Fix to checkBounds

Need to account for the fact that some argument types may be TypeBoudns themselves.
The change makes Jason's latest example work.

Author: odersky

src/dotty/tools/dotc/transform/FirstTransform.scala

@@ -141,8 +141,12 @@ class FirstTransform extends MiniPhaseTransform with IdentityDenotTransformer wi
       val tparams = tycon.tpe.typeSymbol.typeParams
       val bounds = tparams.map(tparam =>
         tparam.info.asSeenFrom(tycon.tpe.normalizedPrefix, tparam.owner.owner).bounds)
-      Checking.checkBounds(
-        args, bounds, (tp, argTypes) => tp.substDealias(tparams, argTypes))
+      def instantiateUpperBound(tp: Type, argTypes: List[Type]): Type = {
+        tp.substDealias(tparams, argTypes).bounds.hi
+          // not that argTypes can contain a TypeBounds type for arguments that are
+          // not fully determined. In that case we need to check against the hi bound.
+      }
+      Checking.checkBounds(args, bounds, instantiateUpperBound)
       normalizeType(tree)
     case tree =>
       normalizeType(tree)

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

@@ -33,9 +33,9 @@ object Checking {
   /** A general checkBounds method that can be used for TypeApply nodes as
    *  well as for AppliedTypeTree nodes.
    */
-  def checkBounds(args: List[tpd.Tree], bounds: List[TypeBounds], instantiate: (Type, List[Type]) => Type)(implicit ctx: Context) = {
+  def checkBounds(args: List[tpd.Tree], boundss: List[TypeBounds], instantiate: (Type, List[Type]) => Type)(implicit ctx: Context) = {
     val argTypes = args.tpes
-    for ((arg, bounds) <- args zip bounds) {
+    for ((arg, bounds) <- args zip boundss) {
       def notConforms(which: String, bound: Type) = {
         ctx.error(
           d"Type argument ${arg.tpe} does not conform to $which bound $bound ${err.whyNoMatchStr(arg.tpe, bound)}",

tests/pos/boundspropagation.scala

@@ -15,4 +15,12 @@ object test1 {
     }
   }
 }
+object test2 {
+  class Tree[S, T <: S]
 
+  class Base {
+    def g(x: Any): Tree[_, _ <: Int] = x match {
+      case y: Tree[Int @unchecked, _] => y
+    }
+  }
+}