Streamline approximateWildcards logic

Author: odersky

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

@@ -79,9 +79,8 @@ trait ConstraintHandling {
   def fullBounds(param: TypeParamRef)(using Context): TypeBounds =
     nonParamBounds(param).derivedTypeBounds(fullLowerBound(param), fullUpperBound(param))
 
-  /** If true: eliminate wildcards in bounds by avoidance.
-   *  Otherwise replace them by fresh variables, except that
-   *  in mode TypeVarsMissContext, wildcards are always eliminated by approximation.
+  /** If true, eliminate wildcards in bounds by avoidance, otherwise replace
+   *  them by fresh variables.
    */
   protected def approximateWildcards: Boolean = true
 
@@ -96,7 +95,7 @@ trait ConstraintHandling {
         if !isUpper then variance = -1
         def apply(t: Type): Type = t match
           case t: WildcardType =>
-            if approximateWildcards || ctx.mode.is(Mode.TypevarsMissContext) then
+            if approximateWildcards then
               val bounds = t.effectiveBounds
               range(bounds.lo, bounds.hi)
             else

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

@@ -139,7 +139,14 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
     try topLevelSubType(tp1, tp2)
     finally useNecessaryEither = saved
 
-  override protected def approximateWildcards: Boolean = useNecessaryEither
+  /** Use avoidance to get rid of wildcards in constraint bounds if
+   *  we are doing a neccessary comparison, or the mode is TypeVarsMissContext.
+   *  The idea is that under either of these conditions we are not interested
+   *  in creating a fresh type variable to replace the wildcard. I verified
+   *  that several tests break if one or the other part of the disjunction is dropped.
+   */
+  override protected def approximateWildcards: Boolean =
+    useNecessaryEither || ctx.mode.is(Mode.TypevarsMissContext)
 
   def testSubType(tp1: Type, tp2: Type): CompareResult =
     GADTused = false