12.68 GNAT.Exceptions (g-except.ads)

Normally it is not possible to raise an exception with a message from a subprogram in a pure package, since the necessary types and subprograms are in Ada.Exceptions which is not a pure unit. GNAT.Exceptions provides a facility for getting around this limitation for a few predefined exceptions, and for example allows raising Constraint_Error with a message from a pure subprogram.