Refactor special handling for unification in order

Author: Linyxus

compiler/src/dotty/tools/dotc/core/OrderingConstraint.scala

@@ -355,13 +355,25 @@ class OrderingConstraint(private val boundsMap: ParamBounds,
       assert(contains(param2), i"$param2")
       // Is `order` called during parameter unification?
       val unifying = isLess(param2, param1)
-      val newUpper = param2 :: exclusiveUpper(param2, param1).filterNot(_ eq param1)
-      val newLower =
+      val newUpper = {
+        val up = exclusiveUpper(param2, param1)
+        if unifying then
+          // Since param2 <:< param1 already holds now, filter out param1 to avoid adding
+          //   duplicated orderings.
+          param2 :: up.filterNot(_ eq param1)
+        else
+          param2 :: up
+      }
+      val newLower = {
+        val lower = exclusiveLower(param1, param2)
         if unifying then
           // Do not add bounds for param1 since it will be unified to param2 soon.
-          exclusiveLower(param1, param2).filterNot(_ eq param2)
+          // And, similarly filter out param2 from lowerly-ordered parameters
+          //   to avoid duplicated orderings.
+          lower.filterNot(_ eq param2)
         else
-          param1 :: exclusiveLower(param1, param2).filterNot(_ eq param2)
+          param1 :: lower
+      }
       val current1 = newLower.foldLeft(current)(upperLens.map(this, _, _, newUpper ::: _))
       val current2 = newUpper.foldLeft(current1)(lowerLens.map(this, _, _, newLower ::: _))
       current2