Private discrete type as index

["AG" <ang@xtra.co.nz>]

Hi all,

Guess I have a question that I'm a bit puzzled about.
Consider the boiled-down code snippet:

package test is
  type x is limited private;
  type test is array(x) of boolean;
private
    type x is new boolean;
end test;

Trying to compile that fails on GNAT
with "Discrete type required" error.
Can't blame it, of course, since it's
correct according to LRM. But...

the obvious intention was to provide
a set of "+/-" operations etc on the
limited type (which are not trivial)
and prevent the out-of-package clients
from messing with the index.

Since the compiler obviously knows
the implementation details, why not?

Of course, it can be implemented as
an iterator, but what's wrong with
the example as written? After all,
the client modules can't use that
index other than through the type
and operation provided by the
package. Compiler/linker sure knows
what the type is. So, why not?