Treat variances in type accumulators and type maps the same

Bonus point: No need to override applyPrefix in variances
accumulator. This means we have reduced three different ways
to compute the current variance to one.

Author: odersky

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

@@ -3924,9 +3924,9 @@ object Types {
 
     protected var variance = 1
 
-    protected def applyToPrefix(x: T, tp: NamedType) = {
+    protected final def applyToPrefix(x: T, tp: NamedType) = {
       val saved = variance
-      variance = 0
+      variance = variance max 0 // see remark on NamedType case in TypeMap
       val result = this(x, tp.prefix)
       variance = saved
       result

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

@@ -319,8 +319,6 @@ object Inferencing {
         case _ =>
           foldOver(vmap, t)
       }
-      override def applyToPrefix(vmap: VarianceMap, t: NamedType) =
-        apply(vmap, t.prefix)
     }
 
     /** Include in `vmap` type variables occurring in the constraints of type variables