a.ts(17,14): error TS2322: Type 'Num' is not assignable to type 'Runtype<any>'.
  The types of 'constraint.constraint' are incompatible between these types.
    Type 'Constraint<Constraint<Num>>' is not assignable to type 'Constraint<Constraint<Runtype<any>>>'.
      Type 'Constraint<Runtype<any>>' is not assignable to type 'Constraint<Num>'.
        Types of property 'underlying' are incompatible.
          Property 'tag' is missing in type 'Runtype<any>' but required in type 'Num'.
