The Ada front end works by doing an unchecked conversion of X to
t'Base, followed by a range check. The issue is to stop range
propagation through the unchecked conversion.
And as we discussed on the phone, the core of the problem is that Gigi
treats unchecked conversions of numeric types as if they were normal
conversions (in order to make them work for different size types) and
GCC is (properly) propagating information through those conversions.
What has to happen is that we need some sort of way of indicating that it's
not permissible to derive information through a particular conversion.
