Why can't I constrain a Boolean type in EK9?

  Boolean — already maximally constrained (two values)
  JSON — dynamic structure, no defined comparison
  Abstract classes — cannot be instantiated
  Traits — cannot be instantiated
  Functions — not value types

  String — constrain with regex or equality
  Integer — constrain with range (> 0 and < 100)
  Float — constrain with range
  Date, Time, DateTime — constrain with range
  Money — constrain with range
  Colour — constrain with specific values
  Records/Classes with comparison operators

#!ek9
defines module qa.advancedtypes.constraintinvalid

  defines type

    ValidAge as Integer constrain as
      > 0 and < 150

    ShortName as String constrain as
      matches /^[A-Za-z]{1,20}$/

  defines function

    //Opaque, so the constraint is checked at RUNTIME.
    ageFromRuntimeInput()
      <- rtn as Integer: 200

  defines program

    TypeConstraintDemo()
      stdout <- Stdout()

      if validAge <- ValidAge(25)
        stdout.println(`Valid age: ${validAge}`)

      if name <- ShortName("Alice")
        stdout.println(`Valid name: ${name}`)

      //This value is NOT statically known, so the constraint is checked at runtime and a breach
      //Panics here. Had it been knowable — a literal, a constant, or a variable carrying one —
      //it would instead be the compile error E08260, before the program ever ran.
      badAge <- ValidAge(ageFromRuntimeInput())
      stdout.println(`Bad age set: ${badAge?}`)

    ValidAge as Boolean constrain as

    ValidAge as Integer constrain as