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!standard 13.14(3/4)          15-03-05 AI05-0155-1/01
!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
When a profile is frozen at the end of a declarative part or package specification, any incomplete types that the profile contains are not frozen.
!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? (Yes.)
!recommendation
(See Summary.)
!wording
Modify 13.14(3/3) as modified by AI12-0103-1:
The end of a declarative_part, protected_body, or a declaration of a library package or generic library package, causes freezing of each entity and profile declared within it, except for incomplete types. A proper_body, body_stub, or entry_body causes freezing of each entity and profile declared before it within the same declarative_part that is not an incomplete type; it only causes freezing of an incomplete type if the body is within the immediate scope of the incomplete type. {Freezing of a profile in these cases only freezes an incomplete subtype of the profile if the corresponding incomplete type would be frozen.}
[RLB Note: I'm not sure we need the exception here; perhaps the simpler "Freezing of a profile in these cases only freezes subtypes of the profile that are not incomplete." is sufficient. I included the exception for the cases in the latter rule where the incomplete type is indeed frozen; however, I think that in such cases the incomplete type is frozen itself and thus it doesn't matter whether freezing the profile freezes it as well. Steve perhaps can create a counter-example where the type isn't frozen but a profile containing it is. If not, then it's impossible. ;-)]
!discussion
13.14(3/4) has an explicit “hole” so that incomplete types whose completion is given in the body (so-called Taft-Amendment types) do not freeze. Otherwise, such incomplete types would be illegal as 13.14(17/3) would be violated when they were frozen at the end of the scope.
Ada 2012 added explicit rules allowing operations like the ones in the question. It seems bizarre that these operations are always illegal because of freezing; it reduces the value of the new rules.
Operations like the ones in the question can be useful to provide operations to child units of a subsystem on a Taft-Amendment type without having to make the full declaration visible.
Such operations are limited to non-primitive operations as any primitive operations of a Taft-Amendment type are illegal by 3.10.1(9.3/2). However, as noted in the question, it's easy to declare non-primitive operations by adding a nested package; this trick is widely known among expert Ada programmers, so it's likely to be used in this case.
----
The reason that we need to freeze incomplete types is discussed in AI05-0017-1. In particular, it's necessary to prevent premature dispatching calls for ordinary (not Taft-Amendment) incomplete types. (Taft-Amendment incomplete types can't have primitive operations, so no dispatching calls can be made on an operations of them.)
----
The author created a complete ACATS-style test for an example like the one in the question. The test uses the operations in a child package where the completion is unknown. (This is usefully possible because the type is tagged incomplete, thus calls can be made so long as the object is passed intact.)
Compiling this program on GNAT shows that GNAT does not detect the freezing violations. However, the child package containing the calls generates a lovely internal error message. Because of this, it is not possible to run the program.
As such, it seems unlikely that anyone is depending on these operations working; neither the RM nor the only known fielded Ada 2012 implementation allows them. That's not particularly surprising; the capability was added by Ada 2012 and Taft-Amendment types aren't that common, so most likely no one has yet run into the intersection of both.
----
** Temporary (?) note **
Interestingly, this fix would not allow the original ACATS test that brought up this question. In that test, a generic with a formal tagged incomplete type is instantiated, passing the Taft-Amendment type and the operations declared in the nested package. This new rule allows the operations to be declared without causing freezing problems. However, an instance freezes the profiles of formal subprograms except for those with untagged formal incomplete parameters. (Subprograms with tagged incomplete parameters freeze as they can be called within the subprogram, and calling an unfrozen profile is bad news.) We did not add any exception for the freezing of such profiles, thus 13.14(17/3) would be violated and the instance is illegal.
Interestingly, this seems to show a problem with calling of subprograms with parameters of tagged incomplete types. Such calls are explicitly allowed by 3.10.1(10/3) [note that it only ban actual parameters of untagged incomplete views]. Since all calls freeze the profiles of the associated subprograms, any call whose parameters are tagged incomplete types without a completion will be illegal because of 3.10.1(9.3/2). That's intentional for cases like the one in AI12-0017-1. And it doesn't seem to be a problem assuming that any entity imported via a with is already frozen (somewhere) - some other unit must have the completion or the partition is illegal. (The language rules don't make this clear, probably they ought to.) With that assumption, any problematic call would either violate some other freezing rule (expression functions) or fail an elaboration check (other subprograms). Thus, we don't need to fix the problem, although it means that a generic cannot be instantiated in the same scope as these operations. (Which is annoying, but not fatal, as the instance could have been in the child unit, where no freezing would apply [we assume!].)
Note that this instance caused an internal error in GNAT very similar to the one caused by the calls in the ACATS-style test for the example in the question.
** End Temporary (?) note **
!corrigendum 13.14(3/4)
Replace the paragraph:
by:
!ASIS
No ASIS effect.
!ACATS test
The problem was originally discovered in ACATS test CC51011, so some version of that test will test the fix. There also is the proposed test named C3A1005, a version of CC51011 without the generic units. If this AI is approved, that test can be issued (it tests some moderate priority 3.10.1 objectives that are currently on the testing list -- this AI would raise that priority somewhat).
!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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