(s"class ${classNameToTest.nameString} not found on classpath")
+        irs.collect { case Failure(st) => t.addSuppressed(st) }
+        throw t
+      }
+    }
+  }
+  val classDef = Await.result(classDefFuture, Duration.Inf)
+
+  def testPos(pos: Position, expectedLine: Int, expectedColumn: Int): Unit = {
+    if (!pos.source.getPath.endsWith("/Main.scala") || pos.line != expectedLine || pos.column != expectedColumn)
+      throw new MessageOnlyException(s"Expected Main.scala@$expectedLine:$expectedColumn but got $pos")
+  }
+
+  testPos(classDef.pos, 5, 7)
+}

sbt-test/scalajs/basic/test

@@ -1 +1,2 @@
 > run
+> testIRPositions