add more comments to code

Author: liufengyun

compiler/src/dotty/tools/dotc/transform/patmat/Space.scala

@@ -582,15 +582,17 @@ class SpaceEngine(implicit ctx: Context) extends SpaceLogic {
     // replace type parameter references with fresh type vars or bounds
     val typeParamMap = new TypeMap {
       def apply(t: Type): Type = t match {
+
         case tp: TypeRef if tp.underlying.isInstanceOf[TypeBounds] =>
+          // See tests/patmat/gadt.scala  tests/patmat/exhausting.scala
           val bound =
-            if (variance == 0) tp.underlying.bounds
+            if (variance == 0) tp.underlying.bounds      // non-variant case is not well-founded
             else if (variance == 1) TypeBounds.upper(tp)
             else TypeBounds.lower(tp)
           newTypeVar(bound)
         case tp: RefinedType if tp.refinedInfo.isInstanceOf[TypeBounds] =>
-          // Ideally, we would expect type inference can do the job
-          //   tests/patmat/t9657.scala
+          // Ideally, we would expect type inference to do the job
+          // Check tests/patmat/t9657.scala
           expose(tp)
         case _ =>
           mapOver(t)