Use substApprox for F-bounds instantiation

Use substApprox for F-bounds instantiation if there are wildcards.

This is the simplest checking algorithm I could find that is not obviously wrong and
that accepts i6146.scala. I can't prove it's correct, though.

Author: odersky

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

@@ -75,8 +75,12 @@ object Checking {
     }.getOrElse(TypeTree(tparam.paramRef))
     val orderedArgs = if (hasNamedArg(args)) tparams.map(argNamed) else args
     val bounds = tparams.map(_.paramInfoAsSeenFrom(tree.tpe).bounds)
-    def instantiate(bound: Type, args: List[Type]) =
-      HKTypeLambda.fromParams(tparams, bound).appliedTo(args)
+    def instantiate(bound: Type, args: List[Type]) = tparams match {
+      case (_: Symbol) :: _ if args.exists(_.isInstanceOf[TypeBounds]) =>
+        bound.substApprox(tparams.asInstanceOf[List[Symbol]], args)
+      case _ =>
+        HKTypeLambda.fromParams(tparams, bound).appliedTo(args)
+    }
     if (boundsCheck) checkBounds(orderedArgs, bounds, instantiate)
 
     def checkWildcardApply(tp: Type): Unit = tp match {