Defined substitution which follows aliases

Used in FullParameterization to substitute type parameters.
Fixes test failure for t2399.scala

Author: odersky

src/dotty/tools/dotc/core/Substituters.scala

@@ -89,6 +89,40 @@ trait Substituters { this: Context =>
     }
   }
 
+  final def substDealias(tp: Type, from: List[Symbol], to: List[Type], theMap: SubstDealiasMap): Type = {
+    tp match {
+      case tp: NamedType =>
+        val sym = tp.symbol
+        var fs = from
+        var ts = to
+        while (fs.nonEmpty) {
+          if (fs.head eq sym) return ts.head
+          fs = fs.tail
+          ts = ts.tail
+        }
+        if (sym.isStatic && !existsStatic(from)) tp
+        else {
+          val prefix1 = substDealias(tp.prefix, from, to, theMap)
+          if (prefix1 ne tp.prefix) tp.derivedSelect(prefix1)
+          else if (sym.isAliasType) {
+            val hi = sym.info.bounds.hi
+            val hi1 = substDealias(hi, from, to, theMap)
+            if (hi1 eq hi) tp else hi1
+          }
+          else tp
+        }
+      case _: ThisType | _: BoundType | NoPrefix =>
+        tp
+      case tp: RefinedType =>
+        tp.derivedRefinedType(substDealias(tp.parent, from, to, theMap), tp.refinedName, substDealias(tp.refinedInfo, from, to, theMap))
+      case tp: TypeBounds if tp.lo eq tp.hi =>
+        tp.derivedTypeAlias(substDealias(tp.lo, from, to, theMap))
+      case _ =>
+        (if (theMap != null) theMap else new SubstDealiasMap(from, to))
+          .mapOver(tp)
+    }
+  }
+
   final def substSym(tp: Type, from: List[Symbol], to: List[Symbol], theMap: SubstSymMap): Type =
     tp match {
       case tp: NamedType =>
@@ -205,11 +239,11 @@ trait Substituters { this: Context =>
   final class SubstMap(from: List[Symbol], to: List[Type]) extends DeepTypeMap {
     def apply(tp: Type): Type = subst(tp, from, to, this)
   }
-/* not needed yet
+
   final class SubstDealiasMap(from: List[Symbol], to: List[Type]) extends SubstMap(from, to) {
-    override def apply(tp: Type): Type = subst(tp.dealias, from, to, this)
+    override def apply(tp: Type): Type = substDealias(tp, from, to, this)
   }
-*/
+
   final class SubstSymMap(from: List[Symbol], to: List[Symbol]) extends DeepTypeMap {
     def apply(tp: Type): Type = substSym(tp, from, to, this)
   }

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

@@ -818,10 +818,17 @@ object Types {
         }
       }
 
-/* Not needed yet:
+    /** Same as `subst` but follows aliases as a fallback. When faced with a reference
+     *  to an alias type, where normal substiution does not yield a new type, the
+     *  substitution is instead applied to the alias. If that yields a new type,
+     *  this type is returned, outherwise the original type (not the alias) is returned.
+     *  A use case for this method is if one wants to substitute the type parameters
+     *  of a class and also wants to substitute any parameter accessors that alias
+     *  the type parameters.
+     */
     final def substDealias(from: List[Symbol], to: List[Type])(implicit ctx: Context): Type =
-      new ctx.SubstDealiasMap(from, to).apply(this)
-*/
+      ctx.substDealias(this, from, to, null)
+
     /** Substitute all types of the form `PolyParam(from, N)` by
      *  `PolyParam(to, N)`.
      */

src/dotty/tools/dotc/transform/FullParameterization.scala

@@ -104,7 +104,7 @@ trait FullParameterization {
     /** Replace class type parameters by the added type parameters of the polytype `pt` */
     def mapClassParams(tp: Type, pt: PolyType): Type = {
       val classParamsRange = (mtparamCount until mtparamCount + ctparams.length).toList
-      tp.subst(clazz.typeParams, classParamsRange map (PolyParam(pt, _)))
+      tp.substDealias(clazz.typeParams, classParamsRange map (PolyParam(pt, _)))
     }
 
     /** The bounds for the added type paraneters of the polytype `pt` */
@@ -201,7 +201,7 @@ trait FullParameterization {
 
       new TreeTypeMap(
         typeMap = rewireType(_)
-          .subst(origTParams, trefs)
+          .substDealias(origTParams, trefs)
           .subst(origVParams, argRefs.map(_.tpe))
           .substThisUnlessStatic(origClass, thisRef.tpe),
         ownerMap = (sym => if (sym eq origMeth) derived else sym),