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