Avoid replacing type lambda params with normal params

Type lambdas shgould be short lived, so we should avoid
instantiating normal poly params to type lambda params.

Author: odersky

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

@@ -89,7 +89,11 @@ trait ConstraintHandling {
   protected def addLess(p1: PolyParam, p2: PolyParam): Boolean = {
     def description = i"ordering $p1 <: $p2 to\n$constraint"
     val res =
-      if (constraint.isLess(p2, p1)) unify(p2, p1)
+      if (constraint.isLess(p2, p1))
+        if (p2.binder.isInstanceOf[TypeLambda] && !p1.binder.isInstanceOf[TypeLambda])
+          unify(p1, p2)
+        else
+          unify(p2, p1)
       else {
         val down1 = p1 :: constraint.exclusiveLower(p1, p2)
         val up2 = p2 :: constraint.exclusiveUpper(p2, p1)
@@ -109,7 +113,6 @@ trait ConstraintHandling {
    */
   private def unify(p1: PolyParam, p2: PolyParam): Boolean = {
     constr.println(s"unifying $p1 $p2")
-    assert(constraint.isLess(p1, p2))
     val down = constraint.exclusiveLower(p2, p1)
     val up = constraint.exclusiveUpper(p1, p2)
     needsCleanup = true