This also ignores and deprecates the -fdouble-square-bracket-attributes command line flag, which seems to not be used anywhere. At least a code search exclusively found mentions of it in documentation: https://sourcegraph.com/search?q=context:global+-fdouble-square-bracket-attributes+-file:clang/*+-file:test/Sema/*+-file:test/Parser/*+-file:test/AST/*+-file:test/Preprocessor/*+-file:test/Misc/*+archived:yes&patternType=standard&sm=0&groupBy=repo
This enables [] attributes in all C and C++ language modes without warning by default. -Wc++-extensions does warn. GCC has enabled this extension in all C modes since GCC 10.