Hi Alexandre,
> I'm upgrading a project that used a 8-month-old copy of gnulib to
> today's version
Since then, gnulib has improved its support for clang.
> I compile with -Werror.
Gnulib generally does not support -Werror on arbitrary platforms.
We try to make for a warning-free compilation on glibc systems with GCC,
as far as it is reasonable (but it does not work always, due to GCC bugs).
> Testing the result with clang++ 11.0.1-2 (Debian unstable)
Now, using clang with -Wall and -Werror is a recipe for failures, for sure.
This is because clang has _many_ warnings, and
- Some of the warnings are of the kind "tell me when you did a certain
optimization". Most of the warnings in this category are not helpful.
- Some of the warnings merely enforce certain coding styles. You can
try to enforce your preferred coding style on your code, but enforcing
it on Gnulib code is a non-starter.
- Some of the warnings are contradictory.
You are supposed to choose the warnings that are reasonable for your
project. For some project of mine, I had to disable 20-40 warning options
before I could get reasonable output.
> > ../lib/gettext.h:234:22: error: zero as null pointer constant
> > [-Werror,-Wzero-as-null-pointer-constant]
> > if (msg_ctxt_id != NULL)
> > ^~~~
This will not be changed. NULL is the preferred way to denote a null pointer.
> Just replacing these two tests with "if (msg_ctxt_id)" gets rid of the issue.
... but makes the code harder to understand.
> (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.
We prefer to use _Static_assert with 2 arguments, since it causes the compiler
to give the diagnostic specified by the programmer. A macro or built-in
that accepts only 1 argument is an inferior solution for this case.
> > 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
Thanks for the suggestion. Committed through the patch below.
> 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.
These lines define our own witnesses whether a certain feature is available,
rather than merely relying on __cpp_static_assert. This allows us to
cater with compilers which lie about their features (e.g. HP compilers
are frequently broken in this way) or consider GCC's -pedantic option.
> Those macros check for
> __cpp_static_assert to assume something about _Static_assert, which
> seems dubious.
Paul can tell more about this one.
2021-01-12 Bruno Haible <br...@clisp.org>
verify: Use C++11 static_assert when available.
Reported by Alexandre Duret-Lutz <a...@lrde.epita.fr> in
<https://lists.gnu.org/archive/html/bug-gnulib/2021-01/msg00177.html>.
* lib/verify.h (_GL_HAVE_STATIC_ASSERT_CXX11): New macro.
(_GL_HAVE_STATIC_ASSERT_CXX17): Renamed from _GL_HAVE_STATIC_ASSERT1.
(_GL_VERIFY): Use static_assert when available with C++11 syntax.
diff --git a/lib/verify.h b/lib/verify.h
index 3cdcdca..a9e7589 100644
--- a/lib/verify.h
+++ b/lib/verify.h
@@ -29,7 +29,11 @@
per C2X. This is supported by GCC 9.1 and later, and by clang in
C++1z mode.
- Define _GL_HAVE_STATIC_ASSERT1 if static_assert (R) works as per
+ Define _GL_HAVE_STATIC_ASSERT_CXX11 if static_assert (R, DIAGNOSTIC)
+ works as per C++11. This is supported by GCC 6.1 and later, and by
+ clang in C++11 mode.
+
+ Define _GL_HAVE_STATIC_ASSERT_CXX17 if static_assert (R) works as per
C++17. This is supported by GCC 9.1 and later, and by clang in
C++1z mode.
@@ -54,10 +58,15 @@
# if 4 <= __clang_major__ && 201411 <= __cpp_static_assert
# define _GL_HAVE__STATIC_ASSERT1 1
# endif
+# if 201103L <= __cplusplus \
+ || 6 <= __GNUC__ \
+ || (4 <= __clang_major__ && 200410 <= __cpp_static_assert)
+# define _GL_HAVE_STATIC_ASSERT_CXX11 1
+# endif
# if 201703L <= __cplusplus \
|| 9 <= __GNUC__ \
|| (4 <= __clang_major__ && 201411 <= __cpp_static_assert)
-# define _GL_HAVE_STATIC_ASSERT1 1
+# define _GL_HAVE_STATIC_ASSERT_CXX17 1
# endif
#endif
@@ -225,7 +234,9 @@ template <int w>
Unfortunately, unlike C11, this implementation must appear as an
ordinary declaration, and cannot appear inside struct { ... }. */
-#if defined _GL_HAVE__STATIC_ASSERT
+#if defined _GL_HAVE_STATIC_ASSERT_CXX11
+# 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, ...) \
@@ -239,7 +250,7 @@ template <int w>
# define _Static_assert(...) \
_GL_VERIFY (__VA_ARGS__, "static assertion failed", -)
# endif
-# if !defined _GL_HAVE_STATIC_ASSERT1 && !defined static_assert
+# if !defined _GL_HAVE_STATIC_ASSERT_CXX17 && !defined static_assert
# define static_assert _Static_assert /* C11 requires this #define. */
# endif
#endif
