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Implement some changes to the currently supported C2x standard attributes that have been made to the specification since they were first implemented in GCC, and some consequent changes: * maybe_unused is now supported on labels. In fact that was already accidentally supported in GCC as a result of sharing the implementation with __attribute__ ((unused)), but needed to be covered in the tests. * As part of the support for maybe_unused on labels, its __has_c_attribute value changed. * The issue of maybe_unused accidentally being already supported on labels showed up the lack of tests for other standard attributes being incorrectly applied to labels; add such tests. * Use of fallthrough or nodiscard attributes on labels already properly resulted in a pedwarn. For the deprecated attribute, however, there was only a warning, and the wording "'deprecated' attribute ignored for 'void'" included an unhelpful "for 'void'". Arrange for the case of the deprecated attribute on a label to be checked for separately and result in a pedwarn. As with inappropriate uses of fallthrough (see commit 6c80b1b56dec2691436f3e2676e3d1b105b01b89), it seems reasonable for this pedwarn to apply regardless of whether [[]] or __attribute__ was used and regardless of whether C or C++ is being compiled. * Attributes on case or default labels (the standard syntax supports attributes on all kinds of labels) were quietly ignored, whether or not appropriate for use in such a context, because they weren't passed to decl_attributes at all. (Note where I'm changing the do_case prototype that such a function is actually only defined in the C front end, not for C++, despite the declaration being in c-common.h.) * A recent change as part of the editorial review in preparation for the C2x CD ballot has changed the __has_c_attribute value for fallthrough to 201910 to reflect when that attribute was actually voted into the working draft. Bootstrapped with no regressions for x86_64-pc-linux-gnu. gcc/c-family/ * c-attribs.cc (handle_deprecated_attribute): Check and pedwarn for LABEL_DECL. * c-common.cc (c_add_case_label): Add argument ATTRS. Call decl_attributes. * c-common.h (do_case, c_add_case_label): Update declarations. * c-lex.cc (c_common_has_attribute): For C, produce a result of 201910 for fallthrough and 202106 for maybe_unused. gcc/c/ * c-parser.cc (c_parser_label): Pass attributes to do_case. * c-typeck.cc (do_case): Add argument ATTRS. Pass it to c_add_case_label. gcc/testsuite/ * gcc.dg/c2x-attr-deprecated-2.c, gcc.dg/c2x-attr-fallthrough-2.c, gcc.dg/c2x-attr-maybe_unused-1.c, gcc.dg/c2x-attr-nodiscard-2.c: Add tests of attributes on labels. * gcc.dg/c2x-has-c-attribute-2.c: Update expected results for maybe_unused and fallthrough.
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