User-defined type attributes

In light of the recent discussion of formal package parameters, I have
been wondering about user-defined type attributes.  They don't
currently exist in any version of Ada.

First, a historical question: Was there a proposal for user attributes
that was considered for Ada 2005?

My interest in them is akin to my interest in formal package
parameters.  The prefixed views for functions and procedures (dot-
notation) of Ada 2005 are directly analogous to what would be required
for attribute syntax.  This notation hints at the possibility of a
kind of visibility related to formal type parameters, one which would
make ordinary subprograms and attribute functions visible without
making the entire definition package visible.  The idea is that a
formal type parameter create a visibility related only to that type
itself.  In order to make this really useful, attribute functions
would be needed to forestall the use of an entire package, rather than
only a single type.

If this seems abstruse, just consider the character traits that the C+
+ standard library uses.

And a related historical question: Why does the prefixed view of a
subprogram only apply to tagged types?  It seems like a syntax
eminently suited for any subprogram.

An attribute function must evaluate at compile-time, or perhaps more
subtly, prior to elaboration time.  Much of the requisite apparatus is
already present with static expressions and preelaborable packages.
The standard would require an additional definition of the ability to
pre-elaborate a function body.  While such a body might be restricted
to returning a single static expression, that seem a bit restrictive.

My overall motivation (as before) is to enable better, more flexible
library sup****t.  Library sup****t gates usability and enables cost-
justifiable selection of Ada in a greater number of application
domains.  These kinds of features are not so critical for a single
team writing their own software, but rather come into their own when
writing libraries for other people to use in not-yet-anticipated ways.

Eric