In ConstrainResult mode, prefer widening over narrowing bounds

The same should also probably be done in GADT mode.

Author: smarter

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

@@ -511,9 +511,13 @@ trait ConstraintHandling[AbstractContext] {
           constraint.entry(bound) match {
             case NoType => pruneLambdaParams(bound)
             case _: TypeBounds =>
-              if (!addParamBound(bound)) NoType
-              else if (fromBelow) defn.NothingType
-              else defn.AnyType
+              if (ctx.mode.is(Mode.ConstrainResult))
+                NoType
+              else {
+                if (!addParamBound(bound)) NoType
+                else if (fromBelow) defn.NothingType
+                else defn.AnyType
+              }
             case inst =>
               prune(inst)
           }