Fix problems with TypeVar instantiation

1) Simplify skipped one level over arguments of AndType/OrType.
2) variances needs to follow instantiated typevars

Author: odersky

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

@@ -631,11 +631,14 @@ class TypeComparer(initctx: Context) extends DotClass {
       def comparePolyParam = {
         tp1 == tp2 ||
         isSubTypeWhenFrozen(bounds(tp1).hi, tp2) || {
-          if (!frozenConstraint &&
-              (tp2 isRef defn.NothingClass) &&
-              ctx.typerState.isGlobalCommittable)
-            ctx.log(s"!!! instantiating to Nothing: $tp1")
-          if (isConstrained(tp1)) addConstraint(tp1, tp2, fromBelow = false)
+          if (isConstrained(tp1))
+            addConstraint(tp1, tp2, fromBelow = false) && {
+              if ((!frozenConstraint) &&
+                  (tp2 isRef defn.NothingClass) &&
+                  ctx.typerState.isGlobalCommittable)
+                ctx.log(s"!!! instantiated to Nothing: $tp1, constraint = ${constraint.show}")
+              true
+            }
           else (ctx.mode is Mode.TypevarsMissContext) || thirdTry(tp1, tp2)
         }
       }

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

@@ -828,7 +828,7 @@ object Types {
     def variances(include: TypeVar => Boolean)(implicit ctx: Context): VarianceMap = track("variances") {
       val accu = new TypeAccumulator[VarianceMap] {
         def apply(vmap: VarianceMap, t: Type): VarianceMap = t match {
-          case t: TypeVar if include(t) =>
+          case t: TypeVar if !t.isInstantiated && include(t) =>
             val v = vmap(t)
             if (v == null) vmap.updated(t, variance)
             else if (v == variance) vmap
@@ -852,9 +852,9 @@ object Types {
       class Simplify extends TypeMap {
         def apply(tp: Type): Type = tp match {
           case AndType(l, r) =>
-            mapOver(l) & mapOver(r)
+            this(l) & this(r)
           case OrType(l, r) =>
-            mapOver(l) | mapOver(r)
+            this(l) | this(r)
           case tp: PolyParam =>
             ctx.typerState.constraint.typeVarOfParam(tp) orElse tp
           case _ =>

src/dotty/tools/dotc/typer/Typer.scala

@@ -1225,8 +1225,8 @@ class Typer extends Namer with Applications with Implicits {
         if (tree.tpe <:< pt) tree
         else if (ctx.mode is Mode.Pattern) tree // no subtype check for pattern
         else {
-          typr.println(s"adapt to subtype ${tree.tpe} !<:< $pt")
-          typr.println(TypeComparer.explained(implicit ctx => tree.tpe <:< pt))
+          typr.println(s"adapt to subtype ${tree.tpe.show} !<:< ${pt.show}")
+          //typr.println(TypeComparer.explained(implicit ctx => tree.tpe <:< pt))
           adaptToSubType(wtp)
         }
     }

tests/pos/collections.scala

@@ -2,6 +2,9 @@ import scala.collection.generic.CanBuildFrom
 
 object collections {
 
+  val arr = Array("a", "b")
+  val aa = arr ++ arr
+ 
   List(1, 2, 3) map (x => 2)
 
   val s = Set(1, 2, 3)