AI22-0016-1
!standard 4.5.9(6/5) 22-02-03 AI22-0016-1/02
!class binding interpretation 21-11-12
!status Amendment 1-2022 22-02-03
!status WG9 Approved 22-10-18
!status ARG Approved 14-0-0 22-02-03
!status work item 21-11-12
!status received 21-09-16
!submitter Stephen Baird
!priority Low
!difficulty Easy
!qualifier Omission
!subject Newly constructed objects in a declare expression
Disallow renaming in a declare expression any part of a newly constructed object or value conversion of a limited type, to avoid having to worry about tasks and build-in-place.
In 4.5.9(6/5) we have:
A declare_item that is an object_renaming_declaration (see 8.5.1) shall not rename an object of a limited type if any operative constituent of the object_name is a value conversion or is newly constructed (see 4.4).
So I think this allows (inside a declare expression) something like
X : Limited_Type renames Function_Returning_Array_Of_Tasks (1 .. 10);
This is a slice of a newly constructed object, which seems just as bad as a newly constructed object, since it still might contain a "new" task. Do we want to allow this? (No)
We should also disallow cases where we are renaming any part of a newly constructed object that might contain a new task.
Modify 4.5.9(6/5):
A declare_item that is an object_renaming_declaration (see 8.5.1) shall not rename an object of a limited type if any operative constituent of the object_name is {part of} a value conversion or is {part of a} newly constructed {object} (see 4.4).
We could not accomplish what we want by changing the definition of "newly constructed", because it is already exactly what we want when we are saying that something must be (as opposed to must not be) newly constructed. So instead we have changed this use of the term newly constructed object, to include parts thereof. We have also disallowed renaming parts of a value conversion of a limited type, since the operand of a value conversion might be newly constructed.
@drepl
A @fa{declare_item} that is an @fa{object_renaming_declaration} (see @ref{8.5.1}) shall
not rename an object of a limited type if any operative constituent
of the @i{object_}@fa{name} is a value conversion or is newly constructed
(see @ref{4.4}).
@dby
A @fa{declare_item} that is an @fa{object_renaming_declaration} (see @ref{8.5.1}) shall
not rename an object of a limited type if any operative constituent
of the @i{object_}@fa{name} is part of a value conversion or is part of a newly
constructed object (see @ref{4.4}).
An ACATS B-Test should check the modified rule with cases like that in the example.
From: Steve Baird [privately]
Sent: Thursday, September 16, 2021 7:12 PM
We've got
A declare_item that is an object_renaming_declaration (see 8.5.1) shall
not rename an object of a limited type if any operative constituent of
the object_name is a value conversion or is newly constructed (see 4.4).
Note that an indexed component or a selected component or a slice
is never newly constructed. This is fine when we are requiring that something must
be newly constructed, but not so good (I think) in the opposite case.
So I think this allows (inside a declare expression) something like
X : Limited_Type renames Function_Returning_Array_Of_Tasks (1 .. 10);
which I don't think we wanted to allow.
Is there a problem here, or am I confused?
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From: Tucker Taft [privately]
Sent: Thursday, September 16, 2021 7:27 PM
Good point. Probably need to add to the definition of "newly constructed"
something like "or part thereof". E.g.:
9.7/5 {AI12-0317-1} In certain contexts, we specify that an operative
constituent shall (or shall not) be newly constructed. This means the
operative constituent shall (or shall not) be an aggregate or a
function_call{, or part thereof}; in either case, a raise_expression is
permitted.
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From: Steve Baird [privately]
Sent: Thursday, September 16, 2021 8:01 PM
We don't want to allow
function F return Limited_Type is
begin
return Limited_Type'(<some aggregate>)(1 .. 10);
end;
I believe the wording change you proposed would allow this case.
I don't think we can accomplish what we want by changing the definition of
"newly constructed", because it is already exactly what we want when we
are saying that something must be (as opposed to must not be) newly
constructed.
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From: Tucker Taft [privately]
Sent: Thursday, September 16, 2021 8:41 PM
Make sense. So the fix needs to be a restriction in declare expressions,
where we are trying to avoid having anything that needs finalization. Hence:
6/5{AI12-0236-1} {AI12-0317-1} A declare_item that is an
object_renaming_declaration (see 8.5.1) shall not rename an object of a
limited type if any operative constituent of the object_name is {part of}
a value conversion or is {part of a} newly constructed {object} (see 4.4).
I am not sure about the value conversion case, since I don't remember why
it is there in the first place. But adding "part of" in seems prudent
there as well.
****************************************************************
From: Steve Baird [privately]
Sent: Thursday, September 16, 2021 9:39 PM
Sounds good. I think the value conversion part of the wording is there to
avoid another case where a new object that requires finalization can be
created.
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