Note that this attribute is now standard in Ada 202x and is available as an implementation defined attribute for earlier Ada versions.
For every enumeration subtype S, S'Enum_Rep denotes a
function with the following spec:
function S'Enum_Rep (Arg : S'Base) return <Universal_Integer>;
It is also allowable to apply Enum_Rep directly to an object of an
enumeration type or to a non-overloaded enumeration
literal. In this case S'Enum_Rep is equivalent to
typ'Enum_Rep(S) where typ is the type of the
enumeration literal or object.
The function returns the representation value for the given enumeration
value. This will be equal to value of the Pos attribute in the
absence of an enumeration representation clause. This is a static
attribute (i.e., the result is static if the argument is static).
S'Enum_Rep can also be used with integer types and objects,
in which case it simply returns the integer value. The reason for this
is to allow it to be used for (<>) discrete formal arguments in
a generic unit that can be instantiated with either enumeration types
or integer types. Note that if Enum_Rep is used on a modular
type whose upper bound exceeds the upper bound of the largest signed
integer type, and the argument is a variable, so that the universal
integer calculation is done at run time, then the call to Enum_Rep
may raise Constraint_Error.