Re: [PATCH] gdb: Fix build error on macOS

[Bruno Haible <bruno@clisp.org>]

Enze Li wrote:
> > PR build/28413 notes that the gdb master fails to build on macOS.

The build fails due to the combination of '-Werror' and a useful
warning emitted by Gnulib.

But Gnulib does not support '-Werror'. To fix the issue, you need to
filter out the '-Werror' option. Or not add it in the first place.

> > Based on De Morgan's law, the expression
> > !(defined __GNUC__ && !defined __clang__ && __OPTIMIZE__)
> > is equal to (!defined __GNUC__ || defined __clang__ || !__OPTIMIZE).
> > The expression above looks weird, maybe there are some problems with
> > this way of judging. When any one of the conditions is established, the
> > subsequent judgment will be ignored.
> > 
> > This patch works around the issue by rewriting the expression of
> > judgment after referring to the nearby notes.

This text does not explain what the patch does.

> > --- a/gnulib/import/c++defs.h
> > +++ b/gnulib/import/c++defs.h
> > @@ -286,7 +286,7 @@
> >     _GL_CXXALIASWARN_2 (func, namespace)
> >  /* To work around GCC bug <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=43881>,
> >     we enable the warning only when not optimizing.  */
> > -# if !(defined __GNUC__ && !defined __clang__ && __OPTIMIZE__)
> > +# if defined __GNUC__ && !defined __clang__ && !__OPTIMIZE__
> >  #  define _GL_CXXALIASWARN_2(func,namespace) \
> >      _GL_WARN_ON_USE (func, \
> >                       "The symbol ::" #func " refers to the system function. " \

The previous code enables a useful warning for clang, and does not enable it
for GCC because that would trigger a GCC bug.

Your patch swaps the cases of clang and GCC. Thus it removes a useful warning
for clang builds, and triggers a known GCC bug in the GCC builds.

> > @@ -314,7 +314,7 @@
> >     _GL_CXXALIASWARN1_2 (func, rettype, parameters_and_attributes, namespace)
> >  /* To work around GCC bug <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=43881>,
> >     we enable the warning only when not optimizing.  */
> > -# if !(defined __GNUC__ && !defined __clang__ && __OPTIMIZE__)
> > +# if defined __GNUC__ && !defined __clang__ && !__OPTIMIZE__
> >  #  define _GL_CXXALIASWARN1_2(func,rettype,parameters_and_attributes,namespace) \
> >      _GL_WARN_ON_USE_CXX (func, rettype, rettype, parameters_and_attributes, \
> >                           "The symbol ::" #func " refers to the system function. " \

Likewise.

So, the patch is bad on all accounts.

Bruno