Document and polish CheckCaptures

Author: odersky

compiler/src/dotty/tools/dotc/cc/CaptureOps.scala

@@ -127,10 +127,6 @@ end CCState
 def ccState(using Context) =
   Phases.checkCapturesPhase.asInstanceOf[CheckCaptures].ccState1
 
-class NoCommonRoot(rs: Symbol*)(using Context) extends Exception(
-  i"No common capture root nested in ${rs.mkString(" and ")}"
-)
-
 extension (tree: Tree)
 
   /** Map tree with CaptureRef type to its type,

compiler/src/dotty/tools/dotc/cc/CaptureSet.scala

@@ -185,10 +185,12 @@ sealed abstract class CaptureSet extends Showable:
 
   /** A more optimistic version of subCaptures used to choose one of two typing rules
    *  for selections and applications. `cs1 mightSubcapture cs2` if `cs2` might account for
-   *  every element currently known to be in `cs1`.
+   *  every element currently known to be in `cs1`, and the same is not true in reverse
+   *  when we compare elements of cs2 vs cs1.
    */
   def mightSubcapture(that: CaptureSet)(using Context): Boolean =
     elems.forall(that.mightAccountFor)
+    && !that.elems.forall(this.mightAccountFor)
 
   /** The subcapturing test.
    *  @param frozen   if true, no new variables or dependent sets are allowed to