https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114007
--- Comment #16 from Joseph S. Myers <jsm28 at gcc dot gnu.org> --- I think it's clear that __has_c_attribute(gnu::unused) should only return 1 if the [[gnu::unused]] syntax is actually parsed. (An unavoidable limitation if it might return 1 in pre-C23 modes is that if it's used with -pedantic / -pedantic-errors, such a usage of [[gnu::unused]] would then be diagnosed - since another principle is that -pedantic / -pedantic-errors should not affect semantics, in particular not change the return value of __has_c_attribute.) I also think that __has_c_attribute(gnu::unused) should always parse successfully in #if, even if it returns 0 in some cases. It's probably reasonable to accept :: in [[]] attributes in pre-C23 standard modes where those are two consecutive : tokens (the use of the [[]] syntax at all would result in a pedwarn-if-pedantic). Ideally there might be a marker on the tokens as suggested to indicate whether two such tokens would have been :: if in C23 mode, to avoid accepting other variants where the two tokens are separated in the sources. (The only reason it's not valid to produce a single preprocessing token in pre-C23 mode is to deal with corner cases such as ##-concatenating < :: > where the concatenations are valid in pre-C23 mode, producing digraphs equivalent to [], but invalid in C23 mode. Once the preprocessing tokens are converted to tokens, pre-C23 doesn't have any valid cases of two consecutive : tokens.)
