Optimize replaceSingletons

Avoid formation of full bounds.

Author: odersky

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

@@ -149,11 +149,16 @@ object Inferencing {
         tp match {
           case param: TypeParamRef =>
             val constraint = ctx.typerState.constraint
-            if (constraint.contains(param) &&
-                (constraint.fullUpperBound(param) frozen_<:< constraint.fullLowerBound(param))) {
-              typr.println(i"replace singleton $param := ${constraint.fullLowerBound(param)}")
-              ctx.typerState.constraint = constraint.replace(param,
-                ctx.typeComparer.approximation(param, fromBelow = true))
+            constraint.entry(param) match {
+              case TypeBounds(lo, hi)
+              if constraint.lower(param).isEmpty && constraint.upper(param).isEmpty &&
+                 hi <:< 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)}")
+                ctx.typerState.constraint = constraint.replace(param,
+                  ctx.typeComparer.approximation(param, fromBelow = true))
+              case _ =>
             }
           case _ =>
         }