Alternative fix

The original fix made run/hmap-covariant fail because a type variable
representing a dependent result parameter was instantiated. Trying something
else now.

Author: odersky

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

@@ -159,7 +159,11 @@ trait TypeOps { this: Context => // TODO: Make standalone object.
         case tp1 => tp1
       }
     case tp: TypeParamRef =>
-      typerState.constraint.typeVarOfParam(tp) orElse tp
+      if (tp.paramName.is(DepParamName)) {
+        val bounds = ctx.typeComparer.bounds(tp)
+        if (bounds.lo.isRef(defn.NothingClass)) bounds.hi else bounds.lo
+      }
+      else typerState.constraint.typeVarOfParam(tp) orElse tp
     case  _: ThisType | _: BoundType | NoPrefix =>
       tp
     case tp: RefinedType =>
@@ -170,9 +174,6 @@ trait TypeOps { this: Context => // TODO: Make standalone object.
       simplify(l, theMap) & simplify(r, theMap)
     case OrType(l, r) =>
       simplify(l, theMap) | simplify(r, theMap)
-    case tp: TypeVar if tp.origin.paramName.is(DepParamName) =>
-      val effectiveVariance = if (theMap == null) 1 else theMap.variance
-      tp.instanceOpt orElse tp.instantiate(fromBelow = effectiveVariance != -1)
     case _ =>
       (if (theMap != null) theMap else new SimplifyMap).mapOver(tp)
   }