Avoid clang warnings regarding [[__nodiscard__]]

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

Re: Avoid clang warnings regarding [[__nodiscard__]]

[Paul Eggert]

[-- Attachment #1: Type: text/plain, Size: 387 bytes --]

On 2023-01-28 02:57, Bruno Haible wrote:
> +  /* With clang up to 15.0.6 (at least), in C++ mode, [[__nodiscard__]] produces
> +     a warning.  */
> +#  if __clang_major__ >= 1000

I found this a little hard to follow, as I don't know __clang_major__ by 
heart and at first misread that as version 10.0.0. How about something 
like the attached, to simplify this for Clang non-experts?

[-- Attachment #2: nodiscard.diff --]
[-- Type: text/x-patch, Size: 876 bytes --]

diff --git a/m4/gnulib-common.m4 b/m4/gnulib-common.m4
index d632819864..bb5a2029af 100644
--- a/m4/gnulib-common.m4
+++ b/m4/gnulib-common.m4
@@ -379,14 +379,11 @@ 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
-# 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__)
+# ifdef __has_c_attribute
+#  if (__has_c_attribute (__nodiscard__) \
+       && ! (defined __clang__ && defined __cplusplus))
 #   define _GL_ATTRIBUTE_NODISCARD [[__nodiscard__]]
 #  endif
 # endif

Re: Avoid clang warnings regarding [[__nodiscard__]]

[Bruno Haible]

Paul Eggert wrote:
> > +  /* With clang up to 15.0.6 (at least), in C++ mode, [[__nodiscard__]] produces
> > +     a warning.  */
> > +#  if __clang_major__ >= 1000
> 
> I found this a little hard to follow

Indeed, I wondered whether I should add a comment. Guess I should have :)

> How about something like the attached, to simplify this for Clang non-experts?

I wanted the code to be prepared for the day when clang++ actually adds
support for this syntax [[__nodiscard__]]. The code you are proposing
optimizes the current needs, but not the future needs.

I'm adding this comment instead. I hope that's clear enough.


2023-01-28  Bruno Haible  <bruno@clisp.org>

	Clarify _GL_ATTRIBUTE_NODISCARD code.
	Reported by Paul Eggert.
	* m4/gnulib-common.m4 (gl_COMMON_BODY): Add comment regarding
	_GL_ATTRIBUTE_NODISCARD definition.

diff --git a/m4/gnulib-common.m4 b/m4/gnulib-common.m4
index d632819864..c0181abdc5 100644
--- a/m4/gnulib-common.m4
+++ b/m4/gnulib-common.m4
@@ -381,7 +381,10 @@ AC_DEFUN([gl_COMMON_BODY], [
 #ifndef _GL_ATTRIBUTE_NODISCARD
 # if defined __clang__ && defined __cplusplus
   /* With clang up to 15.0.6 (at least), in C++ mode, [[__nodiscard__]] produces
-     a warning.  */
+     a warning.
+     The 1000 below means a yet unknown threshold.  When clang++ version X
+     starts supporting [[__nodiscard__]] without warning about it, you can
+     replace the 1000 with X.  */
 #  if __clang_major__ >= 1000
 #   define _GL_ATTRIBUTE_NODISCARD [[__nodiscard__]]
 #  endif