Avoid clang warnings regarding [[__nodiscard__]]

[Bruno Haible]

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  <bruno@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