Introduce tracked class parameters

For a tracked class parameter we add a refinement in the constructor type that
the class member is the same as the parameter. E.g.
```scala
class C { type T }
class D(tracked val x: C) { type T = x.T }
```
This will generate the constructor type:
```scala
(x1: C): D { val x: x1.type }
```
Without `tracked` the refinement would not be added. This can solve several problems with dependent class
types where previously we lost track of type dependencies.

Author: odersky

compiler/src/dotty/tools/dotc/ast/Desugar.scala

@@ -443,13 +443,13 @@ object desugar {
   private def toDefParam(tparam: TypeDef, keepAnnotations: Boolean): TypeDef = {
     var mods = tparam.rawMods
     if (!keepAnnotations) mods = mods.withAnnotations(Nil)
-    tparam.withMods(mods & (EmptyFlags | Sealed) | Param)
+    tparam.withMods(mods & EmptyFlags | Param)
   }
   private def toDefParam(vparam: ValDef, keepAnnotations: Boolean, keepDefault: Boolean): ValDef = {
     var mods = vparam.rawMods
     if (!keepAnnotations) mods = mods.withAnnotations(Nil)
     val hasDefault = if keepDefault then HasDefault else EmptyFlags
-    vparam.withMods(mods & (GivenOrImplicit | Erased | hasDefault) | Param)
+    vparam.withMods(mods & (GivenOrImplicit | Erased | hasDefault | Tracked) | Param)
   }
 
   def mkApply(fn: Tree, paramss: List[ParamClause])(using Context): Tree =
@@ -535,7 +535,7 @@ object desugar {
     // but not on the constructor parameters. The reverse is true for
     // annotations on class _value_ parameters.
     val constrTparams = impliedTparams.map(toDefParam(_, keepAnnotations = false))
-    val constrVparamss =
+    def defVparamss =
       if (originalVparamss.isEmpty) { // ensure parameter list is non-empty
         if (isCaseClass)
           report.error(CaseClassMissingParamList(cdef), namePos)
@@ -546,6 +546,10 @@ object desugar {
         ListOfNil
       }
       else originalVparamss.nestedMap(toDefParam(_, keepAnnotations = true, keepDefault = true))
+    val constrVparamss = defVparamss
+      // defVparamss also needed as separate tree nodes in implicitWrappers below.
+      // Need to be separate because they are `watch`ed in addParamRefinements.
+      // See parsercombinators-givens.scala for a test case.
     val derivedTparams =
       constrTparams.zipWithConserve(impliedTparams)((tparam, impliedParam) =>
         derivedTypeParam(tparam).withAnnotations(impliedParam.mods.annotations))
@@ -623,6 +627,11 @@ object desugar {
       case _ => false
     }
 
+    def isRepeated(tree: Tree): Boolean = stripByNameType(tree) match {
+      case PostfixOp(_, Ident(tpnme.raw.STAR)) => true
+      case _ => false
+    }
+
     def appliedRef(tycon: Tree, tparams: List[TypeDef] = constrTparams, widenHK: Boolean = false) = {
       val targs = for (tparam <- tparams) yield {
         val targ = refOfDef(tparam)
@@ -639,10 +648,13 @@ object desugar {
       appliedTypeTree(tycon, targs)
     }
 
-    def isRepeated(tree: Tree): Boolean = stripByNameType(tree) match {
-      case PostfixOp(_, Ident(tpnme.raw.STAR)) => true
-      case _ => false
-    }
+    def addParamRefinements(core: Tree, paramss: List[List[ValDef]]): Tree =
+      val refinements =
+        for params <- paramss; param <- params; if param.mods.is(Tracked) yield
+          ValDef(param.name, SingletonTypeTree(TermRefTree().watching(param)), EmptyTree)
+            .withSpan(param.span)
+      if refinements.isEmpty then core
+      else RefinedTypeTree(core, refinements).showing(i"refined result: $result", Printers.desugar)
 
     // a reference to the class type bound by `cdef`, with type parameters coming from the constructor
     val classTypeRef = appliedRef(classTycon)
@@ -864,18 +876,17 @@ object desugar {
         Nil
       }
       else {
-        val defParamss = constrVparamss match {
+        val defParamss = defVparamss match
           case Nil :: paramss =>
             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
         // implicit wrapper is typechecked in same scope as constructor, so
         // we can reuse the constructor parameters; no derived params are needed.
         DefDef(
           className.toTermName, joinParams(constrTparams, defParamss),
-          classTypeRef, creatorExpr)
+          addParamRefinements(classTypeRef, defParamss), creatorExpr)
           .withMods(companionMods | mods.flags.toTermFlags & (GivenOrImplicit | Inline) | finalFlag)
           .withSpan(cdef.span) :: Nil
       }
@@ -904,7 +915,9 @@ object desugar {
       }
       if mods.isAllOf(Given | Inline | Transparent) then
         report.error("inline given instances cannot be trasparent", cdef)
-      val classMods = if mods.is(Given) then mods &~ (Inline | Transparent) | Synthetic else mods
+      var classMods = if mods.is(Given) then mods &~ (Inline | Transparent) | Synthetic else mods
+      if vparamAccessors.exists(_.mods.is(Tracked)) then
+        classMods |= Dependent
       cpy.TypeDef(cdef: TypeDef)(
         name = className,
         rhs = cpy.Template(impl)(constr, parents1, clsDerived, self1,

compiler/src/dotty/tools/dotc/ast/untpd.scala

@@ -231,6 +231,8 @@ object untpd extends Trees.Instance[Untyped] with UntypedTreeInfo {
 
     case class Infix()(implicit @constructorOnly src: SourceFile) extends Mod(Flags.Infix)
 
+    case class Tracked()(implicit @constructorOnly src: SourceFile) extends Mod(Flags.Tracked)
+
     /** Used under pureFunctions to mark impure function types `A => B` in `FunctionWithMods` */
     case class Impure()(implicit @constructorOnly src: SourceFile) extends Mod(Flags.Impure)
   }

compiler/src/dotty/tools/dotc/core/Flags.scala

@@ -377,6 +377,9 @@ object Flags {
   /** Symbol cannot be found as a member during typer */
   val (Invisible @ _, _, _) = newFlags(45, "<invisible>")
 
+  /** Tracked modifier for class parameter / a class with some tracked parameters */
+  val (Tracked @ _, _, Dependent @ _) = newFlags(46, "tracked")
+
   // ------------ Flags following this one are not pickled ----------------------------------
 
   /** Symbol is not a member of its owner */
@@ -452,7 +455,7 @@ object Flags {
     CommonSourceModifierFlags.toTypeFlags | Abstract | Sealed | Opaque | Open
 
   val TermSourceModifierFlags: FlagSet =
-    CommonSourceModifierFlags.toTermFlags | Inline | AbsOverride | Lazy
+    CommonSourceModifierFlags.toTermFlags | Inline | AbsOverride | Lazy | Tracked
 
   /** Flags representing modifiers that can appear in trees */
   val ModifierFlags: FlagSet =
@@ -466,7 +469,7 @@ object Flags {
   val FromStartFlags: FlagSet = commonFlags(
     Module, Package, Deferred, Method, Case, Enum, Param, ParamAccessor,
     Scala2SpecialFlags, MutableOrOpen, Opaque, Touched, JavaStatic,
-    OuterOrCovariant, LabelOrContravariant, CaseAccessor,
+    OuterOrCovariant, LabelOrContravariant, CaseAccessor, Tracked,
     Extension, NonMember, Implicit, Given, Permanent, Synthetic, Exported,
     SuperParamAliasOrScala2x, Inline, Macro, ConstructorProxy, Invisible)
 
@@ -477,7 +480,7 @@ object Flags {
    */
   val AfterLoadFlags: FlagSet = commonFlags(
     FromStartFlags, AccessFlags, Final, AccessorOrSealed,
-    Abstract, LazyOrTrait, SelfName, JavaDefined, JavaAnnotation, Transparent)
+    Abstract, LazyOrTrait, SelfName, JavaDefined, JavaAnnotation, Transparent, Tracked)
 
   /** A value that's unstable unless complemented with a Stable flag */
   val UnstableValueFlags: FlagSet = Mutable | Method

compiler/src/dotty/tools/dotc/core/NamerOps.scala

@@ -15,9 +15,14 @@ object NamerOps:
    *  @param ctor the constructor
    */
   def effectiveResultType(ctor: Symbol, paramss: List[List[Symbol]])(using Context): Type =
-    paramss match
-      case TypeSymbols(tparams) :: _ => ctor.owner.typeRef.appliedTo(tparams.map(_.typeRef))
-      case _ => ctor.owner.typeRef
+    val (resType, termParamss) = paramss match
+      case TypeSymbols(tparams) :: rest =>
+        (ctor.owner.typeRef.appliedTo(tparams.map(_.typeRef)), rest)
+      case _ =>
+        (ctor.owner.typeRef, paramss)
+    termParamss.flatten.foldLeft(resType): (rt, param) =>
+      if param.is(Tracked) then RefinedType(rt, param.name, param.termRef)
+      else rt
 
   /** Split dependent class refinements off parent type. Add them to `refinements`,
    *  unless it is null.

compiler/src/dotty/tools/dotc/core/PatternTypeConstrainer.scala

@@ -88,11 +88,6 @@ trait PatternTypeConstrainer { self: TypeComparer =>
       }
     }
 
-    def stripRefinement(tp: Type): Type = tp match {
-      case tp: RefinedOrRecType => stripRefinement(tp.parent)
-      case tp => tp
-    }
-
     def tryConstrainSimplePatternType(pat: Type, scrut: Type) = {
       val patCls = pat.classSymbol
       val scrCls = scrut.classSymbol
@@ -181,14 +176,14 @@ trait PatternTypeConstrainer { self: TypeComparer =>
       case AndType(scrut1, scrut2) =>
         constrainPatternType(pat, scrut1) && constrainPatternType(pat, scrut2)
       case scrut: RefinedOrRecType =>
-        constrainPatternType(pat, stripRefinement(scrut))
+        constrainPatternType(pat, scrut.stripRefinement)
       case scrut => dealiasDropNonmoduleRefs(pat) match {
         case OrType(pat1, pat2) =>
           either(constrainPatternType(pat1, scrut), constrainPatternType(pat2, scrut))
         case AndType(pat1, pat2) =>
           constrainPatternType(pat1, scrut) && constrainPatternType(pat2, scrut)
         case pat: RefinedOrRecType =>
-          constrainPatternType(stripRefinement(pat), scrut)
+          constrainPatternType(pat.stripRefinement, scrut)
         case pat =>
           tryConstrainSimplePatternType(pat, scrut)
           || classesMayBeCompatible && constrainUpcasted(scrut)

compiler/src/dotty/tools/dotc/core/StdNames.scala

@@ -623,6 +623,7 @@ object StdNames {
     val toString_ : N           = "toString"
     val toTypeConstructor: N    = "toTypeConstructor"
     val tpe : N                 = "tpe"
+    val tracked: N              = "tracked"
     val transparent : N         = "transparent"
     val tree : N                = "tree"
     val true_ : N               = "true"

compiler/src/dotty/tools/dotc/core/SymDenotations.scala

@@ -1190,13 +1190,17 @@ object SymDenotations {
     final def isExtensibleClass(using Context): Boolean =
       isClass && !isOneOf(FinalOrModuleClass) && !isAnonymousClass
 
-    /** A symbol is effectively final if it cannot be overridden in a subclass */
+    /** A symbol is effectively final if it cannot be overridden */
     final def isEffectivelyFinal(using Context): Boolean =
       isOneOf(EffectivelyFinalFlags)
       || is(Inline, butNot = Deferred)
       || is(JavaDefinedVal, butNot = Method)
       || isConstructor
-      || !owner.isExtensibleClass
+      || !owner.isExtensibleClass && !is(Deferred)
+      	// Deferred symbols can arise through parent refinements.
+      	// For them, the overriding relationship reverses anyway, so
+      	// being in a final class does not mean the symbol cannot be
+      	// implemented concretely in a superclass.
 
     /** A class is effectively sealed if has the `final` or `sealed` modifier, or it
      *  is defined in Scala 3 and is neither abstract nor open.

compiler/src/dotty/tools/dotc/core/TypeUtils.scala

@@ -6,11 +6,11 @@ import TypeErasure.ErasedValueType
 import Types.*, Contexts.*, Symbols.*, Flags.*, Decorators.*
 import Names.Name
 
-class TypeUtils {
+class TypeUtils:
   /** A decorator that provides methods on types
    *  that are needed in the transformer pipeline.
    */
-  extension (self: Type) {
+  extension (self: Type)
 
     def isErasedValueType(using Context): Boolean =
       self.isInstanceOf[ErasedValueType]
@@ -150,5 +150,11 @@ class TypeUtils {
       case _ =>
         val cls = self.underlyingClassRef(refinementOK = false).typeSymbol
         cls.isTransparentClass && (!traitOnly || cls.is(Trait))
-  }
-}
+
+    /** Strip all outer refinements off this type */
+    def stripRefinement: Type = self match
+      case self: RefinedOrRecType => self.parent.stripRefinement
+      case seld => self
+
+end TypeUtils
+

compiler/src/dotty/tools/dotc/core/tasty/TreePickler.scala

@@ -798,6 +798,7 @@ class TreePickler(pickler: TastyPickler, attributes: Attributes) {
     if (flags.is(Exported)) writeModTag(EXPORTED)
     if (flags.is(Given)) writeModTag(GIVEN)
     if (flags.is(Implicit)) writeModTag(IMPLICIT)
+    if (flags.is(Tracked)) writeModTag(TRACKED)
     if (isTerm) {
       if (flags.is(Lazy, butNot = Module)) writeModTag(LAZY)
       if (flags.is(AbsOverride)) { writeModTag(ABSTRACT); writeModTag(OVERRIDE) }

compiler/src/dotty/tools/dotc/core/tasty/TreeUnpickler.scala

@@ -742,6 +742,7 @@ class TreeUnpickler(reader: TastyReader,
           case INVISIBLE => addFlag(Invisible)
           case TRANSPARENT => addFlag(Transparent)
           case INFIX => addFlag(Infix)
+          case TRACKED => addFlag(Tracked)
           case PRIVATEqualified =>
             readByte()
             privateWithin = readWithin