Overlay allowability

["Marc A. Criley" <marc.a.criley@lmco.com>]

A few times in my career I've encountered situations where two different
representations of the same set of bits are desired in a high
performance application, and this has been effected with the aid of the
'Address representation attribute.  In Ada 95, an example of this would
be:

  N : Natural;

  SN : Stream_Element_Array(1..N'Size / System.Storage_Unit);
  for SN'Address use N;

This has always struck me as somewhat iffy, but I confess I've used
it as well on a couple occasions with Ada 83.  I've always found that
it works as one intuitively expects, so long as all alignment, layout
and sizing aspects are fully thought through and accommodated.

So, is this within the definition of Ada 95?  Or is it well-defined
only for certain constructs, say scalars and non-tagged types, and
dicey for others?  Are there type constructs for which relying on it
is clearly a reliance on undefined behavior, is the whole construct a
reliance on undefined behavior?  Is there a de facto, in place of a
formal, expectation that this should work as expected?

Marc A. Criley