https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103869
Bug ID: 103869 Summary: better diagnostics surrounding uses of -fms-extensions Product: gcc Version: 12.0 Status: UNCONFIRMED Keywords: diagnostic, documentation Severity: normal Priority: P3 Component: c Assignee: unassigned at gcc dot gnu.org Reporter: egallager at gcc dot gnu.org Target Milestone: --- I am putting this under the "C" component because the option is documented under the list of C Dialect Options: https://gcc.gnu.org/onlinedocs/gcc/C-Dialect-Options.html#C-Dialect-Options However, it could also be considered a C++ bug as per bug 71283. Let's start with the example code provided for the option: $ cat ms-extensions.c typedef int UOW; struct ABC { UOW UOW; }; $ The manual says, "In C++ code, this allows member names in structures to be similar to previous types declarations." However, when compiling with the C front-end, -Wc++-compat doesn't say anything about -fms-extensions allowing it, nor does explicitly providing -fms-extensions silence it: $ /usr/local/bin/gcc -c -Wall -Wextra -pedantic -Wshadow -Wc++-compat ms-extensions.c ms-extensions.c:3:7: warning: using 'UOW' as both field and typedef name is invalid in C++ [-Wc++-compat] 3 | UOW UOW; | ^~~ $ /usr/local/bin/gcc -c -Wall -Wextra -pedantic -Wshadow -Wc++-compat -fms-extensions ms-extensions.c ms-extensions.c:3:7: warning: using 'UOW' as both field and typedef name is invalid in C++ [-Wc++-compat] 3 | UOW UOW; | ^~~ $ Meanwhile, with the C++ front-end, the error message only notes -fpermissive as a way to allow it, but not -fms-extensions: $ /usr/local/bin/g++ -c -Wall -Wextra -pedantic -Wshadow ms-extensions.c ms-extensions.c:3:7: error: declaration of 'UOW ABC::UOW' changes meaning of 'UOW' [-fpermissive] 3 | UOW UOW; | ^~~ ms-extensions.c:1:13: note: 'UOW' declared here as 'typedef int UOW' 1 | typedef int UOW; | ^~~ $ -fpermissive turns the error into a warning, but providing -fms-extensions removes even the warning: $ /usr/local/bin/g++ -c -Wall -Wextra -pedantic -Wshadow -fpermissive ms-extensions.c ms-extensions.c:3:7: warning: declaration of 'UOW ABC::UOW' changes meaning of 'UOW' [-fpermissive] 3 | UOW UOW; | ^~~ ms-extensions.c:1:13: note: 'UOW' declared here as 'typedef int UOW' 1 | typedef int UOW; | ^~~ $ /usr/local/bin/g++ -c -Wall -Wextra -pedantic -Wshadow -fpermissive -fms-extensions ms-extensions.c $ /usr/local/bin/g++ -c -Wall -Wextra -pedantic -Wshadow -fms-extensions ms-extensions.c $ IMO there should be a separate pedwarn even with -fms-extensions controlled by a separate -Wms-extensions to complain in the -pedantic -fms-extensions case, which could then be disabled with -pedantic -fms-extensions -Wno-ms-extensions (or just -Wno-pedantic), e.g.: $ /usr/local/bin/g++ -c -Wall -Wextra -pedantic -Wshadow -fms-extensions ms-extensions.c ms-extensions.c:3:7: warning: declaration of 'UOW ABC::UOW' is only being allowed due to -fms-extensions [-Wms-extensions] 3 | UOW UOW; | ^~~ ms-extensions.c:1:13: note: 'UOW' declared here as 'typedef int UOW' 1 | typedef int UOW; | ^~~ $ Alternatively, the additional complaint could come from -Wshadow instead. Anyways, there are lots of possibilities here; let's try to come up with a consensus as to what would work best.