cc1plus：对任何其他警告的无法识别的命令行选项警告

Question

I have a strange behavior of g++ that it shows a warning about unrecognized command line option, when any other warning is shown.

Example:

struct Foo{virtual int bar() = 0;};
struct Bar:public Foo{int bar() {return 0;} };

int main(){}

Compiling with g++-5 -Wsuggest-override -Wno-c99-extensions -std=c++11 a.cpp or even g++-5 -Wsuggest-override -Wno-c99-extensions a.cpp shows:

a.cpp:2:27: warning: ‘virtual int Bar::bar()’ can be marked override [-Wsuggest-override]
 struct Bar:public Foo{int bar() {return 0;} };
                           ^
cc1plus: warning: unrecognized command line option ‘-Wno-c99-extensions’

ADDITION: There is no warning/error when I compile with g++-5 -Wno-c99-extensions a.cpp hence that option passes CMAKEs check using CHECK_CXX_COMPILER_FLAG

This bothers me quite a lot, as we use Werror but with exceptions via Wno-error=... It then bails out when any of the (non-error) warnings is shown with the "unrecognized command line option"

Is this known/expected? How can it be prevented?

Answer 1

If you don't want warnings about unrecognised command-line options, don't use unrecognised command-line options: -Wno-c99-extensions has never been a valid GCC option (I believe it's a clangism). Simply remove it from your build command.

As for why the warning is only emitted when you have another warning present, this behaviour seems counter-intuitive but it is actually deliberate and documented :

The warning "unrecognized command-line option" is not given for -Wno-foo

Since GCC 4.4, and as explained in the GCC manual: when an unrecognized warning option is requested ( -Wunknown-warning ), GCC emits a diagnostic stating that the option is not recognized. However, if the -Wno- form is used, the behavior is slightly different: no diagnostic is produced for -Wno-unknown-warning unless other diagnostics are being produced. This allows the use of new -Wno- options with old compilers, but if something goes wrong, the compiler warns that an unrecognized option is present. (See PR28322 for the history of this change)

This might break configure tests that check for -Wno-foo options. The solution is to either test for the positive form ( -Wfoo ) or test using a testcase that triggers some other warning.

There are in fact a few results on Google of mailing list threads where software developers have faced this "problem", specifically with CMake, and made trivial fixes to their build scripts to "fix" it.