Coarser variance checking for match types

Only check the bound, instead of all branches, in the definition of a match type.

Author: odersky

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

@@ -95,6 +95,8 @@ class VarianceChecker()(implicit ctx: Context) {
             this(status, tp.resultType) // params will be checked in their TypeDef or ValDef nodes.
           case AnnotatedType(_, annot) if annot.symbol == defn.UncheckedVarianceAnnot =>
             status
+          case tp: MatchType =>
+            apply(status, tp.bound)
           case tp: ClassInfo =>
             foldOver(status, tp.classParents)
           case _ =>