Even in ConstrainResult mode, pruning can succeed

This happens when the param we're trying to prune is already a bound of
the current param, so `addParamBound` succeeds without adding any
constraint.

Author: smarter

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

@@ -511,13 +511,16 @@ trait ConstraintHandling[AbstractContext] {
           constraint.entry(bound) match {
             case NoType => pruneLambdaParams(bound)
             case _: TypeBounds =>
-              if (ctx.mode.is(Mode.ConstrainResult))
+              val savedConstraint = constraint
+              val added = addParamBound(bound)
+              if (!added)
+                NoType
+              else if (constraint != savedConstraint && ctx.mode.is(Mode.ConstrainResult)) {
+                constraint = savedConstraint
                 NoType
-              else {
-                if (!addParamBound(bound)) NoType
-                else if (fromBelow) defn.NothingType
-                else defn.AnyType
               }
+              else if (fromBelow) defn.NothingType
+              else defn.AnyType
             case inst =>
               prune(inst)
           }