Fix of too strict variance checking.

When following an alias type, go directly to aliased type.
Going via the TypeAlias link causes the current variance ot be
narrowed to 0.

Author: odersky

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

@@ -84,7 +84,7 @@ class VarianceChecker()(implicit ctx: Context) {
         case tp: TypeRef =>
           val sym = tp.symbol
           if (sym.variance != 0 && base.isContainedIn(sym.owner)) checkVarianceOfSymbol(sym)
-          else if (sym.isAliasType) this(status, sym.info)
+          else if (sym.isAliasType) this(status, sym.info.bounds.hi)
           else foldOver(status, tp)
         case tp: MethodType =>
           this(status, tp.resultType) // params will be checked in their TypeDef nodes.