[2.x] Show warnings when scalaVersion is missing

**Problem**
There's a disconnect between what is perceived to be the current
Scala version, and what sbt uses internally, and thus what it
chooses to be the default scalaVersion.

**Solution**
This displays a warning if scalaVersion setting is missing.

build.sbt

@@ -290,8 +290,11 @@ lazy val utilPosition = (project in file("internal") / "util-position")
     utilCommonSettings,
     name := "Util Position",
     scalacOptions += "-language:experimental.macros",
-    libraryDependencies ++= Seq(scalatest % "test"),
+    libraryDependencies ++= Seq(hedgehog % Test),
     mimaSettings,
+    mimaBinaryIssueFilters ++= Seq(
+      exclude[ReversedMissingMethodProblem]("sbt.internal.util.FilePosition.sourceCode"),
+    ),
   )
 
 lazy val utilCore = project

internal/util-position/src/main/scala-3/sbt/internal/util/SourcePositionMacro.scala

@@ -23,13 +23,14 @@ abstract class SourcePositionImpl {
 object SourcePositionImpl {
 
   def fromEnclosingImpl(using Quotes): Expr[SourcePosition] = {
-    val x = quotes.reflect.Position.ofMacroExpansion
-
-    '{
-      LinePosition(
-        path = ${ Expr(x.sourceFile.name) },
-        startLine = ${ Expr(x.startLine + 1) }
-      )
-    }
+    val pos = quotes.reflect.Position.ofMacroExpansion
+    if pos.startLine >= 0 then
+      '{
+        LinePosition(
+          path = ${ Expr(pos.sourceFile.name) },
+          startLine = ${ Expr(pos.startLine) }
+        ).withSourceCode(${ Expr(pos.sourceCode) })
+      }
+    else '{ NoPosition }
   }
 }

internal/util-position/src/main/scala/sbt/internal/util/Positions.scala

@@ -13,18 +13,40 @@ sealed trait SourcePosition
 sealed trait FilePosition extends SourcePosition {
   def path: String
   def startLine: Int
+  def sourceCode: Option[String]
 }
 
 case object NoPosition extends SourcePosition
 
-final case class LinePosition(path: String, startLine: Int) extends FilePosition
+final case class LinePosition(path: String, startLine: Int) extends FilePosition {
+  private var _sourceCode: Option[String] = None
+  def sourceCode: Option[String] = _sourceCode
+  def withSourceCode(code: String): LinePosition =
+    val o = copy()
+    o._sourceCode = Some(code)
+    o
+  def withSourceCode(c: Option[String]): LinePosition =
+    c match
+      case Some(code) => this.withSourceCode(code)
+      case None       => this
+}
 
 final case class LineRange(start: Int, end: Int) {
   def shift(n: Int) = new LineRange(start + n, end + n)
 }
 
 final case class RangePosition(path: String, range: LineRange) extends FilePosition {
+  private var _sourceCode: Option[String] = None
   def startLine = range.start
+  def sourceCode: Option[String] = _sourceCode
+  def withSourceCode(code: String): RangePosition =
+    val o = copy()
+    o._sourceCode = Some(code)
+    o
+  def withSourceCode(c: Option[String]): RangePosition =
+    c match
+      case Some(code) => this.withSourceCode(code)
+      case None       => this
 }
 
 object SourcePosition extends SourcePositionImpl

internal/util-position/src/test/scala/sbt/internal/util/SourcePositionSpec.scala

@@ -8,19 +8,29 @@
 
 package sbt.internal.util
 
-import org.scalatest.flatspec.AnyFlatSpec
+import hedgehog.*
+import hedgehog.runner.*
 
-class SourcePositionSpec extends AnyFlatSpec {
-  "SourcePosition()" should "return a sane SourcePosition" in {
-    val filename = "SourcePositionSpec.scala"
-    val lineNumber = 17
-    SourcePosition.fromEnclosing() match {
-      case LinePosition(path, startLine) => assert(path === filename && startLine === lineNumber)
-      case RangePosition(path, range)    => assert(path === filename && inRange(range, lineNumber))
-      case NoPosition                    => fail("No source position found")
-    }
-  }
+object SourcePositionSpec extends Properties:
+  override def tests: List[Test] = List(
+    example(
+      "SourcePosition() should return a SourcePosition", {
+        val filename = "SourcePositionSpec.scala"
+        val lineNumber = 19
+        SourcePosition.fromEnclosing() match {
+          case pos @ LinePosition(path, startLine) =>
+            Result.assert(path == filename && startLine == lineNumber).log(pos.toString())
+            Result
+              .assert(pos.sourceCode == Some("SourcePosition.fromEnclosing()"))
+              .log(pos.sourceCode.toString())
+          case pos @ RangePosition(path, range) =>
+            Result.assert(path == filename && inRange(range, lineNumber)).log(pos.toString())
+          case NoPosition => Result.assert(false).log("No source position found")
+        }
+      }
+    )
+  )
 
   private def inRange(range: LineRange, lineNo: Int) =
     range.start until range.end contains lineNo
-}
+end SourcePositionSpec

main-settings/src/main/scala/sbt/std/TaskMacro.scala

@@ -12,7 +12,7 @@ package std
 import Def.{ Initialize, Setting }
 import sbt.internal.util.Types.Id
 import sbt.internal.util.appmacro.{ Cont, ContextUtil, ContextUtil0 }
-import sbt.internal.util.{ LinePosition, NoPosition, SourcePosition }
+import sbt.internal.util.{ SourcePosition, SourcePositionImpl }
 
 import language.experimental.macros
 import scala.quoted.*
@@ -184,13 +184,7 @@ object TaskMacro:
         }
 
   private[sbt] def sourcePosition(using qctx: Quotes): Expr[SourcePosition] =
-    import qctx.reflect.*
-    val pos = Position.ofMacroExpansion
-    if pos.startLine >= 0 && pos.sourceCode != None then
-      val name = Expr(pos.sourceCode.get)
-      val line = Expr(pos.startLine)
-      '{ LinePosition($name, $line) }
-    else '{ NoPosition }
+    SourcePositionImpl.fromEnclosingImpl
 
 end TaskMacro
 

main/src/main/scala/sbt/Main.scala

@@ -973,7 +973,11 @@ object BuiltinCommands {
       st => setupGlobalFileTreeRepository(Clean.addCacheStoreFactoryFactory(st))
     )
     val s4 = s3.put(Keys.useLog4J.key, Project.extract(s3).get(Keys.useLog4J))
-    addSuperShellParams(CheckBuildSources.init(LintUnused.lintUnusedFunc(s4)))
+    addSuperShellParams(
+      CheckBuildSources.init(
+        LintUnused.lintScalaVersion(LintUnused.lintUnusedFunc(s4))
+      )
+    )
   }
 
   private val setupGlobalFileTreeRepository: State => State = { state =>

main/src/main/scala/sbt/internal/LintUnused.scala

@@ -10,7 +10,7 @@ package sbt
 package internal
 
 import Keys.*
-import sbt.internal.util.{ FilePosition, NoPosition, SourcePosition }
+import sbt.internal.util.{ FilePosition, LinePosition, NoPosition, SourcePosition }
 import java.io.File
 import ProjectExtra.{ extract, scopedKeyData }
 import Scope.Global
@@ -165,6 +165,40 @@ object LintUnused {
     unusedKeys.map(u => (u.scoped, display.show(u.scoped), u.positions)).sortBy(_._2)
   }
 
+  def lintScalaVersion(state: State): State = {
+    val log = state.log
+    val extracted = Project.extract(state)
+    val structure = extracted.structure
+    val comp = structure.compiledMap
+    for
+      p <- structure.allProjectRefs
+      scope = Scope.Global.rescope(p)
+      key = scalaVersion.rescope(scope)
+      data = Project.scopedKeyData(structure, key.scopedKey)
+      sv = extracted.get(key)
+      isPlugin = extracted.get(sbtPlugin.rescope(scope))
+      mb = extracted.get(isMetaBuild.rescope(scope))
+      auto = extracted.get(autoScalaLibrary.rescope(scope))
+      msi = extracted.get(managedScalaInstance.rescope(scope))
+      (_, sk) = extracted.runTask(skip.rescope(scope.rescope(publish.key)), state)
+      display = p match
+        case ProjectRef(_, id) => id
+        case _ | null          => Reference.display(p)
+      c <- comp.get(data.map(_.definingKey).getOrElse(key.scopedKey))
+      setting <- c.settings.headOption
+    do
+      if auto && msi && !isPlugin && !mb && !sk then
+        setting.pos match
+          case LinePosition(path, _) if path.endsWith("Defaults.scala") =>
+            log.warn(
+              s"""scalaVersion for subproject $display fell back to a default value $sv; declare it explicitly in build.sbt:
+  scalaVersion := "$sv""""
+            )
+          case _ => ()
+      else ()
+    state
+  }
+
   private case class UnusedKey(
       scoped: ScopedKey[?],
       positions: Seq[SourcePosition],

sbt-app/src/sbt-test/project/scala-version/build.sbt

@@ -0,0 +1,39 @@
+@transient
+lazy val checkScalaVersionWarning = taskKey[Unit]("")
+
+// exempt publish skipped projects
+lazy val `scala-version-root` = (project in file("."))
+  .settings(
+    name := "scala-version-root",
+    checkScalaVersionWarning := {
+      val state = Keys.state.value
+      val logging = state.globalLogging
+      val sv = scalaVersion.value
+      val contents = IO.read(logging.backing.file)
+      assert(contents.contains(s"""scalaVersion for subproject nievab1 fell back to a default value $sv"""))
+      assert(!contents.contains(s"""scalaVersion for subproject scala-version-root fell back to a default value $sv"""))
+      assert(!contents.contains(s"""scalaVersion for subproject nievab2 fell back to a default value $sv"""))
+      assert(!contents.contains(s"""scalaVersion for subproject nievab3 fell back to a default value $sv"""))
+      assert(!contents.contains(s"""scalaVersion for subproject nievab4 fell back to a default value $sv"""))
+      ()
+    },
+    publish / skip := true,
+  )
+
+lazy val nievab1 = project
+
+// exempt plugin projects
+lazy val nievab2 = project
+  .enablePlugins(SbtPlugin)
+
+// exempt Java projects
+lazy val nievab3 = project
+  .settings(
+    autoScalaLibrary := false,
+  )
+
+// exempt SCALA_HOME projects
+lazy val nievab4 = project
+  .settings(
+    managedScalaInstance := false,
+  )

sbt-app/src/sbt-test/project/scala-version/test

@@ -0,0 +1 @@
+> checkScalaVersionWarning