P2156 Allow Duplicate Attributes

[wg21bot]

P2156R0 Allow Duplicate Attributes (Erich Keane)

[jfbastien]

Discussed in EWG telecon on July 2nd:

P2156R0 is tentatively ready to move to CWG, once we can forward to Core (modulo no_unique_addr wording)

SF	F	N	A	SA
5	5	1	0	0

[wg21bot]

P2156R1 Allow Duplicate Attributes (Erich Keane)

[AaronBallman]

This paper does not need to be seen by SG22 as it was already seen by WG14 as part of WG14 N2557, which was approved at the Oct 2020 meeting.

Straw Poll: Is the committee in favor of adopting N2557 into C23? 19-0-0

[jfbastien]

EWG February 2021 polls:

Poll: Forward P2156R1 "Allow Duplicate Attributes" to Core, thereby also fixing [CWG1914].

SF	F	N	A	SA
12	11	3	1	1

Poll outcome: ✅ consensus.

Salient comments:

• (Against / Strongly against) The use case (macro replacement) is questionable, especially for cases where the same attribute has different values. It also further complicates future extensions for reflection on attributes. We can resolved CWG1914 by declaring that the particular attributes mentioned there could not be specified more than once on a declaration, even in separate attribute specifiers.
• In practice, attributes are often hidden in macros and it is easy to get duplicates, and fixing the duplication is challenging in code that supports many compilers and standards.
• Especially useful for auto-generated code.
• It is easier on developers if we allow duplicate attributes.
• I see no reason for preventing [[X, X]] but allowing [[X]] [[X]] as these variants should be equivalent.
• Compilers could still warn about duplicated attributes.
• This unifies the behavior with what’s already been adopted into C.
• This makes the language more regular.