Re: GDB 8.1 build error

Pedro Alves
2018-05-02T09:42:20Z

On 04/27/2018 09:41 PM, Simon Marchi wrote:
> On 2018-04-27 16:24, Pedro Alves wrote:
>> -Wnarrowing is on by default on gcc, and -w disables it.
>
> But it's a warning by default, not an error (like clang).Â  I think that's the important distinction.

OK, That suggests to me that clang has -Werror=narrowing enabled by
default, instead of -Wnarrowing.

But I still think that the real issue is that gcc and clang
behave differently wrt "-w" precedence.

Note, with:

$ cat narrow.cc
int return_int () { return 42; }
char buf[2] = { return_int () };

#1 - even if you make gcc error out with -Werror=narrowing, a subsequent "-w"
cancels the error/warning, not so with clang:

$ g++ -std=gnu++17 narrow.cc -c -Werror=narrowing -w
$ clang++ -std=gnu++17 narrow.cc -c -Werror=narrowing -w
narrow.cc:2:17: error: non-constant-expression cannot be narrowed from type 'int' to 'char' in initializer list [-Wc++11-narrowing]

#2 - If you put "-Wno-error=narrowing", after the "-w", then both compilers
suppress the warning/error:

$ clang++ -std=gnu++17 narrow.cc -c -Werror=narrowing -w -Wno-error=narrowing
$ g++ -std=gnu++17 narrow.cc -c -Werror=narrowing -w -Wno-error=narrowing

#3 - For completeness, adding "-Werror=narrowing" after the -w, shows #1 again:

$ g++ -std=gnu++17 narrow.cc -c -w -Werror=narrowing
$ clang++ -std=gnu++17 narrow.cc -c -w -Werror=narrowing
narrow.cc:2:17: error: non-constant-expression cannot be narrowed from type 'int' to 'char' in initializer list [-Wc++11-narrowing]

$ g++ -std=gnu++17 narrow.cc -c -Werror=narrowing -w -Werror=narrowing
$ clang++ -std=gnu++17 narrow.cc -c -Werror=narrowing -w -Werror=narrowing
narrow.cc:2:17: error: non-constant-expression cannot be narrowed from type 'int' to 'char' in initializer list [-Wc++11-narrowing]

So it seems to be the issue here is more about "-w" precedence over all
warning switches than about what is enabled by default.

To confirm this, we can see the same thing with any other warning:

~~~~~~~~~~~~
$ gcc unused.c -c -Werror=unused-variable
unused.c: In function ‘function’:
unused.c:1:23: error: unused variable ‘i’ [-Werror=unused-variable]
int function () { int i; return 42; }
^
cc1: some warnings being treated as errors
~~~~~~~~~~~~

~~~~~~~~~~~~
$ clang unused.c -c -Werror=unused-variable
unused.c:1:23: error: unused variable 'i' [-Werror,-Wunused-variable]
int function () { int i; return 42; }
^
1 error generated.
~~~~~~~~~~~~

vs

$ gcc unused.c -c -Werror=unused-variable -w
$ clang unused.c -c -Werror=unused-variable -w
unused.c:1:23: error: unused variable 'i' [-Werror,-Wunused-variable]

So seems like we could handle this my making --disable-build-warnings
use "-Wno-error -w" instead of just "-w". But I'd suggest checking with
clang and/or gcc folks to confirm the difference is intentional too.

Thanks,
Pedro Alves