Building a Gnulib testdir on FreeBSD 12.0, which uses clang 6.0.1, I see
several warnings such as
../../gltests/../gllib/gl_list.h:642:1: warning: unknown attribute
'__nodiscard__' ignored [-Wunknown-attributes]
The cause is that in this version of clang, __has_c_attribute(__nodiscard__)
apparently returns true, but the use of [[__nodiscard__]] elicits a warning.
Here's how it depends on the clang version:
clang result
----- -------
5.0.2 error
6.0.1 warning
7.0.1 warning
8.0.0 warning
9.0.0 warning
10.0.0 warning
11.0.0 warning
12.0.1 warning
13.0.0 warning
14.0.0 warning
15.0.6 warning
This patch fixes the warnings, by choosing a different (more conservative)
expansion of _GL_ATTRIBUTE_NODISCARD.
2023-01-28 Bruno Haible <br...@clisp.org>
Avoid clang warnings regarding [[__nodiscard__]].
* m4/gnulib-common.m4 (gl_COMMON_BODY): For clang, in C++ mode, ignore
the __has_c_attribute value and define _GL_ATTRIBUTE_NODISCARD to
__attribute__ ((__warn_unused_result__)), not [[__nodiscard__]].
diff --git a/m4/gnulib-common.m4 b/m4/gnulib-common.m4
index cf0fc5a1e7..d632819864 100644
--- a/m4/gnulib-common.m4
+++ b/m4/gnulib-common.m4
@@ -1,4 +1,4 @@
-# gnulib-common.m4 serial 79
+# gnulib-common.m4 serial 80
dnl Copyright (C) 2007-2023 Free Software Foundation, Inc.
dnl This file is free software; the Free Software Foundation
dnl gives unlimited permission to copy and/or distribute it,
@@ -379,7 +379,13 @@ AC_DEFUN([gl_COMMON_BODY], [
the return value, unless the caller uses something like ignore_value. */
/* Applies to: function, enumeration, class. */
#ifndef _GL_ATTRIBUTE_NODISCARD
-# ifdef __has_c_attribute
+# if defined __clang__ && defined __cplusplus
+ /* With clang up to 15.0.6 (at least), in C++ mode, [[__nodiscard__]]
produces
+ a warning. */
+# if __clang_major__ >= 1000
+# define _GL_ATTRIBUTE_NODISCARD [[__nodiscard__]]
+# endif
+# elif defined __has_c_attribute
# if __has_c_attribute (__nodiscard__)
# define _GL_ATTRIBUTE_NODISCARD [[__nodiscard__]]
# endif
