!standard 12.5.1(5.1/2)                              11-03-11    AI05-0237-1/02
!class binding interpretation 11-01-30
!status Amendment 2012 11-03-11
!status ARG Approved  7-0-1  10-02-20
!status work item 11-01-30
!status received 10-07-14
!priority Low
!difficulty Easy
!qualifier Omission
!subject Ancestor types for synchronized formal types
!summary

The ancestor type for a synchronized formal derived type must be a limited
interface.

!question

There appears to be no rule forbidding a declaration like:

   generic
      type T is synchronized new Record_Type and Interface_Type with private;

But the actual type for synchronized type has to be a task type, protected type,
or a synchronized interface. A record type is not allowed. Should this be
fixed? (Yes.)

!recommendation

(See Summary.)

!wording

Modify 12.5.1(5/2):

...  The reserved word limited or synchronized shall appear only if the ancestor type
Redundant[and any progenitor types] are limited types. The reserved word synchronized shall
appear (rather than limited) if the ancestor type or any of the progenitor types are
synchronized interfaces. {The ancestor type shall be a limited interface if the reserved
word synchonized appears.}

!discussion

Even without any changes, any instantiation of such a generic would necessarily fail.
But that seems silly; the error should be diagnosed earlier rather than later.

Moreover, the similar case for private extensions is illegal by 7.3(8.1/2). Generally,
we want to rules for generic formal types to mirror the rules for private types.
So we add wording to make this illegal.

!corrigendum 12.5.1(5/2)

@drepl
The @i<ancestor subtype> of a formal derived type is the subtype denoted by the
@fa<subtype_mark> of the @fa<formal_derived_type_definition>. For a formal
derived type declaration, the reserved words @b<with private> shall appear if
and only if the ancestor type is a tagged type; in this case the formal derived
type is a private extension of the ancestor type and the ancestor shall not be
a class-wide type. Similarly, an @fa<interface_list> or the optional reserved
words @b<abstract> or @b<synchronized> shall appear only if the ancestor type
is a tagged type. The reserved word @b<limited> or @b<synchronized> shall
appear only if the ancestor type and any progenitor types are limited types.
The reserved word @b<synchronized> shall appear (rather than @b<limited>) if
the ancestor type or any of the progenitor types are synchronized interfaces.
@dby
The @i<ancestor subtype> of a formal derived type is the subtype denoted by the
@fa<subtype_mark> of the @fa<formal_derived_type_definition>. For a formal
derived type declaration, the reserved words @b<with private> shall appear if
and only if the ancestor type is a tagged type; in this case the formal derived
type is a private extension of the ancestor type and the ancestor shall not be
a class-wide type. Similarly, an @fa<interface_list> or the optional reserved
words @b<abstract> or @b<synchronized> shall appear only if the ancestor type
is a tagged type. The reserved word @b<limited> or @b<synchronized> shall
appear only if the ancestor type and any progenitor types are limited types.
The reserved word @b<synchronized> shall appear (rather than @b<limited>) if
the ancestor type or any of the progenitor types are synchronized interfaces.
The ancestor type shall be a limited interface if the reserved
word @b<synchonized> appears.

!ACATS test

Add an ACATS B-test to check this issue.


!appendix

!topic Synchronized formal types derived from records?
!reference RM05 12.5.1
!from Adam Beneschan 10-07-14
!discussion

I think this is an error of omission:  Nothing prevents a declaration like 

   generic
      type T is synchronized new Record_Type and Interface_Type with private;

even though this makes no sense---a synchronized type is a task or protected type
and can't be derived from a record.  The error is probably harmless since such a
generic can't be instantiated anyway, but it certainly seems like this ought
to be illegal.

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