Another optimization

In fact, the isLess relation is irrelevant here. If

  p >: L <: H   and  L =:= H

and the constraint is satisfiable, then it's safe to replace p by L or H
anyway, no matter what other parameters are known to be smaller or larger
than p.

Author: odersky

compiler/src/dotty/tools/dotc/typer/Inferencing.scala

@@ -151,8 +151,7 @@ object Inferencing {
             val constraint = ctx.typerState.constraint
             constraint.entry(param) match {
               case TypeBounds(lo, hi)
-              if constraint.lower(param).isEmpty && constraint.upper(param).isEmpty &&
-                 (hi frozen_<:< lo) =>
+              if (hi frozen_<:< lo) =>
                 // if lower or upper is nonEmpty, the full bounds can't be equal, since
                 // common type params in lower and upper are eliminated through unification
                 typr.println(i"replace singleton $param := ${constraint.fullLowerBound(param)}")