On Wed, May 19, 2021 at 12:33 PM Jonathan Wakely via Gcc-patches <gcc-patches@gcc.gnu.org> wrote: > > This adds new warning flags, enabled by default: -Wc++11-extensions, > -Wc++14-extensions, -Wc++17-extensions, and -Wc++20-extensions. The > names of the flags are copied from Clang, which already has similar > options. > > No new diagnostics are added, but the new OPT_Wxxx variables are used to > control existing pedwarns about occurences of new C++ constructs in code > using an old C++ standard dialect. This allows several existing warnings > that cannot currently be disabled to be controlled by the appropriate > -Wno-xxx flag. For example, it will now be possible to disable warnings > about using variadic templates in C++98 code, by using the new > -Wno-c++11-extensions option. This will allow libstdc++ headers to > disable those warnings unconditionally by using diagnostic pragmas, so > that they are not emitted even if -Wsystem-headers is used. > > Some of the affected diagnostics are currently only given when > -Wpedantic is used. Now that we have a more specific warning flag, we > could consider making them not depend on -Wpedantic, and only on the new > flag. This patch does not do that, as it intends to make no changes to > what is accepted/rejected by default. The only effect should be that > the new option is shown when -fdiagnostics-show-option is active, and > that some warnings can be disabled by using the new flags (and for the > warnings that previously only dependend on -Wpedantic, it will now be > possible to disable just those warnings while still using -Wpedantic for > its other benefits). > > A new helper function, warn_about_dialect_p, is introduced to avoid the > repetition of `if (cxx_dialect < cxxNN && warn_cxxNN_extensions)` > everywhere. > > gcc/c-family/ChangeLog: > > * c.opt (Wc++11-extensions, Wc++14-extensions) > (Wc++17-extensions, Wc++20-extensions): New options. > > gcc/cp/ChangeLog: > > * call.c (maybe_warn_array_conv): Use new function and option. > * cp-tree.h (warn_about_dialect_p): Declare new function. > * error.c (maybe_warn_cpp0x): Use new function and options. > (warn_about_dialect_p): Define new function. > * parser.c (cp_parser_unqualified_id): Use new function and > option. > (cp_parser_pseudo_destructor_name): Likewise. > (cp_parser_lambda_introducer): Likewise. > (cp_parser_lambda_declarator_opt): Likewise. > (cp_parser_init_statement): Likewise. > (cp_parser_decomposition_declaration): Likewise. > (cp_parser_function_specifier_opt): Likewise. > (cp_parser_static_assert): Likewise. > (cp_parser_namespace_definition): Likewise. > (cp_parser_initializer_list): Likewise. > (cp_parser_member_declaration): Likewise. > * pt.c (check_template_variable): Likewise. > > Tested x86_64-linux. OK for trunk? > >
Thank you for doing this! One thing I'm wondering about, is that I'm pretty sure clang also allows at least some of these to be used with plain C as well, for example for things like the old use of "auto" in C conflicting with the newer C++11 meaning of "auto". Would it be possible to do likewise for GCC as well? Just an idea. Thanks, Eric Gallager
