A slightly more conservative version of #14218 (#18352)

Author: dwijnand

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

@@ -251,6 +251,11 @@ object Types {
  case _ => false
  }
 
+ /** Is this type exactly `Any`, or a type lambda ending in `Any`? */
+ def isTopOfSomeKind(using Context): Boolean = dealias match
+ case tp: TypeLambda => tp.resType.isTopOfSomeKind
+ case _ => isExactlyAny
+
  def isBottomType(using Context): Boolean =
  if ctx.mode.is(Mode.SafeNulls) && !ctx.phase.erasedTypes then hasClassSymbol(defn.NothingClass)
  else isBottomTypeAfterErasure
@@ -4896,7 +4901,7 @@ object Types {
  def hasLowerBound(using Context): Boolean = !currentEntry.loBound.isExactlyNothing
 
  /** For uninstantiated type variables: Is the upper bound different from Any? */
- def hasUpperBound(using Context): Boolean = !currentEntry.hiBound.finalResultType.isExactlyAny
+ def hasUpperBound(using Context): Boolean = !currentEntry.hiBound.isTopOfSomeKind
 
  /** Unwrap to instance (if instantiated) or origin (if not), until result
  * is no longer a TypeVar

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

@@ -187,7 +187,11 @@ object Inferencing {
  // else hold off instantiating unbounded unconstrained variable
  else if direction != 0 then
  instantiate(tvar, fromBelow = direction < 0)
- else if variance >= 0 && (force.ifBottom == IfBottom.ok && !tvar.hasUpperBound || tvar.hasLowerBound) then
+ else if variance >= 0 && tvar.hasLowerBound then
+ instantiate(tvar, fromBelow = true)
+ else if (variance > 0 || variance == 0 && !tvar.hasUpperBound)
+ && force.ifBottom == IfBottom.ok
+ then // if variance == 0, prefer upper bound if one is given
  instantiate(tvar, fromBelow = true)
  else if variance >= 0 && force.ifBottom == IfBottom.fail then
  fail = true