First, the newBuilder function in object PriorityQueue is defined as

   def newBuilder[A](implicit ord: Ordering[A]) = new PriorityQueue[A]

The lack of a return type means that the type in the API comes out as

   def newBuilder[A](implicit ord: Ordering[A]): PriorityQueue[A]

rather than the expected

   def newBuilder[A](implicit ord: Ordering[A]): Builder[A, PriorityQueue[A]]

The fix is to explicitly add the appropriate return type to the method declaration.

Second, the current algorithm for creating a priority queue from initial elements inserts the elements one at a time. This takes O(N log N) time. Substituting the well-known "heapify" algorithm would improve this to O(N) time. Similar improvements can be made to enqueue, ++=, and reverse.

I am currently working on implementing these changes.