improve clang support (12)

[Bruno Haible]

clang does not have the attributes __warning__ and __error__. But it has the
attribute __diagnose_if__, that is a generalization of both.
See <https://clang.llvm.org/docs/AttributeReference.html>.


2020-08-09  Bruno Haible  <bruno@clisp.org>

        Add ability to emit user-defined warnings and errors with clang.
        * m4/gnulib-common.m4 (gl_COMMON_BODY): Define _GL_ATTRIBUTE_ERROR,
        _GL_ATTRIBUTE_WARNING using an attribute for clang.
        * lib/warn-on-use.h (_GL_WARN_ON_USE, _GL_WARN_ON_USE_ATTRIBUTE,
        _GL_WARN_ON_USE_CXX): Define using an attribute for clang.
        * lib/cdefs.h (__warndecl, __warnattr, __errordecl): Define using an
        attribute for clang.

diff --git a/lib/cdefs.h b/lib/cdefs.h
index b034c0b..801753c 100644
--- a/lib/cdefs.h
+++ b/lib/cdefs.h
@@ -129,6 +129,12 @@
 # define __warnattr(msg) __attribute__((__warning__ (msg)))
 # define __errordecl(name, msg) \
   extern void name (void) __attribute__((__error__ (msg)))
+#elif __clang_major__ >= 4
+# define __warndecl(name, msg) \
+  extern void name (void) __attribute__((__diagnose_if__ (1, msg, "warning")))
+# define __warnattr(msg) __attribute__((__diagnose_if__ (1, msg, "warning")))
+# define __errordecl(name, msg) \
+  extern void name (void) __attribute__((__diagnose_if__ (1, msg, "error")))
 #else
 # define __warndecl(name, msg) extern void name (void)
 # define __warnattr(msg)
diff --git a/lib/warn-on-use.h b/lib/warn-on-use.h
index 291e709..a39e3a3 100644
--- a/lib/warn-on-use.h
+++ b/lib/warn-on-use.h
@@ -87,6 +87,13 @@
 extern __typeof__ (function) function __attribute__ ((__warning__ (message)))
 #  define _GL_WARN_ON_USE_ATTRIBUTE(message) \
   __attribute__ ((__warning__ (message)))
+# elif __clang_major__ >= 4
+/* Another compiler attribute is available in clang.  */
+#  define _GL_WARN_ON_USE(function, message) \
+extern __typeof__ (function) function \
+  __attribute__ ((__diagnose_if__ (1, message, "warning")))
+#  define _GL_WARN_ON_USE_ATTRIBUTE(message) \
+  __attribute__ ((__diagnose_if__ (1, message, "warning")))
 # elif __GNUC__ >= 3 && GNULIB_STRICT_CHECKING
 /* Verify the existence of the function.  */
 #  define _GL_WARN_ON_USE(function, message) \
@@ -111,9 +118,15 @@ _GL_WARN_EXTERN_C int _gl_warn_on_use
      _GL_WARN_ON_USE (function, msg)
 # else
 #  if 4 < __GNUC__ || (__GNUC__ == 4 && 3 <= __GNUC_MINOR__)
+/* A compiler attribute is available in gcc versions 4.3.0 and later.  */
+#   define _GL_WARN_ON_USE_CXX(function,rettype,parameters_and_attributes,msg) 
\
+extern rettype function parameters_and_attributes \
+  __attribute__ ((__warning__ (msg)))
+#  elif __clang_major__ >= 4
+/* Another compiler attribute is available in clang.  */
 #   define _GL_WARN_ON_USE_CXX(function,rettype,parameters_and_attributes,msg) 
\
 extern rettype function parameters_and_attributes \
-     __attribute__ ((__warning__ (msg)))
+  __attribute__ ((__diagnose_if__ (1, msg, "warning")))
 #  elif __GNUC__ >= 3 && GNULIB_STRICT_CHECKING
 /* Verify the existence of the function.  */
 #   define _GL_WARN_ON_USE_CXX(function,rettype,parameters_and_attributes,msg) 
\
diff --git a/m4/gnulib-common.m4 b/m4/gnulib-common.m4
index 50acc0a..e42b80c 100644
--- a/m4/gnulib-common.m4
+++ b/m4/gnulib-common.m4
@@ -1,4 +1,4 @@
-# gnulib-common.m4 serial 53
+# gnulib-common.m4 serial 54
 dnl Copyright (C) 2007-2020 Free Software Foundation, Inc.
 dnl This file is free software; the Free Software Foundation
 dnl gives unlimited permission to copy and/or distribute it,
@@ -76,6 +76,7 @@ AC_DEFUN([gl_COMMON_BODY], [
 # define _GL_ATTR_cold _GL_GNUC_PREREQ (4, 3)
 # define _GL_ATTR_const _GL_GNUC_PREREQ (2, 95)
 # define _GL_ATTR_deprecated _GL_GNUC_PREREQ (3, 1)
+# define _GL_ATTR_diagnose_if 0
 # define _GL_ATTR_error _GL_GNUC_PREREQ (4, 3)
 # define _GL_ATTR_externally_visible _GL_GNUC_PREREQ (4, 1)
 # define _GL_ATTR_fallthrough _GL_GNUC_PREREQ (7, 0)
@@ -149,6 +150,9 @@ AC_DEFUN([gl_COMMON_BODY], [
 #if _GL_HAS_ATTRIBUTE (error)
 # define _GL_ATTRIBUTE_ERROR(msg) __attribute__ ((__error__ (msg)))
 # define _GL_ATTRIBUTE_WARNING(msg) __attribute__ ((__warning__ (msg)))
+#elif _GL_HAS_ATTRIBUTE (diagnose_if)
+# define _GL_ATTRIBUTE_ERROR(msg) __attribute__ ((__diagnose_if__ (1, msg, 
"error")))
+# define _GL_ATTRIBUTE_WARNING(msg) __attribute__ ((__diagnose_if__ (1, msg, 
"warning")))
 #else
 # define _GL_ATTRIBUTE_ERROR(msg)
 # define _GL_ATTRIBUTE_WARNING(msg)