!standard 6.5(8/3)                                     14-07-15  AI05-0097-1/02
!class binding interpretation 14-02-12
!status Corrigendum 2015 14-07-15
!status ARG Approved 7-0-0  14-06-28
!status work item 14-02-12
!status received 14-01-29
!priority Low
!difficulty Easy
!qualifier Clarification
!subject Tag of the return object of a simple return expression
!summary

The tag of a return object is that of the return type if that is specific.
Otherwise, it is that of the return expression of a simple return statement,
or the initializing expression of an extended return statement, unless
that return statement has a subtype_indication of a specific type, in which
case the tag of that type is used.

!question

The second sentence of 6.5(8/3) was modified by AI05-0032-1 to say:

"If the result type is class-wide, the tag of the return object is that of the
type of the subtype_indication if it is specific, or otherwise that of the
value of the expression."

Consider the case of a simple return statement (this rule applies to any return
statement from a function). What "subtype_indication" are we talking about?
There could be one somewhere in the expression (say in an allocator or slice),
or even more than one, or there may not be one at all. This wording seems to
imply that there is only one.

Probably this wording was crafted assuming that it only applies to an
extended_return_statement, but that's not the case. Should the wording be
clarified? (Yes.)

!recommendation

(See Summary.)

!wording

Modify 6.5(8/3):

If the result type of a function is a specific tagged type, the tag of the
return object is that of the result type. If the result type is class-wide,
the tag of the return object is that of the {value of the expression, unless
the return object is defined by an extended_return_object_declaration with a
subtype_indication that is specific, in which case it is that of the type of
the subtype_indication}[type of the subtype_indication if it is specific, or
otherwise that of the value of the expression]. A check is
made that the master of the type identified by the tag of the result includes
the elaboration of the master that elaborated the function body. If this check
fails, Program_Error is raised.

Add an AARM Note:

AARM To Be Honest: The expression here is the return expression if the return
statement is a simple_return_statement, and the initializing expression
of the extended_return_object_declaration if the return statement is an
extended_return_statement (ignoring any inner simple_return_statements, which
necessarily cannot have an expression, and any other expressions inside of the
extended_return_statement).

!discussion

Clearly, we have to cover the case of a simple_return_statement, and we do not
want the effect of that to be different than it was in Ada 2005. So the wording
has to make it clear that the only subtype_indication being talked about is
the one directly part of an extended_return_statement.

Note that we use the relatively new syntax term extended_return_object_declaration
in this wording, so there is no confusion about which subtype_indication we're
talking about -- the current syntax of an extended_return_statement doesn't
directly include any subtype_indication.

!corrigendum 6.5(8/3)

@drepl
If the result type of a function is a specific
tagged type, the tag of the return object is that
of the result type. If the result type is class-wide, the tag of the
return object is that of the type of the @fa<subtype_indication>
if it is specific, or otherwise that of the value of the @fa<expression>. A
check is made that the master of the type identified by the tag of the result
includes the elaboration of the master that elaborated the function body. If
this check fails, Program_Error is raised.
@dby
If the result type of a function is a specific tagged type, the tag of the
return object is that of the result type. If the result type is class-wide,
the tag of the return object is that of the value of the @fa<expression>,
unless the return object is defined by an @fa<extended_return_object_declaration>
with a @fa<subtype_indication> that is specific, in which case it is that of
the type of the @fa<subtype_indication>. A check is made that the master of
the type identified by the tag of the result includes the elaboration of the
master that elaborated the function body. If this check fails, Program_Error
is raised.

!ASIS

No ASIS effect.

!ACATS

Existing ACATS test C390004 contains a subtest that the tag is correct for
a simple return statement returning a class-wide type. We could try cases
with slices or allocators to provide a subtype_indication to improperly
use, but someone actually implementing the incorrect rule verbatum seems
unlikely (as it clearly violates the Dewar rule).

!appendix

From: Randy Brukardt
Sent: Wednesday, January 29, 2014  9:57 PM

The second sentence of 6.5(8/3) was modified by AI05-0032-1 to say:

"If the result type is class-wide, the tag of the return object is that of the
type of the subtype_indication if it is specific, or otherwise that of the
value of the expression."

Consider the case of a simple return statement (this rule applies to any return
statement from a function). What "subtype_indication" are we talking about?
There could be one somewhere in the expression (say in an allocator or slice),
or even more than one, or there may not be one at all. This wording seems to
imply that there is only one.

There is also the case of a parameterless return inside of an
extended_return_statement; in that case, we are interested in the subtype
of the extended return and not the non-existent one of the inner return.

I think we meant the subtype_indication of an
extended_return_object_declaration, which always exists. AI05-0032-1 never
appears to even consider that 6.5(8/3) applies to any kind of return statement.

It's also not 100% clear which expression is being talked about. In the case of
the extended return, it's the initializing expression, and in the case of a
simple return, it's the return expression. These aren't the same thing!

I think we need to clarify this wording somehow. The obvious fix isn't very
appealing:

"If the result type is class-wide, the tag of the return object is that of
the type of the subtype_indication{ of the extended_return_object_declaration}
if {one is present and }it is specific, or otherwise that of the value of the
expression."

Perhaps we should stick in an "unless" instead:

"If the result type is class-wide, the tag of the return object is that [of
the type of the subtype_indication if it is specific,] of the value of the
expression{, unless the return is an extended_return_statement and the type
of the subtype_indication is specific, in which case is that of the type of
that subtype_indication."

Ugh, that doesn't work very well, either. And I'm not sure either of these
are quite right in the case of an inner (parameterless) return that ends an
outer extended_return. Nor does either clarify the expression in question.
Oh joy. :-)

Probably just have to give up on the combination and separate the parts
completely:

If the result type is class-wide, for a simple_return_statement with an
expression, the tag of the return object is that of the expression; otherwise
the tag of the return object is that of the type of the subtype_indication
of the applicable extended_return_object_declaration if it is specific, or
otherwise is that of the value of the expression of the applicable
extended_return_object_declaration.

AARM Ramification: The return statement could be a simple_return_statement
without an expression that occurs inside of an extended_return_statement.
In this case, the tag of the result comes from the extended_return_statement;
there can only be one such applicable statement in a legal function.

Perhaps there is a way to simplify this mess.

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

From: Tucker Taft
Sent: Thursday, January 30, 2014  3:23 PM

> The second sentence of 6.5(8/3) was modified by AI05-0032-1 to say:
>
> "If the result type is class-wide, the tag of the return object is 
> that of the type of the subtype_indication if it is specific, or 
> otherwise that of the value of the expression."
>
> Consider the case of a simple return statement (this rule applies to 
> any return statement from a function). ...
> Perhaps there is a way to simplify this mess.

Either of your suggestions seem just about right.  Here is another attempt,
which is almost identical to one of yours:

    If the result type of a function is a specific tagged type,
    the tag of the return object is that of the result type.
    If the result type is class-wide, the tag of the return
    object is that of the value of the expression, unless the
    return object is defined by an extended_return_statement
    with a subtype_indication that is specific, in which case
    it is that of the type of the subtype_indication.  ...

I don't see a problem with the expression-less return statement inside an
extended return, because we are talking about the "return object" and those
nested return statements have nothing to say about the return object.

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