Address review comment

Author: odersky

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

@@ -232,11 +232,12 @@ class TyperState() {
             tvars.foreach(tvar =>
               if !tvar.inst.exists then
                 if !isOwnedAnywhere(this, tvar) then includeVar(tvar)
-                if constraint.isHard(tvar) then constraint = constraint.withHard(tvar))
           typeComparer.addToConstraint(tl, tvars)
         }) &&
         // Integrate the additional constraints on type variables from `other`
+        // and merge hardness markers
         constraint.uninstVars.forall(tv =>
+          if other.isHard(tv) then constraint = constraint.withHard(tv)
           val p = tv.origin
           val otherLos = other.lower(p)
           val otherHis = other.upper(p)