What would be the Ada solution?

[KK6GM <mjsilva@scriptoriumdesigns.com>]

A colleague is working on some old code, written in C, that uses an
"out of range" integer value to indicate no valid value.  Thus, a
default value (in this case 0x7FFFFFFF) means no value has been
entered.  All code that uses any values should check for this no-value
and act accordingly, but of course not all the code actually does
that, and there are odd cases where the no-value value gets processed
as a valid value and then Bad Things Happen.

I'm curious what the Ada approach to this issue would be, the issue
being to differentiate between valid and invalid values, and to catch
(compile time or run time) any manipulation of an invalid value as if
it were a valid value.