From zefram@fysh.org

The Range class allows most values to be used as endpoints, even things
such as type objects. But certain type objects get mistaken for defined
objects, causing silly error results​:

Range.new(List, Pair).perl
List..Pair
Range.new(Seq, List).perl
Seq objects are not valid endpoints for Ranges
  in block <unit> at <unknown file> line 1
Range.new(List, Int).perl
List..Int
Range.new(Int, List).perl
Use of uninitialized value of type List in numeric context in block <unit> at <unknown file> line 1
Use of uninitialized value of type List in numeric context in block <unit> at <unknown file> line 1
Invocant requires an instance of type Int, but a type object was passed. Did you forget a .new?
  in block <unit> at <unknown file> line 1

These type objects should be either successfully allowed or cleanly
prohibited. They don't have the disadvantages that the corresponding
defined objects would have, so they should probably be allowed.

-zefram