Version 1.3 of ais/ai-00411.txt

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!standard 7.5(4)          05-02-26 AI95-00411/02
!standard 3.9.4(1)
!class amendment 05-02-07
!status Amendment 200Y 05-02-26
!status WG9 Approved 06-06-09
!status ARG Approved 10-0-0 05-02-12
!status work item 05-02-07
!status received 05-02-07
!priority High
!difficulty Easy
!subject Predefined equality for types derived from interfaces
!summary
No additional wording is required to ensure that the "=" operator works as expected for nonlimited types whose parent type is an interface.
!problem
There is some concern that we have forgotten about the "=" operator and how it relates to nonlimited interfaces. In particular, unless declared explicitly as abstract, the primitive equality operators for an interface will be non-abstract. In fact, they will be the only non-abstract primitive functions, since any others are necessarily user-declared, and will necessarily be declared as abstract (since only procedures can be declared "null").
!proposal
No additional wording is needed, though this is not obvious, and we will need to double check the sections indicated in the discussion after the final revisions to be sure everything still works.
It would be useful to mention in a note when talking about interfaces that they have non-abstract equality operators, if nonlimited.
There also is a wording error in 7.5 - limited interfaces are not included in the list of limited things. (While a declared limited interface includes the word "limited" and thus is covered by the existing wording, task and protected interfaces are not.)
!wording
Add a note in the new 3.9.4 proposed by AI-251 and AI-345:
NOTE 1: Nonlimited interface types have predefined nonabstract equality
operators. These may be overridden with user-defined abstract equality operators. Such operators will then require an explicit overriding for any nonabstract descendant of the interface.
Add after 7.5(4):
* a limited interface;
!discussion
Here are some relevant paragraphs, which combine to make "=" work properly for types derived from interfaces. I believe we need
3.4(16):
For each predefined operator of the parent type, there is a corresponding predefined operator of the derived type.
3.4(17):
Primitive user-defined equality operators of the parent type are also inherited by the derived type, except when the derived type is a nonlimited record extension, and the inherited operator would have a profile that is type conformant with the profile of the corresponding predefined equality operator; in this case, the user-defined equality operator is not inherited, but is rather incorporated into the implementation of the predefined equality operator of the record extension (see 4.5.2).
3.9.4(7/2):
All user-defined primitive subprograms of an interface type shall be abstract subprograms or null procedures.
4.5.2(6-7):
The equality operators are predefined for every specific type T that is not limited, and not an anonymous access type, with the following specifications:
function "=" (Left, Right : T) return Boolean function "/="(Left, Right : T) return Boolean
4.5.2(14):
For a type extension, predefined equality is defined in terms of the primitive [(possibly user-defined)] equals operator of the parent type and of any tagged components of the extension part, and predefined equality for any other components not inherited from the parent type.
4.5.2(22):
If there are no components, the result is defined to be True;
4.5.2(25):
The predefined "/=" operator gives the complementary result to the predefined "=" operator.
7.5(3-8):
A type is limited if it is a descendant of one of the following:
* a type with the reserved word limited in its definition;
* a task or protected type;
* a composite type with a limited component.
Otherwise, the type is nonlimited.
[There are no predefined equality operators for a limited type.]
!comment !corrigendum 3.9.4(1) We've done this in the Conflict text without
!comment a formal reference (so we don't have to allow 4-way conflicts).
!corrigendum 7.5(4)
Insert after the paragraph:
the new paragraph:
!ACATS test
!appendix

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