Workaround scala#592 Insert casts all over the place in specialised methods.

The way how this fix is structures, is that if 592 ever gets fixed, no casts will be inserted.

Author: DarkDimius

src/dotty/tools/dotc/ast/TreeTypeMap.scala

@@ -33,7 +33,7 @@ import dotty.tools.dotc.transform.SymUtils._
  *  gets two different denotations in the same period. Hence, if -Yno-double-bindings is
  *  set, we would get a data race assertion error.
  */
-final class TreeTypeMap(
+class TreeTypeMap(
   val typeMap: Type => Type = IdentityTypeMap,
   val treeMap: tpd.Tree => tpd.Tree = identity _,
   val oldOwners: List[Symbol] = Nil,

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

@@ -176,7 +176,8 @@ class TypeSpecializer extends MiniPhaseTransform  with InfoTransformer {
               val tmap: (Tree => Tree) = _ match {
                 case Return(t, from) if from.symbol == tree.symbol => Return(t, ref(newSym))
                 case t: TypeApply => transformTypeApply(t)
-                case t: Apply => transformApply(t)
+                case t: Apply =>
+                  transformApply(t)
                 case t => t
               }
 
@@ -188,7 +189,21 @@ class TypeSpecializer extends MiniPhaseTransform  with InfoTransformer {
                 ,
                 oldOwners = tree.symbol :: Nil,
                 newOwners = newSym :: Nil
-              ).transform(tree.rhs)
+              ) {
+                // needed to workaround https://github.com/lampepfl/dotty/issues/592
+                override def transform(t: Tree)(implicit ctx: Context) = super.transform(t) match {
+                  case t @ Apply(fun, args) =>
+                    val newArgs = (args zip fun.tpe.firstParamTypes).map{case(t, tpe) => t.ensureConforms(tpe)}
+                    if (sameTypes(args, newArgs)) {
+                      t
+                    } else tpd.Apply(fun, newArgs)
+                  case t: ValDef =>
+                    cpy.ValDef(t)(rhs = t.rhs.ensureConforms(t.tpe.widen))
+                  case t: DefDef =>
+                    cpy.DefDef(t)(rhs = t.rhs.ensureConforms(t.tpe.finalResultType))
+                  case t => t
+                }
+              }.transform(tree.rhs)
             }})
           }
         } else Nil

tests/pos/specialization/this_specialization.scala

@@ -1,8 +1,14 @@
-sealed abstract class Foo[@specialized +A] {
+trait Foo[@specialized +A] {
+  // all those examples trigger bugs due to https://github.com/lampepfl/dotty/issues/592
   def bop[@specialized B >: A]: Foo[B] = new Bar[B](this)
+  def gwa[@specialized B >: A]: Foo[B] = this
+  def gwd[@specialized B >: A]: Foo[B] = {
+    val d: Foo[B] = this
+    d
+  }
   //def bip[@specialized C >: A, @specialized D >: A]: Foo[D] = new Cho[D, C](new Bar[C](this))
 }
 
 case class Bar[@specialized a](tl: Foo[a]) extends Foo[a]
 
-//case class Cho[@specialized c, @specialized d](tl: Bar[d]) extends Foo[c]
+//case class Cho[@specialized c, @specialized d](tl: Bar[d]) extends Foo[c]