GB-055 9.12.1p6 [dcl.attr.grammar] Ignoring standard attributes

[wg21bot]

It is unclear whether implementations can ignore standard attributes.
The standard states: "Any attribute-token that is not recognized by the implementation is ignored."
It is unclear whether this applies to attributes listed in the standard itself.

See CWG2538 and P2552R0.

[jfbastien]

Paper: cplusplus/papers#1212 (updated draft https://isocpp.org/files/papers/D2552R1.pdf)

[erichkeane]

From D2552R1, we support the adoption of the proposed wording for option 1.A on Syntactic ignorability: Specify that standard attributes cannot be syntactically ignored for C++23 (= adopt proposed resolution for CWG2538 as recommended by CWG).

SF	F	N	A	SA
14	14	6	1	6

Result: Consensus

From D2552R1, we support the adoption of the proposed wording for option 1.B on Syntactic ignorability: Specify that standard attributes can be syntactically ignored for C++23 (= parsed as a balanced-token-sequence and then skipped entirely; argument clause and appertainment do not need to be syntax checked; entities inside the argument clause do not need to be ODR-used).

SF	F	N	A	SA
9	6	9	11	6

Result: No Consensus

From D2552R1, we want to adopt wording for option 2.A on Semantic ignorability: Specify that standard attributes are meant to be semantically ignorable, and what this means exactly, normatively in the C++ standard, for C++23.

SF	F	N	A	SA
3	7	16	6	4

Result: No Consensus

From D2552R1, we want to adopt wording for option 2.B on Semantic ignorability: Specify this in a separate new Standing Document (SD) instead. This SD might also contain other design principles for new language features.

SF	F	N	A	SA
1	11	14	9	1

Result: No Consensus

From D2552R1, we would want to adopt wording for option 3.A on __has_cpp_attribute: Specify that __has_cpp_attribute should return a positive value for a standard attribute only if an an implementation has a useful implementation of its semantics (GCC behaviour).

SF	F	N	A	SA
8	15	11	7	2

Result: No Consensus

From D2552R1, we would want to adopt wording for option 3.B on __has_cpp_attribute: Specify that __has_cpp_attribute should also return a positive value for a standard attribute if an implementation can syntax-parse it, even if it does not implement any useful semantics (Clang and ICC behaviour).

SF	F	N	A	SA
9	7	12	11	2

Result: No Consensus

[jensmaurer]

CWG2538

[jensmaurer]

CWG 2022-11-08: Approved CWG2538.

Accepted with Modifications. Attributes specified by the standard cannot be syntactically ignored; see CWG2538 for details.