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From: stt@houdini.camb.inmet.com (Tucker Taft)
Subject: Re: Generic Formal With Discrim Used As Actual - How?
Date: 1998/10/25
Message-ID: <F1DwDG.2qr.0.-s@inmet.camb.inmet.com>#1/1
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References: <m3af2l77o2.fsf@mheaney.ni.net>
Organization: Intermetrics, Inc.
Newsgroups: comp.lang.ada
Date: 1998-10-25T00:00:00+00:00
List-Id: <comp.lang.ada>

Matthew Heaney (matthew_heaney@acm.org) wrote:

: I'm having trouble passing generic formal types --one of which has an
: access discriminant-- as generic actuals to another generic having an
: identical profile.  

I believe you have bumped into a compiler bug.  My favorite Ada 95
front end has no problem with your code.

: ... Something like:

: generic
:    type T1 (<>) is limited private;
:    type T2 (O : access T1) is limited private;
: procedure GP;

: generic
:    type T1 (<>) is limited private;
:    type T2 (O : access T1) is limited private;
: procedure GQ;

: with GQ;
: procedure GP is
:   procedure Q is new GQ (T1, T2);
: begin
:    null;
: end GP;


: When I try to compile the body of GP, I get the following message:

: (start of compiler message)
: gnatf /home/matt/acl/rings/gp.adb
: gp.adb:4:30: types of actual discriminants must match formal
: gp.adb:4:30: instantiation abandoned

This may be due to the fact that each access discriminant conceptually
declares a new access type.  However, subtype matching is
defined in the RM so as to bypass this problem, but perhaps your
compiler is falling into the trap.

: Compilation exited abnormally with code 1 at Sun Oct 25 01:13:09
: (end of compiler message)


: Can someone explain what's going on here?  Why doesn't this code compile?

: Is there any way to pass a formal type with a discriminant as the actual
: type of another instantiation?

You are doing the right thing.  Your compiler is tripping over it...

You might try using a named access type rather than an anonymous
access type for the discriminant, and see whether your compiler swallows it.

: Thanks,
: Matt

--
-Tucker Taft   stt@inmet.com   http://www.inmet.com/~stt/
Intermetrics, Inc.  Burlington, MA  USA
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