Backport "chore: add scala.language.2.13" to 3.7.1 (#23232)

WojciechMazur · web-flow · commit 2aee56dada09 · 2025-05-22T15:01:30.000+02:00
Backports #23219 to the 3.7.1-RC2. PR submitted by the release tooling.
diff --git a/compiler/src/dotty/tools/dotc/ast/Desugar.scala b/compiler/src/dotty/tools/dotc/ast/Desugar.scala
@@ -703,7 +703,7 @@ object desugar {
     def isNonEnumCase = !isEnumCase && (isCaseClass || isCaseObject)
     val isValueClass = parents.nonEmpty && isAnyVal(parents.head)
       // This is not watertight, but `extends AnyVal` will be replaced by `inline` later.
-    val caseClassInScala2Library = isCaseClass && ctx.settings.YcompileScala2Library.value
+    val caseClassInScala2Library = isCaseClass && Feature.shouldBehaveAsScala2
 
     val originalTparams = constr1.leadingTypeParams
     val originalVparamss = asTermOnly(constr1.trailingParamss)
@@ -922,7 +922,7 @@ object desugar {
           val copyRestParamss = derivedVparamss.tail.nestedMap(vparam =>
             cpy.ValDef(vparam)(rhs = EmptyTree))
           var flags = Synthetic | constr1.mods.flags & copiedAccessFlags
-          if ctx.settings.YcompileScala2Library.value then flags &~= Private
+          if Feature.shouldBehaveAsScala2 then flags &~= Private
           DefDef(
             nme.copy,
             joinParams(derivedTparams, copyFirstParams :: copyRestParamss),
@@ -983,7 +983,7 @@ object desugar {
           else {
             val appMods =
               var flags = Synthetic | constr1.mods.flags & copiedAccessFlags
-              if ctx.settings.YcompileScala2Library.value then flags &~= Private
+              if Feature.shouldBehaveAsScala2 then flags &~= Private
               Modifiers(flags).withPrivateWithin(constr1.mods.privateWithin)
             val appParamss =
               derivedVparamss.nestedZipWithConserve(constrVparamss)((ap, cp) =>
@@ -1066,7 +1066,7 @@ object desugar {
             paramss // drop leading () that got inserted by class
                     // TODO: drop this once we do not silently insert empty class parameters anymore
           case paramss => paramss
-        val finalFlag = if ctx.settings.YcompileScala2Library.value then EmptyFlags else Final
+        val finalFlag = if Feature.shouldBehaveAsScala2 then EmptyFlags else Final
         // implicit wrapper is typechecked in same scope as constructor, so
         // we can reuse the constructor parameters; no derived params are needed.
         DefDef(
diff --git a/compiler/src/dotty/tools/dotc/ast/TreeInfo.scala b/compiler/src/dotty/tools/dotc/ast/TreeInfo.scala
@@ -9,6 +9,7 @@ import Annotations.Annotation
 import NameKinds.ContextBoundParamName
 import typer.ConstFold
 import reporting.trace
+import config.Feature
 
 import Decorators.*
 import Constants.Constant
@@ -466,7 +467,7 @@ trait UntypedTreeInfo extends TreeInfo[Untyped] { self: Trees.Instance[Untyped]
    */
   private def defKind(tree: Tree)(using Context): FlagSet = unsplice(tree) match {
     case EmptyTree | _: Import => NoInitsInterface
-    case tree: TypeDef if ctx.settings.YcompileScala2Library.value =>
+    case tree: TypeDef if Feature.shouldBehaveAsScala2 =>
       if (tree.isClassDef) EmptyFlags else NoInitsInterface
     case tree: TypeDef => if (tree.isClassDef) NoInits else NoInitsInterface
     case tree: DefDef =>
@@ -479,7 +480,7 @@ trait UntypedTreeInfo extends TreeInfo[Untyped] { self: Trees.Instance[Untyped]
         NoInitsInterface
       else if tree.mods.is(Given) && tree.paramss.isEmpty then
         EmptyFlags // might become a lazy val: TODO: check whether we need to suppress NoInits once we have new lazy val impl
-      else if ctx.settings.YcompileScala2Library.value then
+      else if Feature.shouldBehaveAsScala2 then
         EmptyFlags
       else
         NoInits
diff --git a/compiler/src/dotty/tools/dotc/config/Feature.scala b/compiler/src/dotty/tools/dotc/config/Feature.scala
@@ -154,6 +154,10 @@ object Feature:
       case Some(v) => v
       case none => sourceVersionSetting
 
+  /* Should we behave as scala 2?*/
+  def shouldBehaveAsScala2(using Context): Boolean =
+    ctx.settings.YcompileScala2Library.value || sourceVersion.isScala2
+
   def migrateTo3(using Context): Boolean =
     sourceVersion == `3.0-migration`
 
diff --git a/compiler/src/dotty/tools/dotc/config/SourceVersion.scala b/compiler/src/dotty/tools/dotc/config/SourceVersion.scala
@@ -17,7 +17,9 @@ enum SourceVersion:
   case `3.6-migration`, `3.6`
   case `3.7-migration`, `3.7`
   case `3.8-migration`, `3.8`
+  // Add 3.x-migration and 3.x here
   // !!! Keep in sync with scala.runtime.stdlibPatches.language !!!
+  case `2.13`
   case `future-migration`, `future`
 
   case `never`    // needed for MigrationVersion.errorFrom if we never want to issue an error
@@ -34,6 +36,8 @@ enum SourceVersion:
 
   def isAtMost(v: SourceVersion) = stable.ordinal <= v.ordinal
 
+  def isScala2 = this == `2.13`
+
   def enablesFewerBraces = isAtLeast(`3.3`)
   def enablesClauseInterleaving = isAtLeast(`3.6`)
   def enablesNewGivens = isAtLeast(`3.6`)
@@ -46,7 +50,7 @@ object SourceVersion extends Property.Key[SourceVersion]:
   val defaultSourceVersion = `3.7`
 
   /* Illegal source versions that may not appear in the settings `-source:<...>` */
-  val illegalInSettings = List(`3.1-migration`, `never`)
+  val illegalInSettings = List(`2.13`, `3.1-migration`, `never`)
 
   /* Illegal source versions that may not appear as an import `import scala.language.<...>` */
   val illegalInImports  = List(`3.1-migration`, `never`)
diff --git a/compiler/src/dotty/tools/dotc/core/Definitions.scala b/compiler/src/dotty/tools/dotc/core/Definitions.scala
@@ -1537,9 +1537,7 @@ class Definitions {
       denot.sourceModule.info = denot.typeRef // we run into a cyclic reference when patching if this line is omitted
       patch2(denot, patchCls)
 
-    if ctx.settings.YcompileScala2Library.value then
-      ()
-    else if denot.name == tpnme.Predef.moduleClassName && denot.symbol == ScalaPredefModuleClass then
+    if denot.name == tpnme.Predef.moduleClassName && denot.symbol == ScalaPredefModuleClass then
       patchWith(ScalaPredefModuleClassPatch)
     else if denot.name == tpnme.language.moduleClassName && denot.symbol == LanguageModuleClass then
       patchWith(LanguageModuleClassPatch)
@@ -1880,7 +1878,7 @@ class Definitions {
     || tp.derivesFrom(defn.PolyFunctionClass)   // TODO check for refinement?
 
   private def withSpecMethods(cls: ClassSymbol, bases: List[Name], paramTypes: Set[TypeRef]) =
-    if !ctx.settings.YcompileScala2Library.value then
+    if !Feature.shouldBehaveAsScala2 then
       for base <- bases; tp <- paramTypes do
         cls.enter(newSymbol(cls, base.specializedName(List(tp)), Method, ExprType(tp)))
     cls
@@ -1923,7 +1921,7 @@ class Definitions {
       case List(x, y) => Tuple2SpecializedParamClasses().contains(x.classSymbol) && Tuple2SpecializedParamClasses().contains(y.classSymbol)
       case _          => false
     && base.owner.denot.info.member(base.name.specializedName(args)).exists // when dotc compiles the stdlib there are no specialised classes
-    && !ctx.settings.YcompileScala2Library.value // We do not add the specilized TupleN methods/classes when compiling the stdlib
+    && !Feature.shouldBehaveAsScala2 // We do not add the specilized TupleN methods/classes when compiling the stdlib
 
   def isSpecializableFunction(cls: ClassSymbol, paramTypes: List[Type], retType: Type)(using Context): Boolean =
     paramTypes.length <= 2
@@ -1945,7 +1943,7 @@ class Definitions {
       case _ =>
         false
     })
-    && !ctx.settings.YcompileScala2Library.value // We do not add the specilized FunctionN methods/classes when compiling the stdlib
+    && !Feature.shouldBehaveAsScala2 // We do not add the specilized FunctionN methods/classes when compiling the stdlib
 
   @tu lazy val Function0SpecializedApplyNames: List[TermName] =
     for r <- Function0SpecializedReturnTypes
diff --git a/compiler/src/dotty/tools/dotc/transform/Pickler.scala b/compiler/src/dotty/tools/dotc/transform/Pickler.scala
@@ -291,7 +291,7 @@ class Pickler extends Phase {
       val isOutline = isJavaAttr // TODO: later we may want outline for Scala sources too
       val attributes = Attributes(
         sourceFile = sourceRelativePath,
-        scala2StandardLibrary = ctx.settings.YcompileScala2Library.value,
+        scala2StandardLibrary = Feature.shouldBehaveAsScala2,
         explicitNulls = ctx.settings.YexplicitNulls.value,
         captureChecked = Feature.ccEnabled,
         withPureFuns = Feature.pureFunsEnabled,
diff --git a/compiler/src/dotty/tools/dotc/transform/PostTyper.scala b/compiler/src/dotty/tools/dotc/transform/PostTyper.scala
@@ -101,7 +101,7 @@ class PostTyper extends MacroTransform with InfoTransformer { thisPhase =>
   private var compilingScala2StdLib = false
   override def initContext(ctx: FreshContext): Unit =
     initContextCalled = true
-    compilingScala2StdLib = ctx.settings.YcompileScala2Library.value(using ctx)
+    compilingScala2StdLib = Feature.shouldBehaveAsScala2(using ctx)
 
   val superAcc: SuperAccessors = new SuperAccessors(thisPhase)
   val synthMbr: SyntheticMembers = new SyntheticMembers(thisPhase)
diff --git a/compiler/src/dotty/tools/dotc/transform/SpecializeApplyMethods.scala b/compiler/src/dotty/tools/dotc/transform/SpecializeApplyMethods.scala
@@ -5,6 +5,7 @@ import ast.Trees.*, ast.tpd, core.*
 import Contexts.*, Types.*, Decorators.*, Symbols.*, DenotTransformers.*
 import SymDenotations.*, Scopes.*, StdNames.*, NameOps.*, Names.*
 import MegaPhase.MiniPhase
+import config.Feature
 
 import scala.collection.mutable
 
@@ -25,7 +26,7 @@ class SpecializeApplyMethods extends MiniPhase with InfoTransformer {
   override def description: String = SpecializeApplyMethods.description
 
   override def isEnabled(using Context): Boolean =
-    !ctx.settings.scalajs.value && !ctx.settings.YcompileScala2Library.value
+    !ctx.settings.scalajs.value && !Feature.shouldBehaveAsScala2
 
   private def specApplySymbol(sym: Symbol, args: List[Type], ret: Type)(using Context): Symbol = {
     val name = nme.apply.specializedFunction(ret, args)
diff --git a/compiler/src/dotty/tools/dotc/transform/SyntheticMembers.scala b/compiler/src/dotty/tools/dotc/transform/SyntheticMembers.scala
@@ -10,6 +10,7 @@ import NameOps.*
 import Annotations.Annotation
 import typer.ProtoTypes.constrained
 import ast.untpd
+import config.Feature
 
 import util.Property
 import util.Spans.Span
@@ -79,7 +80,7 @@ class SyntheticMembers(thisPhase: DenotTransformer) {
 
   private def existingDef(sym: Symbol, clazz: ClassSymbol)(using Context): Symbol =
     val existing = sym.matchingMember(clazz.thisType)
-    if ctx.settings.YcompileScala2Library.value && clazz.isValueClass && (sym == defn.Any_equals || sym == defn.Any_hashCode) then
+    if Feature.shouldBehaveAsScala2 && clazz.isValueClass && (sym == defn.Any_equals || sym == defn.Any_hashCode) then
       NoSymbol
     else if existing != sym && !existing.is(Deferred) then
       existing
@@ -168,7 +169,7 @@ class SyntheticMembers(thisPhase: DenotTransformer) {
         case nme.productPrefix if isEnumValue => nameRef
         case nme.productPrefix => ownNameLit
         case nme.productElement =>
-          if ctx.settings.YcompileScala2Library.value then productElementBodyForScala2Compat(accessors.length, vrefss.head.head)
+          if Feature.shouldBehaveAsScala2 then productElementBodyForScala2Compat(accessors.length, vrefss.head.head)
           else productElementBody(accessors.length, vrefss.head.head)
         case nme.productElementName => productElementNameBody(accessors.length, vrefss.head.head)
       }
@@ -720,7 +721,7 @@ class SyntheticMembers(thisPhase: DenotTransformer) {
     val syntheticMembers = serializableObjectMethod(clazz) ::: serializableEnumValueMethod(clazz) ::: caseAndValueMethods(clazz)
     checkInlining(syntheticMembers)
     val impl1 = cpy.Template(impl)(body = syntheticMembers ::: impl.body)
-    if ctx.settings.YcompileScala2Library.value then impl1
+    if Feature.shouldBehaveAsScala2 then impl1
     else addMirrorSupport(impl1)
   }
 
diff --git a/compiler/src/dotty/tools/dotc/typer/Namer.scala b/compiler/src/dotty/tools/dotc/typer/Namer.scala
@@ -247,7 +247,7 @@ class Namer { typer: Typer =>
     tree match {
       case tree: TypeDef if tree.isClassDef =>
         var flags = checkFlags(tree.mods.flags)
-        if ctx.settings.YcompileScala2Library.value then
+        if Feature.shouldBehaveAsScala2 then
           flags |= Scala2x
         val name = checkNoConflict(tree.name, tree.span).asTypeName
         val cls =
diff --git a/compiler/src/dotty/tools/dotc/typer/Typer.scala b/compiler/src/dotty/tools/dotc/typer/Typer.scala
@@ -3206,7 +3206,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
 
     checkEnumParent(cls, firstParent)
 
-    if defn.ScalaValueClasses()(cls) && ctx.settings.YcompileScala2Library.value then
+    if defn.ScalaValueClasses()(cls) && Feature.shouldBehaveAsScala2 then
       constr1.symbol.resetFlag(Private)
 
     val self1 = typed(self)(using ctx.outer).asInstanceOf[ValDef] // outer context where class members are not visible
diff --git a/library/src/scala/runtime/stdLibPatches/language.scala b/library/src/scala/runtime/stdLibPatches/language.scala
@@ -222,6 +222,12 @@ object language:
   @compileTimeOnly("`future-migration` can only be used at compile time in import statements")
   object `future-migration`
 
+  /** Set source version to 2.13. Effectively, this doesn't change the source language,
+   * but rather adapts the generated code as if it was compiled with Scala 2.13
+   */
+  @compileTimeOnly("`2.13` can only be used at compile time in import statements")
+  private[scala] object `2.13`
+
   /** Set source version to 3.0-migration.
     *
     * @see [[https://docs.scala-lang.org/scala3/guides/migration/compatibility-intro.html]]
diff --git a/project/Build.scala b/project/Build.scala
@@ -1237,11 +1237,8 @@ object Build {
     withCommonSettings(Bootstrapped).
     dependsOn(dottyCompiler(Bootstrapped) % "provided; compile->runtime; test->test").
     settings(scala2LibraryBootstrappedSettings).
-    settings(
-      moduleName := "scala2-library-cc",
-      scalacOptions += "-Ycheck:all",
-    )
-
+    settings(moduleName := "scala2-library-cc")
+  
   lazy val scala2LibraryBootstrappedSettings = Seq(
       javaOptions := (`scala3-compiler-bootstrapped` / javaOptions).value,
       Compile / scalacOptions ++= {
diff --git a/project/MiMaFilters.scala b/project/MiMaFilters.scala
@@ -13,6 +13,9 @@ object MiMaFilters {
         // Scala.js-only class
         ProblemFilters.exclude[FinalClassProblem]("scala.scalajs.runtime.AnonFunctionXXL"),
         ProblemFilters.exclude[DirectMissingMethodProblem]("scala.scalajs.runtime.AnonFunctionXXL.this"),
+
+        ProblemFilters.exclude[MissingFieldProblem]("scala.runtime.stdLibPatches.language.2.13"),
+        ProblemFilters.exclude[MissingClassProblem]("scala.runtime.stdLibPatches.language$2$u002E13$")
       ),
 
       // Additions since last LTS
diff --git a/project/scripts/scala2-library-tasty-mima.sh b/project/scripts/scala2-library-tasty-mima.sh
@@ -18,6 +18,6 @@ setTastyVersion $MINOR_TASTY_VERSION_SUPPORTED_BY_TASTY_MIMA 0
 # We clean before to make sure all sources are recompiled using the new TASTY version.
 # We clean after to make sure no other test will use the TASTy generated with this version.
 # We set -Ycheck:all to check that -Ycompile-scala2-library does not gererate inconsistent trees.
-"$SBT" 'clean; scala2-library-bootstrapped/clean; reload; set `scala2-library-bootstrapped`/scalacOptions += "-Ycheck:all"; scala2-library-bootstrapped/tastyMiMaReportIssues; clean; scala2-library-bootstrapped/clean'
+"$SBT" 'clean; scala2-library-bootstrapped/clean; reload; scala2-library-bootstrapped/tastyMiMaReportIssues; clean; scala2-library-bootstrapped/clean'
 
 setTastyVersion $MINOR_TASTY_VERSION $EXPERIMENTAL_TASTY_VERSION
