Fixed type adaptation problem in checkBounds

We need to adapt type parameter bounds with an as-ssen-from to the
prefix of the type constructor.

Makes pos/boundspropagation pass.

Author: odersky

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

@@ -126,8 +126,10 @@ class FirstTransform extends MiniPhaseTransform with IdentityDenotTransformer {
     case tree: NamedArg => transform(tree.arg)
     case AppliedTypeTree(tycon, args) =>
       val tparams = tycon.tpe.typeSymbol.typeParams
+      val bounds = tparams.map(tparam =>
+        tparam.info.asSeenFrom(tycon.tpe.normalizedPrefix, tparam.owner.owner).bounds)
       Checking.checkBounds(
-        args, tparams.map(_.info.bounds), (tp, argTypes) => tp.substDealias(tparams, argTypes))
+        args, bounds, (tp, argTypes) => tp.substDealias(tparams, argTypes))
       normalizeType(tree)
     case tree =>
       normalizeType(tree)

tests/pos/boundspropagation.scala

@@ -0,0 +1,18 @@
+// test contributed by @retronym
+object test1 {
+  class Base {
+    type N
+
+    class Tree[-T >: N]
+
+    def f(x: Any): Tree[N] = x match {
+      case y: Tree[_] => y
+    }
+  }
+  class Derived extends Base {
+    def g(x: Any): Tree[N] = x match {
+      case y: Tree[_] => y // now succeeds in dotc
+    }
+  }
+}
+