clang++ 11 compilation issues

[Alexandre Duret-Lutz]

Hi,

I'm upgrading a project that used a 8-month-old copy of gnulib to
today's version, and testing the result with various compilers.

This is a C++17 project that is not internationalized and that
uses the following modules:
  argmatch
  argp
  closeout
  error
  isatty
  mkstemp
  mkstemps
  progname
  secure_getenv
  stpcpy
  strverscmp
  sys_wait

My copy of gnulib is also patched with a patch I sent in 2013:
https://lists.gnu.org/archive/html/bug-gnulib/2013-12/msg00106.html

I compile with -Werror.

Testing the result with clang++ 11.0.1-2 (Debian unstable), I observe
the following two issues:

(1) lib/argmatch.h includes lib/gettext.h which fails as follows

> clang++ -DHAVE_CONFIG_H -I. -I..  -I.. -I.. -I../buddy/src -I../lib -I../lib  
> -W -Wall -Werror -Wint-to-void-pointer-cast -Wzero-as-null-pointer-constant 
> -Wcast-align -Wpointer-arith -Wwrite-strings -Wcast-qual -DXTSTRINGDEFINES 
> -Wdocumentation -Wmissing-declarations -Woverloaded-virtual 
> -Wmisleading-indentation -Wimplicit-fallthrough -Wnull-dereference 
> -Wsuggest-override -Wpedantic -fvisibility=hidden -fvisibility-inlines-hidden 
> -DSPOT_BUILD -std=c++17 -g -O -MT autcross.o -MD -MP -MF .deps/autcross.Tpo 
> -c -o autcross.o autcross.cc
> In file included from autcross.cc:34:
> In file included from ../lib/argmatch.h:31:
> ../lib/gettext.h:234:22: error: zero as null pointer constant 
> [-Werror,-Wzero-as-null-pointer-constant]
>   if (msg_ctxt_id != NULL)
>                      ^~~~
>                      nullptr
> /usr/lib/llvm-11/lib/clang/11.0.1/include/stddef.h:84:18: note: expanded from 
> macro 'NULL'
> #    define NULL __null
>                  ^
> In file included from autcross.cc:34:
> In file included from ../lib/argmatch.h:31:
> ../lib/gettext.h:282:22: error: zero as null pointer constant 
> [-Werror,-Wzero-as-null-pointer-constant]
>   if (msg_ctxt_id != NULL)
>                      ^~~~
>                      nullptr
> /usr/lib/llvm-11/lib/clang/11.0.1/include/stddef.h:84:18: note: expanded from 
> macro 'NULL'
> #    define NULL __null
>                  ^
> 2 errors generated.

Just replacing these two tests with "if (msg_ctxt_id)" gets rid of the issue.


(2) ARGMATCH_VERIFY ends up using _Static_assert which is a C11 keyword
    that does not exist in C++.  However static_assert exists in C++11
    with two args, and since C++17 with one arg.

> clang++ -DHAVE_CONFIG_H -I. -I..  -I.. -I.. -I../buddy/src -I../lib -I../lib  
> -W -Wall -Werror -Wint-to-void-pointer-cast -Wzero-as-null-pointer-constant 
> -Wcast-align -Wpointer-arith -Wwrite-strings -Wcast-qual -DXTSTRINGDEFINES 
> -Wdocumentation -Wmissing-declarations -Woverloaded-virtual 
> -Wmisleading-indentation -Wimplicit-fallthrough -Wnull-dereference 
> -Wsuggest-override -Wpedantic -fvisibility=hidden -fvisibility-inlines-hidden 
> -DSPOT_BUILD -std=c++17 -g -O -MT common_aoutput.o -MD -MP -MF 
> .deps/common_aoutput.Tpo -c -o common_aoutput.o common_aoutput.cc
> common_aoutput.cc:85:1: error: '_Static_assert' is a C11 extension 
> [-Werror,-Wc11-extensions]
> ARGMATCH_VERIFY(check_args, check_types);
> ^
> ../lib/argmatch.h:45:5: note: expanded from macro 'ARGMATCH_VERIFY'
>     verify (ARRAY_CARDINALITY (Arglist) == ARRAY_CARDINALITY (Vallist) + 1)
>     ^
> ../lib/verify.h:289:20: note: expanded from macro 'verify'
> # define verify(R) _GL_VERIFY (R, "verify (" #R ")", -)
>                    ^
> ../lib/verify.h:229:41: note: expanded from macro '_GL_VERIFY'
> # define _GL_VERIFY(R, DIAGNOSTIC, ...) _Static_assert (R, DIAGNOSTIC)
>                                         ^
> common_aoutput.cc:397:7: error: '_Static_assert' is a C11 extension 
> [-Werror,-Wc11-extensions]
>       ARGMATCH_VERIFY(args, format);
>       ^
> ../lib/argmatch.h:45:5: note: expanded from macro 'ARGMATCH_VERIFY'
>     verify (ARRAY_CARDINALITY (Arglist) == ARRAY_CARDINALITY (Vallist) + 1)
>     ^
> ../lib/verify.h:289:20: note: expanded from macro 'verify'
> # define verify(R) _GL_VERIFY (R, "verify (" #R ")", -)
>                    ^
> ../lib/verify.h:229:41: note: expanded from macro '_GL_VERIFY'
> # define _GL_VERIFY(R, DIAGNOSTIC, ...) _Static_assert (R, DIAGNOSTIC)
>                                         ^
> 2 errors generated.

to work around this, I've just changed the definition of _GL_VERIFY in
lib/verify.h from

> #if defined _GL_HAVE__STATIC_ASSERT
> # define _GL_VERIFY(R, DIAGNOSTIC, ...) _Static_assert (R, DIAGNOSTIC)
> #else
> # define _GL_VERIFY(R, DIAGNOSTIC, ...)                                \
>     extern int (*_GL_GENSYM (_gl_verify_function) (void))            \
>       [_GL_VERIFY_TRUE (R, DIAGNOSTIC)]
> #endif

to

> #if defined __cpp_static_assert
> # define _GL_VERIFY(R, DIAGNOSTIC, ...) static_assert (R, DIAGNOSTIC)
> #elif defined _GL_HAVE__STATIC_ASSERT
> # define _GL_VERIFY(R, DIAGNOSTIC, ...) _Static_assert (R, DIAGNOSTIC)
> #else
> # define _GL_VERIFY(R, DIAGNOSTIC, ...)                                \
>     extern int (*_GL_GENSYM (_gl_verify_function) (void))            \
>       [_GL_VERIFY_TRUE (R, DIAGNOSTIC)]
> #endif

However there are a few __cplusplus tests at the top of the file that
attempt to tell when _Static_assert can be used (and failed here), and I
do not really follow that logic.  Those macros check for
__cpp_static_assert to assume something about _Static_assert, which
seems dubious.

-- 
Alexandre Duret-Lutz