Make compareHK work if other side is already a lambda.

There ws a missing case, where a lambda projection `T#Apply` is compared
to something to a type lambda. In that case we should compare
the prefix `T` directly wiht the type lambda.

With that modification i553 works if the canBuildFrom implicit is imported.
But the implicit is not yet found in the type scope.

Author: odersky

src/dotty/tools/dotc/core/TypeComparer.scala

@@ -467,16 +467,23 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
   }
 
   /** If `projection` is of the form T # Apply where `T` is an instance of a Lambda class,
-   *  and `other` is not a type lambda projection, then convert `other` to a type lambda `U`, and
-   *  continue with `T <:< U` if `inOrder` is true and `U <:< T` otherwise.
+   *  then
+   *    (1) if `other` is not a (possibly projected) type lambda, convert `other` to a type lambda `U`,
+   *        and continue with `T <:< U` if `inOrder` is true and `U <:< T` otherwise.
+   *    (2) if `other` is a already a type lambda,
+   *        continue with `T <:< other` if `inOrder` is true and `other <:< T` otherwise.
    */
   def compareHK(projection: NamedType, other: Type, inOrder: Boolean) =
     projection.name == tpnme.Apply && {
       val lambda = projection.prefix.LambdaClass(forcing = true)
-      lambda.exists && !other.isLambda &&
-        other.testLifted(lambda.typeParams,
-          if (inOrder) isSubType(projection.prefix, _) else isSubType(_, projection.prefix),
-          if (inOrder) Nil else classBounds(projection.prefix))
+      lambda.exists && {
+        if (!other.isLambda)
+          other.testLifted(lambda.typeParams,
+            if (inOrder) isSubType(projection.prefix, _) else isSubType(_, projection.prefix),
+            if (inOrder) Nil else classBounds(projection.prefix))
+        else if (inOrder) isSubType(projection.prefix, other)
+        else isSubType(other, projection.prefix)
+      }
     }
 
   /** The class symbols bounding the type of the `Apply` member of `tp` */

tests/pos/i553-shuffle.scala

@@ -0,0 +1,8 @@
+import scala.util.Random
+import scala.collection.immutable.List._
+object Test {
+  def test = {
+    val rand = new Random
+    rand.shuffle(List(1,2))//(List.canBuildFrom[Int]) // fails
+  }
+}