Version 1.1 of ai12s/ai12-0155-1.txt

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!standard 13.14(3/4)          15-02-20 AI05-0155-1/00
!class binding interpretation 15-02-20
!status work item 15-02-20
!status received 15-02-20
!priority Low
!difficulty Easy
!qualifier Omission
!subject Freezing of operations of incomplete types with completions deferred to a body
!summary
** TBD.
!question
Consider:
package Pack is
type Bag_Type is private;
procedure Add (Bag : in out Bag_Type; Elem : Integer);
private
type Set is tagged;
type Bag_Type is access Set;
package Inner is -- These can't be primitive for Set, else 3.10.1(9.3/2) is violated. procedure Add (Elem : Integer; To_Set : in out Set); procedure Union (Left, Right : in Set; Result : out Set); procedure Intersection (Left, Right : Set; Result : out Set); function "=" (Left, Right : Set) return Boolean; end Inner;
end Pack; -- Freezing here.
13.14(3/4) [and earlier versions as well] says that everything exception incomplete types are frozen at the end of Pack. This includes the profiles of any subprograms declared within Pack.
13.14(14/3) tells us that freezing of a profile freezes each subtype of the profile. That means, for example, that freezing the profile of Union freezes Set.
But freezing of Set is illegal by 13.14(17/3) [the type is not yet complete]. [This is why there is an exception for incomplete types in 13.14(3/4), otherwise any incomplete type with the completion deferred to a body would be illegal.] Thus all of the declarations in package Inner are illegal.
The changes to the usage of incomplete types in Ada 2012 (AI05-0151-1) would suggest that these subprograms should be legal. They could be called from a child unit without that child having to see the full type for Set, so they seem potentially useful. Should such subprograms be allowed? (???.)
!recommendation
(See Summary.)
!wording
** TBD.
!discussion
** TBD.
!corrigendum 13.14(3/4)
Replace the paragraph:
by:
!ASIS
No ASIS effect.
!ACATS test
** TBD.
!appendix

From: Randy Brukardt
Sent: Friday, February 28, 2015  5:52 PM

I have had a running conversation with an implementer over a series of tests,
which has led to a question that suggests that there might be a problem
(really an oversight) in the freezing rules. (Imagine that!)

Here's the important part of the example:

package Pack is

   type Bag_Type is private;

   procedure Add (Bag : in out Bag_Type; Elem : Integer);

private

   type Set is tagged;

   type Bag_Type is access Set;

   package Inner is
      -- These can't be primitive for Set, else 3.10.1(9.3/2) is violated.
      procedure Add (Elem : Integer; To_Set : in out Set);
      procedure Union (Left, Right : in Set; Result : out Set);
      procedure Intersection (Left, Right : Set; Result : out Set);
      function "=" (Left, Right : Set) return Boolean;
   end Inner;

end Pack; -- Freezing here.

The implementer's complaints led me to look at how this freezes.

13.14(3/4) [and earlier versions as well] says that everything exception
incomplete types are frozen at the end of Pack. This includes the profiles of
any subprograms declared within Pack.

13.14(14/3) tells us that freezing of a profile freezes each subtype of the
profile. That means, for example, that freezing the profile of Union freezes
Set.

But freezing of Set is illegal by 13.14(17/3) [the type is not yet complete].
[This is why there is an exception for incomplete types in 13.14(3/4),
otherwise any Taft-Amendment type would be illegal.] Thus all of the
declarations in package Inner are illegal.


This only matters to Taft-Amendment types like Set in the example. For a type
that is completed in the package specification, 13.14(17/3) does not apply at
the end of the package, because the type is in fact complete. For an
incomplete view, one assumes that types that are imported were previously
frozen by the end of the compilation unit that contains them and thus don't
need any freezing. [BTW, I can't find any justification for this view in the
RM wording; it's something that I wish we'd fix because it comes up
periodically, but it also seems to fit the Dewar rule as no other
interpretation makes sense (at least to me). And since it fits the Duff rule
as well (no one would ever change behavior because of the fix), I've never
made an AI for it. Interestingly, I made this very same point in the e-mail
in AI12-0151-1 (June 6, 2009), so maybe it's mainly this case where it
matters. That e-mail specifically argued that there was no freezing problem
for incomplete views from limited withs for this sort of case, in response to
a question from Tucker. It must have satisified everyone, because no further
discussion occurred on that point.]


Note that there is no problem with these profiles freezing without knowing the
full type. We allow calls on similarly defined subprograms when the parameter
type is a tagged incomplete view defined by a limited with. The parameters
have to be passed by reference, and dispatching calls are prevented by
3.10.1(9.3/2) [primitives aren't allowed].

Such routines can be used in child packages without having to make the actual
implementation visible. That seems useful to me. (Maybe not useful enough to
bother changing the language, but certainly useful enough to disqualify this
case from the ARG gallery of pathologies, honoring Steve Baird. :-)


The freezing requirement here says that the expansions in usage of incomplete
types defined in Ada 2012 are completely unusable for Taft-Amendment types.
Any use of a Taft-Amendment incomplete type directly as a parameter or result
will necessarily result in a freezing violation. It's hard to believe we
intended that.

OTOH, AI12-0151-1 talks exclusively about incomplete views coming from limited
views. As such, it's possible that we didn't care at all about any changes that
happened for any other kind of access type.


Either way, I'd like to have a decision. If the freezing rule stands, then a
couple existing tests and a number of objectives are unnecessary, and I need to
clean up the ACATS and its documentation to reflect that. And if it is going to
be relaxed somehow, I'm sure the implementer would be interested, and the test
objectives probably will need a couple of additional cases.

For what it's worth, GNAT appears to accept the example package (and its body)
without complaint. I didn't try to create a runnable example, so I don't know
if the result would actually work or if I would get to see one of GNAT's famous
bug boxes. I'll do that IFF there is some interest in exploring whether the
freezing rules should allow examples like this. (Of course, if we confirm the
language, meaning the above is illegal, I'll make a B-Test out of the example
to ensure that it is getting rejected.)

****************************************************************


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