Fix docs and naming

Author: odersky

src/dotty/tools/dotc/config/Config.scala

@@ -15,11 +15,12 @@ object Config {
    */
   final val checkConstraintsNonCyclic = false
 
-  /** Like `checkConstraintsNonCyclic`, but all constrained parameters
-   *  are tested for direct or indirect dependencies, each time a
-   *  constraint is added in TypeComparer.
+  /** Make sure none of the bounds in an OrderingConstraint contains
+   *  another constrained parameter at its toplevel (i.e. as an operand
+   *  of a combination of &'s and |'s.). The check is performed each time
+   *  a new bound is added to the constraint.
    */
-  final val checkConstraintsNonCyclicTrans = false
+  final val checkConstraintsSeparated = false
 
   /** Check that each constraint resulting from a subtype test
    *  is satisfiable.

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

@@ -45,7 +45,7 @@ trait ConstraintHandling {
           }
         }
       }
-      if (Config.checkConstraintsNonCyclicTrans)
+      if (Config.checkConstraintsSeparated)
         assert(!occursIn(bound), s"$param occurs in $bound")
       val c1 = constraint.narrowBound(param, bound, isUpper)
       (c1 eq constraint) || {
@@ -252,11 +252,13 @@ trait ConstraintHandling {
        *    Some of the Ti are constrained parameters
        *
        *  In each case we cannot record the relationship as an isLess, because
-       *  of he outer |/&. But we should not leave it in the constraint either
+       *  of the outer |/&. But we should not leave it in the constraint either
        *  because that would risk making a parameter a subtype or supertype of a bound
        *  where the parameter occurs again at toplevel, which leads to cycles
        *  in the subtyping test. So we intentionally loosen the constraint in order
        *  to keep it safe. A test case that demonstrates the problem is i864.scala.
+       *  Turn Config.checkConstraintsSeparated on to get an accurate diagnostic
+       *  of the cycle when it is created.
        */
       def prune(bound: Type): Type = bound match {
         case bound: AndOrType =>