chore: filter allowed source versions by import and by settings

[Cherry-picked 473f8ce]

compiler/src/dotty/tools/dotc/config/ScalaSettingsProperties.scala

@@ -25,7 +25,8 @@ object ScalaSettingsProperties:
     ScalaRelease.values.toList.map(_.show)
 
   def supportedSourceVersions: List[String] =
-    (SourceVersion.values.toList.diff(SourceVersion.illegalSourceVersionNames)).toList.map(_.toString)
+    SourceVersion.values.diff(SourceVersion.illegalInSettings)
+      .map(_.toString).toList
 
   def supportedLanguageFeatures: List[ChoiceWithHelp[String]] =
     Feature.values.map((n, d) => ChoiceWithHelp(n.toString, d))

compiler/src/dotty/tools/dotc/config/SourceVersion.scala

@@ -40,10 +40,18 @@ enum SourceVersion:
   def enablesBetterFors(using Context) = isAtLeast(`3.7`) && isPreviewEnabled
 
 object SourceVersion extends Property.Key[SourceVersion]:
-  def defaultSourceVersion = `3.7`
+
+  /* The default source version used by the built compiler */
+  val defaultSourceVersion = `3.7`
+
+  /* Illegal source versions that may not appear in the settings `-source:<...>` */
+  val illegalInSettings = List(`never`)
+
+  /* Illegal source versions that may not appear as an import `import scala.language.<...>` */
+  val illegalInImports  = List(`never`)
 
   /** language versions that may appear in a language import, are deprecated, but not removed from the standard library. */
-  val illegalSourceVersionNames = List("3.1-migration", "never").map(_.toTermName)
+  val illegalSourceVersionNames = illegalInImports.map(_.toString.toTermName)
 
   /** language versions that the compiler recognises. */
   val validSourceVersionNames = values.toList.map(_.toString.toTermName)

compiler/test/dotty/tools/dotc/config/ScalaSettingsTests.scala

@@ -299,4 +299,11 @@ class ScalaSettingsTests:
     )
     assertEquals(result, Right(reporting.Action.Error))
 
+  @Test def `illegal source versions are not accepted when parsing the settings`: Unit =
+    for source <- SourceVersion.illegalInSettings do
+      val settings = ScalaSettings
+      val result = settings.processArguments(List("-source", source.toString()), true)
+      assertEquals(0, result.warnings.length)
+      assertEquals(1, result.errors.length)
+
 end ScalaSettingsTests