This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 110b. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2023-01-15


2538. Can standard attributes be syntactically ignored?

Section: 9.12.1  [dcl.attr.grammar]     Status: DR     Submitter: Jens Maurer     Date: 2021-12-02     Liaison: EWG

P2720R0 comment GB 055

[Accepted as a DR at the November, 2022 meeting.]

Subclause 9.12.1 [dcl.attr.grammar] paragraph 6 specifies that an unrecognized attribute-token is ignored:

For an attribute-token (including an attribute-scoped-token) not specified in this document, the behavior is implementation-defined. Any attribute-token that is not recognized by the implementation is ignored.

The intent is that only non-standard unrecognized attribute-tokens can be ignored; in particular, an implementation is required to syntax-check all standard attributes, even if the implementation then chooses not to effect any semantics for that attribute.

The paper introducing attributes was N2761; the phrasing in question was introduced by P0283R2 attempting to implement the design change presented in P0283R1.

See also paper P2552 (On the ignorability of standard attributes).

Suggested resolution:

Change in 9.12.1 [dcl.attr.grammar] paragraph 6 as follows:

For an attribute-token (including an attribute-scoped-token) not specified in this document, the behavior is implementation-defined. Any ; any such attribute-token that is not recognized by the implementation is ignored.

EWG telecon 2022-05-26

See paper issue 1252.

There was consensus for the statement "It is EWG's intent that [dcl.attr]/6 ONLY permits an implementation to ignore a standard attribute's effect, but not appertainment and argument parsing." To be confirmed by electronic polling.

Proposed resolution (approved by CWG 2022-07-01) [SUPERSEDED]:

Change in 9.12.1 [dcl.attr.grammar] paragraph 6 as follows:

For an attribute-token (including an attribute-scoped-token) not specified in this document, the behavior is implementation-defined. Any ; any such attribute-token that is not recognized by the implementation is ignored. [ Note: A program is ill-formed if it contains an attribute specified in 9.12 [dcl.attr] that violates the rules to which entity or statement the attribute may apply or the syntax rules for the attribute's attribute-argument-clause, if any. -- end note ]

EWG 2022-06 electronic polling

No consensus. See vote.

EWG 2022-11-08

Approved the direction of the 2022-07-01 proposed resolution.

Proposed resolution (approved by CWG 2022-11-08):

Change in 9.12.1 [dcl.attr.grammar] paragraph 6 as follows:

For an attribute-token (including an attribute-scoped-token) not specified in this document, the behavior is implementation-defined. Any ; any such attribute-token that is not recognized by the implementation is ignored. [ Note: A program is ill-formed if it contains an attribute specified in 9.12 [dcl.attr] that violates the rules specifying to which entity or statement the attribute may apply or the syntax rules for the attribute's attribute-argument-clause, if any. -- end note ]