!standard 7.3.2(17/4)                                 16-12-19  AI12-0210-1/00
!standard 7.3.2(20.1/4)
!class binding interpretation 16-12-19
!status work item 16-12-19
!status received 16-09-29
!priority Low
!difficulty Medium
!qualifier Omission
!subject Type Invariants and Generics
!summary

** TBD **

!question

7.3.2(20.1/4) says
    For a view conversion to a class-wide type occurring within the
    immediate scope of T, from a specific type that is a descendant of T
    (including T itself), a check is performed on the part of the object
    that is of type T.

What does "occurring within the immediate scope" mean in the case where the 
view conversion occurs in an instance of a generic unit? (Who knows?)

One can also have the opposite situation where it is the generic unit body
(or private part, in the case of a generic child unit) which can see the
completion of the invariant-bearing type and the instance which occurs
outside of that scope. 7.3.2(17/4) is about that case, but that rule doesn't
address the other cases in which invariant checking rules depend on being or
not being "in the immediate scope": view conversions (including conversion
to classwide) and deferred constants. Should it? (Yes.)

!recommendation

(See Summary.)

!wording

** TBD **

!discussion

Does anyone care? Pick something. :-)

!ASIS

No ASIS effect.

!ACATS test


!appendix

From: Steve Baird
Sent: Thursday, September 29, 2016  6:31 PM

Ada RM 7.3.2(20.1/4) says
    For a view conversion to a class-wide type occurring within the
    immediate scope of T, from a specific type that is a descendant of T
    (including T itself), a check is performed on the part of the object
    that is of type T.

What does "occurring within the immediate scope" mean in the case where the 
view conversion occurs in an instance of a generic unit?

Consider the following example:

  procedure Invariant_Test is
    pragma Assertion_Policy (Check);

    type T1 is tagged record
       Aaa : Natural := 0;
       Bbb : Natural := 100;
    end record;

    procedure Swap_T1 (X1 : in out T1) is
    begin
       X1 := (Aaa => X1.Bbb, Bbb => X1. Aaa);
    end;

    generic
       type D is new T1 with private;
    procedure Swap_View_Conversion (X : in out D);
    procedure Swap_View_Conversion (X : in out D) is
    begin
       Swap_T1 (T1 (X)); -- a view conversion
    end;

    package Pkg is
       type T2 is private;
    private
       type T2 is new T1 with null record
         with Type_Invariant => T2.Aaa < T2.Bbb;

       package Nested is
          procedure Swap_T2 is new Swap_View_Conversion (T2);
       end;
     end;

     package body Pkg is
        Y : T2;
     begin
        Nested.Swap_T2 (Y);
        pragma Assert (Y.Aaa = 100);
     end;
   begin null; end;

Does the call to Nested.Swap_T2 fail an invariant check?

Tuck said (in private communication):
> Should an external generic, instantiated locally, be able to do 
> something that a normal external package could *not* do. I believe the 
> answer is yes, presuming the type with the invariant is only visible 
> to the generic as a formal type, as opposed to being named directly. 
> When you instantiate a generic with an actual type that has an 
> invariant, and you do it within the full view of the type, then the 
> generic should not be affected by the invariant.

A general design principle is that invariants don't come into play very much
as long as you stay inside the body of the package that implements the
private type and don't call subprograms which are callable (directly or via
overriding) from outside of that package. There are exceptions to that (e.g.,
default initialization of an object declared in the package body), but that's
the general idea. Tuck's suggestion that no exception should be raised in this
case is consistent with that principle.

Note that 7.3.2(20.1/4) is somewhat unusual in talking about being "within"
some scope in the context of a dynamic semantics rule. Any time the RM does
this, there are going to be questions about generics vs. instances.

One can also have the opposite situation where it is the generic unit body (or
private part, in the case of a generic child unit) which can see the
completion of the invariant-bearing type and the instance which occurs
outside of that scope. 7.3.2(17/4) is about that case, but that rule doesn't
address the other cases in which invariant checking rules depend on being or
not being "in the immediate scope": view conversions (including conversion to
classwide) and deferred constants. These cases just need to be well-defined;
all that is needed is a clarification one way or the other. The fact that
7.3.2(17/4) is a rule and not a ramification *suggests* that no checks should
be performed in these cases, but that seems far from conclusive. As usual, we
should decide first what behavior we want before worrying about interpreting
the current wording.

7.3.2(17/4) also doesn't really deal well with a case like
    package I1 is new G1;
    package I2 is new I1.G2;
When it says "and the generic is declared within the immediate scope of type
T", in what scope is the generic I1.G2 declared? Inside (the expansion of) I1
or inside G1? The point of this rule is to deal with generic bodies that can
see the completion of the private type; this suggests that we want to think of
I1.G2 as being declared inside of G1, as opposed to inside I1, but the current
wording seems unclear.

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

From: Randy Brukardt
Sent: Thursday, September 29, 2016  6:41 PM

...
> As usual, we should decide first what behavior we want before worrying 
> about interpreting the current wording.

When it comes to type invariants, pretty much the only thing we care about is
that the rule is well-defined and doesn't cause horrible implementation
problems. These are a distant fourth out of the four kinds of contracts; the
fact that GNAT had completely implemented them wrong until Jeff wrote some
ACATS tests tells me that no real users really care about the details of this
feature.

I suspect that these checks in instances will be hell on generic code sharing,
but I have a hard time caring because I doubt that I'll ever have a customer
who cares. (Ergo, probably these will never be implemented in Janus/Ada.)

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