Add more explanations

Author: odersky

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

@@ -350,7 +350,10 @@ trait TypeAssigner {
           else {
             // Make sure arguments don't contain the type `pt` itself.
             // make a copy of the argument if that's the case.
-            // See pos/i6682a.scala for a test case where this matters.
+            // This is done to compensate for the fact that normally every
+            // reference to a polytype would have to be a fresh copy of that type,
+            // but we want to avoid that because it would increase compilation cost.
+            // See pos/i6682a.scala for a test case where the defensive copying matters.
             val ensureFresh = new TypeMap:
               def apply(tp: Type) = mapOver(
                 if tp eq pt then pt.newLikeThis(pt.paramNames, pt.paramInfos, pt.resType)