Re: GDB 8.1 build error

[Pedro Alves <palves at redhat dot com>]

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).  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