Refine dealiasing operations

 - some keep their annotations
 - some only keep refining annotations
 - most of them still strip all annotations

Author: odersky

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

@@ -78,7 +78,7 @@ object desugar {
           case tp: NamedType if tp.symbol.exists && (tp.symbol.owner eq originalOwner) =>
             val defctx = ctx.outersIterator.dropWhile(_.scope eq ctx.scope).next()
             var local = defctx.denotNamed(tp.name ++ suffix).suchThat(_.isParamOrAccessor).symbol
-            if (local.exists) (defctx.owner.thisType select local).dealias
+            if (local.exists) (defctx.owner.thisType select local).dealiasKeepAnnots
             else {
               def msg =
                 s"no matching symbol for ${tp.symbol.showLocated} in ${defctx.owner} / ${defctx.effectiveScope.toList}"

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

@@ -963,7 +963,7 @@ object tpd extends Trees.Instance[Type] with TypedTreeInfo {
         val alternatives = ctx.typer.resolveOverloaded(allAlts, proto)
         assert(alternatives.size == 1,
           i"${if (alternatives.isEmpty) "no" else "multiple"} overloads available for " +
-          i"$method on ${receiver.tpe.widenDealias} with targs: $targs%, %; args: $args%, % of types ${args.tpes}%, %; expectedType: $expectedType." +
+          i"$method on ${receiver.tpe.widenDealiasKeepAnnots} with targs: $targs%, %; args: $args%, % of types ${args.tpes}%, %; expectedType: $expectedType." +
           i" isAnnotConstructor = $isAnnotConstructor.\n" +
           i"all alternatives: ${allAlts.map(_.symbol.showDcl).mkString(", ")}\n" +
           i"matching alternatives: ${alternatives.map(_.symbol.showDcl).mkString(", ")}.") // this is parsed from bytecode tree. there's nothing user can do about it

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

@@ -275,7 +275,7 @@ trait ConstraintHandling {
       case tp: OrType  => isFullyDefined(tp.tp1) && isFullyDefined(tp.tp2)
       case _ => true
     }
-    def isOrType(tp: Type): Boolean = tp.stripTypeVar.dealias match {
+    def isOrType(tp: Type): Boolean = tp.dealias match {
       case tp: OrType => true
       case tp: RefinedOrRecType => isOrType(tp.parent)
       case AndType(tp1, tp2) => isOrType(tp1) | isOrType(tp2)

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

@@ -387,7 +387,7 @@ class OrderingConstraint(private val boundsMap: ParamBounds,
    *  that are not top-level are not affected.
    */
   def replace(param: TypeParamRef, tp: Type)(implicit ctx: Context): OrderingConstraint = {
-    val replacement = tp.dealias.stripTypeVar
+    val replacement = tp.dealiasKeepAnnots.stripTypeVar
     if (param == replacement) this
     else {
       assert(replacement.isValueTypeOrLambda)

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

@@ -351,7 +351,7 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
         if (tp11.stripTypeVar eq tp12.stripTypeVar) recur(tp11, tp2)
         else thirdTry
       case tp1 @ OrType(tp11, tp12) =>
-        def joinOK = tp2.dealias match {
+        def joinOK = tp2.dealiasKeepRefiningAnnots match {
           case tp2: AppliedType if !tp2.tycon.typeSymbol.isClass =>
             // If we apply the default algorithm for `A[X] | B[Y] <: C[Z]` where `C` is a
             // type parameter, we will instantiate `C` to `A` and then fail when comparing
@@ -510,7 +510,7 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
         }
         compareTypeLambda
       case OrType(tp21, tp22) =>
-        val tp1a = tp1.widenDealias
+        val tp1a = tp1.widenDealiasKeepRefiningAnnots
         if (tp1a ne tp1)
           // Follow the alias; this might avoid truncating the search space in the either below
           // Note that it's safe to widen here because singleton types cannot be part of `|`.
@@ -644,7 +644,7 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
         }
         compareHKLambda
       case AndType(tp11, tp12) =>
-        val tp2a = tp2.dealias
+        val tp2a = tp2.dealiasKeepRefiningAnnots
         if (tp2a ne tp2) // Follow the alias; this might avoid truncating the search space in the either below
           return recur(tp1, tp2a)
 
@@ -690,13 +690,13 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
       */
       def isMatchingApply(tp1: Type): Boolean = tp1 match {
         case AppliedType(tycon1, args1) =>
-          tycon1.dealias match {
+          tycon1.dealiasKeepRefiningAnnots match {
             case tycon1: TypeParamRef =>
               (tycon1 == tycon2 ||
               canConstrain(tycon1) && tryInstantiate(tycon1, tycon2)) &&
               isSubArgs(args1, args2, tp1, tparams)
             case tycon1: TypeRef =>
-              tycon2.dealias match {
+              tycon2.dealiasKeepRefiningAnnots match {
                 case tycon2: TypeRef if tycon1.symbol == tycon2.symbol =>
                   isSubType(tycon1.prefix, tycon2.prefix) &&
                   isSubArgs(args1, args2, tp1, tparams)
@@ -1104,11 +1104,10 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
    *  is some combination of TypeRefs that point to classes, where the
    *  combiners are AppliedTypes, RefinedTypes, RecTypes, And/Or-Types or AnnotatedTypes.
    */
-   private def isCovered(tp: Type): Boolean = tp.dealias.stripTypeVar match {
+   private def isCovered(tp: Type): Boolean = tp.dealiasKeepRefiningAnnots.stripTypeVar match {
     case tp: TypeRef => tp.symbol.isClass && tp.symbol != NothingClass && tp.symbol != NullClass
     case tp: AppliedType => isCovered(tp.tycon)
     case tp: RefinedOrRecType => isCovered(tp.parent)
-    case tp: AnnotatedType => isCovered(tp.underlying)
     case tp: AndType => isCovered(tp.tp1) && isCovered(tp.tp2)
     case tp: OrType  => isCovered(tp.tp1) && isCovered(tp.tp2)
     case _ => false

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

@@ -931,6 +931,11 @@ object Types {
       */
     def stripAnnots(implicit ctx: Context): Type = this
 
+    def rewrapAnnots(tp: Type)(implicit ctx: Context): Type = tp.stripTypeVar match {
+      case AnnotatedType(tp1, annot) => AnnotatedType(rewrapAnnots(tp1), annot)
+      case _ => this
+    }
+
     /** Strip PolyType prefix */
     def stripPoly(implicit ctx: Context): Type = this match {
       case tp: PolyType => tp.resType.stripPoly
@@ -1017,48 +1022,57 @@ object Types {
         this
     }
 
-    private def dealias1(keepAnnots: Boolean)(implicit ctx: Context): Type = this match {
+    private def dealias1(keep: AnnotatedType => Context => Boolean)(implicit ctx: Context): Type = this match {
       case tp: TypeRef =>
         if (tp.symbol.isClass) tp
         else tp.info match {
-          case TypeAlias(alias) => alias.dealias1(keepAnnots): @tailrec
+          case TypeAlias(alias) => alias.dealias1(keep): @tailrec
           case _ => tp
         }
       case app @ AppliedType(tycon, args) =>
-        val tycon1 = tycon.dealias1(keepAnnots)
-        if (tycon1 ne tycon) app.superType.dealias1(keepAnnots): @tailrec
+        val tycon1 = tycon.dealias1(keep)
+        if (tycon1 ne tycon) app.superType.dealias1(keep): @tailrec
         else this
       case tp: TypeVar =>
         val tp1 = tp.instanceOpt
-        if (tp1.exists) tp1.dealias1(keepAnnots): @tailrec else tp
+        if (tp1.exists) tp1.dealias1(keep): @tailrec else tp
       case tp: AnnotatedType =>
-        val tp1 = tp.tpe.dealias1(keepAnnots)
-        if (keepAnnots) tp.derivedAnnotatedType(tp1, tp.annot) else tp1
+        val tp1 = tp.tpe.dealias1(keep)
+        if (keep(tp)(ctx)) tp.derivedAnnotatedType(tp1, tp.annot) else tp1
       case tp: LazyRef =>
-        tp.ref.dealias1(keepAnnots): @tailrec
+        tp.ref.dealias1(keep): @tailrec
       case _ => this
     }
 
+    /** Follow aliases and dereferences LazyRefs, annotated types and instantiated
+     *  TypeVars until type is no longer alias type, annotated type, LazyRef,
+     *  or instantiated type variable.
+     */
+    final def dealias(implicit ctx: Context): Type = dealias1(keepNever)
+
     /** Follow aliases and dereferences LazyRefs and instantiated TypeVars until type
      *  is no longer alias type, LazyRef, or instantiated type variable.
      *  Goes through annotated types and rewraps annotations on the result.
      */
-    final def dealiasKeepAnnots(implicit ctx: Context): Type =
-      dealias1(keepAnnots = true)
+    final def dealiasKeepAnnots(implicit ctx: Context): Type = dealias1(keepAlways)
 
-    /** Follow aliases and dereferences LazyRefs, annotated types and instantiated
-     *  TypeVars until type is no longer alias type, annotated type, LazyRef,
-     *  or instantiated type variable.
-     */
-    final def dealias(implicit ctx: Context): Type =
-      dealias1(keepAnnots = false)
+    /** Like `dealiasKeepAnnots`, but keeps only refining annotations */
+    final def dealiasKeepRefiningAnnots(implicit ctx: Context): Type = dealias1(keepIfRefining)
 
-    /** Perform successive widenings and dealiasings until none can be applied anymore */
-    @tailrec final def widenDealias(implicit ctx: Context): Type = {
-      val res = this.widen.dealias
-      if (res eq this) res else res.widenDealias
+    private def widenDealias1(keep: AnnotatedType => Context => Boolean)(implicit ctx: Context): Type = {
+      val res = this.widen.dealias1(keep)
+      if (res eq this) res else res.widenDealias1(keep)
     }
 
+    /** Perform successive widenings and dealiasings until none can be applied anymore */
+    final def widenDealias(implicit ctx: Context): Type = widenDealias1(keepNever)
+
+    /** Perform successive widenings and dealiasings while rewrapping annotations, until none can be applied anymore */
+    final def widenDealiasKeepAnnots(implicit ctx: Context): Type = widenDealias1(keepAlways)
+
+    /** Perform successive widenings and dealiasings while rewrapping refining annotations, until none can be applied anymore */
+    final def widenDealiasKeepRefiningAnnots(implicit ctx: Context): Type = widenDealias1(keepIfRefining)
+
     /** Widen from constant type to its underlying non-constant
      *  base type.
      */
@@ -1805,8 +1819,8 @@ object Types {
           case arg: TypeBounds =>
             val v = param.paramVariance
             val pbounds = param.paramInfo
-            if (v > 0 && pbounds.loBound.dealias.isBottomType) TypeAlias(arg.hiBound & rebase(pbounds.hiBound))
-            else if (v < 0 && pbounds.hiBound.dealias.isTopType) TypeAlias(arg.loBound | rebase(pbounds.loBound))
+            if (v > 0 && pbounds.loBound.dealiasKeepAnnots.isBottomType) TypeAlias(arg.hiBound & rebase(pbounds.hiBound))
+            else if (v < 0 && pbounds.hiBound.dealiasKeepAnnots.isTopType) TypeAlias(arg.loBound | rebase(pbounds.loBound))
             else arg recoverable_& rebase(pbounds)
           case arg => TypeAlias(arg)
         }
@@ -4165,11 +4179,12 @@ object Types {
      */
     def tryWiden(tp: NamedType, pre: Type): Type = pre.member(tp.name) match {
       case d: SingleDenotation =>
-        d.info.dealias match {
+        val tp1 = d.info.dealiasKeepAnnots
+        tp1.stripAnnots match {
           case TypeAlias(alias) =>
             // if H#T = U, then for any x in L..H, x.T =:= U,
             // hence we can replace with U under all variances
-            reapply(alias)
+            reapply(alias.rewrapAnnots(tp1))
           case TypeBounds(lo, hi) =>
             // If H#T = _ >: S <: U, then for any x in L..H, S <: x.T <: U,
             // hence we can replace with S..U under all variances
@@ -4595,7 +4610,7 @@ object Types {
     case _ => false
   }
 
-  // ----- Decorator implicits --------------------------------------------
+  // ----- Helpers and Decorator implicits --------------------------------------
 
   implicit def decorateTypeApplications(tpe: Type): TypeApplications = new TypeApplications(tpe)
 
@@ -4608,4 +4623,8 @@ object Types {
         else tps2.nonEmpty && tps1.head.equals(tps2.head, bs) && tps1.tail.equalElements(tps2.tail, bs)
       }
   }
+
+  private val keepAlways: AnnotatedType => Context => Boolean = _ => _ => true
+  private val keepNever: AnnotatedType => Context => Boolean = _ => _ => false
+  private val keepIfRefining: AnnotatedType => Context => Boolean = tp => ctx => tp.isRefining(ctx)
 }

compiler/src/dotty/tools/dotc/typer/Applications.scala

@@ -46,7 +46,7 @@ object Applications {
     val ref = extractorMember(tp, name)
     if (ref.isOverloaded)
       errorType(i"Overloaded reference to $ref is not allowed in extractor", errorPos)
-    ref.info.widenExpr.annotatedToRepeated.dealias
+    ref.info.widenExpr.annotatedToRepeated.dealiasKeepAnnots
   }
 
   /** Does `tp` fit the "product match" conditions as an unapply result type

compiler/src/dotty/tools/dotc/typer/Implicits.scala

@@ -763,7 +763,7 @@ trait Implicits { self: Typer =>
     tree match {
       case Select(qual, nme.apply) if defn.isFunctionType(qual.tpe.widen) =>
         val qt = qual.tpe.widen
-        val qt1 = qt.dealias
+        val qt1 = qt.dealiasKeepAnnots
         def addendum = if (qt1 eq qt) "" else (i"\nwhich is an alias of: $qt1")
         em"parameter of ${qual.tpe.widen}$addendum"
       case _ =>

compiler/src/dotty/tools/dotc/typer/Inliner.scala

@@ -224,7 +224,7 @@ class Inliner(call: tpd.Tree, rhsToInline: tpd.Tree)(implicit ctx: Context) {
    */
   private def paramBindingDef(name: Name, paramtp: Type, arg: Tree,
                               bindingsBuf: mutable.ListBuffer[ValOrDefDef]): ValOrDefDef = {
-    val argtpe = arg.tpe.dealias
+    val argtpe = arg.tpe.dealiasKeepAnnots
     val isByName = paramtp.dealias.isInstanceOf[ExprType]
     val inlineFlag = if (paramtp.hasAnnotation(defn.InlineParamAnnot)) Inline else EmptyFlags
     val (bindingFlags, bindingType) =

compiler/src/dotty/tools/dotc/util/common.scala

@@ -8,6 +8,7 @@ import core.Types.WildcardType
 object common {
 
   val alwaysTrue = Function.const(true) _
+  val alwaysFalse = Function.const(false) _
   val alwaysZero = Function.const(0) _
   val alwaysWildcardType = Function.const(WildcardType) _