Update from Gnulib

This incorporates:
2020-08-11 Use expression statements also on clang
2020-08-10 Use many __attribute__s with clang
2020-08-09 Use attribute __aligned__ with clang
2020-08-09 Use __alignof__ with clang
2020-08-09 ignore-value: Simplify on clang
2020-08-09 Use __typeof__ with clang
2020-08-09 intprops: Fix typo in comment
2020-08-09 Silence warnings from clang 10 with -Wimplicit-fallthrough
2020-08-09 count-one-bits: Use __builtin_popcount{,l,ll} on clang
2020-08-09 string: Fix build error in C++ mode with clang
2020-08-09 Add ability to emit user-defined diagnostics with clang
2020-08-07 alloca: No need to compile alloca.c with clang
2020-08-06 Use __builtin_assume with clang
* lib/alloca.in.h, lib/arg-nonnull.h, lib/c++defs.h, lib/cdefs.h:
* lib/count-one-bits.h, lib/dirent.in.h, lib/ignore-value.h:
* lib/intprops.h, lib/malloca.h, lib/regex_internal.h:
* lib/stdalign.in.h, lib/stddef.in.h, lib/stdio.in.h:
* lib/stdlib.in.h, lib/string.in.h, lib/verify.h, lib/warn-on-use.h:
* m4/gnulib-common.m4, m4/stddef_h.m4, m4/stdint.m4:
Copy from Gnulib.

20 files changed, 137 insertions, 75 deletions

diff --git a/lib/alloca.in.h b/lib/alloca.in.h
index 5686b082bbe..c71e9bfed9e 100644
--- a/lib/alloca.in.h
+++ b/lib/alloca.in.h
@@ -44,7 +44,7 @@
 # endif
 #endif
 #ifndef alloca
-# ifdef __GNUC__
+# if defined __GNUC__ || (__clang_major__ >= 4)
 #  define alloca __builtin_alloca
 # elif defined _AIX
 #  define alloca __alloca
diff --git a/lib/arg-nonnull.h b/lib/arg-nonnull.h
index ac26ca8cfed..db9d9ae116a 100644
--- a/lib/arg-nonnull.h
+++ b/lib/arg-nonnull.h
@@ -18,7 +18,7 @@
    that the values passed as arguments n, ..., m must be non-NULL pointers.
    n = 1 stands for the first argument, n = 2 for the second argument etc.  */
 #ifndef _GL_ARG_NONNULL
-# if (__GNUC__ == 3 && __GNUC_MINOR__ >= 3) || __GNUC__ > 3
+# if __GNUC__ > 3 || (__GNUC__ == 3 && __GNUC_MINOR__ >= 3) || defined __clang__
 #  define _GL_ARG_NONNULL(params) __attribute__ ((__nonnull__ params))
 # else
 #  define _GL_ARG_NONNULL(params)
diff --git a/lib/c++defs.h b/lib/c++defs.h
index 182c2b3a88d..90e6fd62e6d 100644
--- a/lib/c++defs.h
+++ b/lib/c++defs.h
@@ -298,7 +298,7 @@
    we enable the warning only when not optimizing.  */
 # if !(defined __GNUC__ && !defined __clang__ && __OPTIMIZE__)
 #  define _GL_CXXALIASWARN1_2(func,rettype,parameters_and_attributes,namespace) \
-    _GL_WARN_ON_USE_CXX (func, rettype, parameters_and_attributes, \
+    _GL_WARN_ON_USE_CXX (func, rettype, rettype, parameters_and_attributes, \
                          "The symbol ::" #func " refers to the system function. " \
                          "Use " #namespace "::" #func " instead.")
 # else
diff --git a/lib/cdefs.h b/lib/cdefs.h
index f6c447ad377..beedd891fb8 100644
--- a/lib/cdefs.h
+++ b/lib/cdefs.h
@@ -34,7 +34,34 @@
 #undef  __P
 #undef  __PMT
 
-#ifdef __GNUC__
+/* Compilers that are not clang may object to
+       #if defined __clang__ && __has_attribute(...)
+   even though they do not need to evaluate the right-hand side of the &&.  */
+#if defined __clang__ && defined __has_attribute
+# define __glibc_clang_has_attribute(name) __has_attribute (name)
+#else
+# define __glibc_clang_has_attribute(name) 0
+#endif
+
+/* Compilers that are not clang may object to
+       #if defined __clang__ && __has_builtin(...)
+   even though they do not need to evaluate the right-hand side of the &&.  */
+#if defined __clang__ && defined __has_builtin
+# define __glibc_clang_has_builtin(name) __has_builtin (name)
+#else
+# define __glibc_clang_has_builtin(name) 0
+#endif
+
+/* Compilers that are not clang may object to
+       #if defined __clang__ && __has_extension(...)
+   even though they do not need to evaluate the right-hand side of the &&.  */
+#if defined __clang__ && defined __has_extension
+# define __glibc_clang_has_extension(ext) __has_extension (ext)
+#else
+# define __glibc_clang_has_extension(ext) 0
+#endif
+
+#if defined __GNUC__ || defined __clang__
 
 /* All functions, except those with callbacks or those that
    synchronize memory, are leaf functions.  */
@@ -51,7 +78,8 @@
    gcc 2.8.x and egcs.  For gcc 3.2 and up we even mark C functions
    as non-throwing using a function attribute since programs can use
    the -fexceptions options for C code as well.  */
-# if !defined __cplusplus && __GNUC_PREREQ (3, 3)
+# if !defined __cplusplus \
+     && (__GNUC_PREREQ (3, 3) || __glibc_clang_has_attribute (__nothrow__))
 #  define __THROW       __attribute__ ((__nothrow__ __LEAF))
 #  define __THROWNL     __attribute__ ((__nothrow__))
 #  define __NTH(fct)    __attribute__ ((__nothrow__ __LEAF)) fct
@@ -70,7 +98,7 @@
 #  endif
 # endif
 
-#else   /* Not GCC.  */
+#else   /* Not GCC or clang.  */
 
 # if (defined __cplusplus                                               \
       || (defined __STDC_VERSION__ && __STDC_VERSION__ >= 199901L))
@@ -83,16 +111,7 @@
 # define __THROWNL
 # define __NTH(fct)     fct
 
-#endif  /* GCC.  */
-
-/* Compilers that are not clang may object to
-       #if defined __clang__ && __has_extension(...)
-   even though they do not need to evaluate the right-hand side of the &&.  */
-#if defined __clang__ && defined __has_extension
-# define __glibc_clang_has_extension(ext) __has_extension (ext)
-#else
-# define __glibc_clang_has_extension(ext) 0
-#endif
+#endif  /* GCC || clang.  */
 
 /* These two macros are not used in glibc anymore.  They are kept here
    only because some other projects expect the macros to be defined.  */
@@ -129,6 +148,12 @@
 # define __warnattr(msg) __attribute__((__warning__ (msg)))
 # define __errordecl(name, msg) \
   extern void name (void) __attribute__((__error__ (msg)))
+#elif __glibc_clang_has_attribute (__diagnose_if__)
+# 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)
@@ -194,17 +219,17 @@
 */
 #endif
 
-/* GCC has various useful declarations that can be made with the
-   `__attribute__' syntax.  All of the ways we use this do fine if
-   they are omitted for compilers that don't understand it. */
-#if !defined __GNUC__ || __GNUC__ < 2
+/* GCC and clang have various useful declarations that can be made with
+   the '__attribute__' syntax.  All of the ways we use this do fine if
+   they are omitted for compilers that don't understand it.  */
+#if !(defined __GNUC__ || defined __clang__)
 # define __attribute__(xyz)     /* Ignore */
 #endif
 
 /* At some point during the gcc 2.96 development the `malloc' attribute
    for functions was introduced.  We don't want to use it unconditionally
    (although this would be possible) since it generates warnings.  */
-#if __GNUC_PREREQ (2,96)
+#if __GNUC_PREREQ (2,96) || __glibc_clang_has_attribute (__malloc__)
 # define __attribute_malloc__ __attribute__ ((__malloc__))
 #else
 # define __attribute_malloc__ /* Ignore */
@@ -222,14 +247,14 @@
 /* At some point during the gcc 2.96 development the `pure' attribute
    for functions was introduced.  We don't want to use it unconditionally
    (although this would be possible) since it generates warnings.  */
-#if __GNUC_PREREQ (2,96)
+#if __GNUC_PREREQ (2,96) || __glibc_clang_has_attribute (__pure__)
 # define __attribute_pure__ __attribute__ ((__pure__))
 #else
 # define __attribute_pure__ /* Ignore */
 #endif
 
 /* This declaration tells the compiler that the value is constant.  */
-#if __GNUC_PREREQ (2,5)
+#if __GNUC_PREREQ (2,5) || __glibc_clang_has_attribute (__const__)
 # define __attribute_const__ __attribute__ ((__const__))
 #else
 # define __attribute_const__ /* Ignore */
@@ -238,7 +263,7 @@
 /* At some point during the gcc 3.1 development the `used' attribute
    for functions was introduced.  We don't want to use it unconditionally
    (although this would be possible) since it generates warnings.  */
-#if __GNUC_PREREQ (3,1)
+#if __GNUC_PREREQ (3,1) || __glibc_clang_has_attribute (__used__)
 # define __attribute_used__ __attribute__ ((__used__))
 # define __attribute_noinline__ __attribute__ ((__noinline__))
 #else
@@ -247,7 +272,7 @@
 #endif
 
 /* Since version 3.2, gcc allows marking deprecated functions.  */
-#if __GNUC_PREREQ (3,2)
+#if __GNUC_PREREQ (3,2) || __glibc_clang_has_attribute (__deprecated__)
 # define __attribute_deprecated__ __attribute__ ((__deprecated__))
 #else
 # define __attribute_deprecated__ /* Ignore */
@@ -270,7 +295,7 @@
    If several `format_arg' attributes are given for the same function, in
    gcc-3.0 and older, all but the last one are ignored.  In newer gccs,
    all designated arguments are considered.  */
-#if __GNUC_PREREQ (2,8)
+#if __GNUC_PREREQ (2,8) || __glibc_clang_has_attribute (__format_arg__)
 # define __attribute_format_arg__(x) __attribute__ ((__format_arg__ (x)))
 #else
 # define __attribute_format_arg__(x) /* Ignore */
@@ -280,7 +305,7 @@
    attribute for functions was introduced.  We don't want to use it
    unconditionally (although this would be possible) since it
    generates warnings.  */
-#if __GNUC_PREREQ (2,97)
+#if __GNUC_PREREQ (2,97) || __glibc_clang_has_attribute (__format__)
 # define __attribute_format_strfmon__(a,b) \
   __attribute__ ((__format__ (__strfmon__, a, b)))
 #else
@@ -291,7 +316,7 @@
    must not be NULL.  Do not define __nonnull if it is already defined,
    for portability when this file is used in Gnulib.  */
 #ifndef __nonnull
-# if __GNUC_PREREQ (3,3)
+# if __GNUC_PREREQ (3,3) || __glibc_clang_has_attribute (__nonnull__)
 #  define __nonnull(params) __attribute__ ((__nonnull__ params))
 # else
 #  define __nonnull(params)
@@ -300,7 +325,7 @@
 
 /* If fortification mode, we warn about unused results of certain
    function calls which can lead to problems.  */
-#if __GNUC_PREREQ (3,4)
+#if __GNUC_PREREQ (3,4) || __glibc_clang_has_attribute (__warn_unused_result__)
 # define __attribute_warn_unused_result__ \
    __attribute__ ((__warn_unused_result__))
 # if defined __USE_FORTIFY_LEVEL && __USE_FORTIFY_LEVEL > 0
@@ -314,7 +339,7 @@
 #endif
 
 /* Forces a function to be always inlined.  */
-#if __GNUC_PREREQ (3,2)
+#if __GNUC_PREREQ (3,2) || __glibc_clang_has_attribute (__always_inline__)
 /* The Linux kernel defines __always_inline in stddef.h (283d7573), and
    it conflicts with this definition.  Therefore undefine it first to
    allow either header to be included first.  */
@@ -327,7 +352,7 @@
 
 /* Associate error messages with the source location of the call site rather
    than with the source location inside the function.  */
-#if __GNUC_PREREQ (4,3)
+#if __GNUC_PREREQ (4,3) || __glibc_clang_has_attribute (__artificial__)
 # define __attribute_artificial__ __attribute__ ((__artificial__))
 #else
 # define __attribute_artificial__ /* Ignore */
@@ -370,7 +395,7 @@
    run in pedantic mode if the uses are carefully marked using the
    `__extension__' keyword.  But this is not generally available before
    version 2.8.  */
-#if !__GNUC_PREREQ (2,8)
+#if !(__GNUC_PREREQ (2,8) || defined __clang__)
 # define __extension__          /* Ignore */
 #endif
 
@@ -401,7 +426,7 @@
 # endif
 #endif
 
-#if (__GNUC__ >= 3) || (__clang_major__ >= 4)
+#if (__GNUC__ >= 3) || __glibc_clang_has_builtin (__builtin_expect)
 # define __glibc_unlikely(cond) __builtin_expect ((cond), 0)
 # define __glibc_likely(cond)   __builtin_expect ((cond), 1)
 #else
@@ -417,7 +442,8 @@
 
 #if (!defined _Noreturn \
      && (defined __STDC_VERSION__ ? __STDC_VERSION__ : 0) < 201112 \
-     &&  !__GNUC_PREREQ (4,7))
+     &&  !(__GNUC_PREREQ (4,7) \
+           || (3 < __clang_major__ + (5 <= __clang_minor__))))
 # if __GNUC_PREREQ (2,8)
 #  define _Noreturn __attribute__ ((__noreturn__))
 # else
diff --git a/lib/count-one-bits.h b/lib/count-one-bits.h
index 6c5b75708cf..a9e166aed8c 100644
--- a/lib/count-one-bits.h
+++ b/lib/count-one-bits.h
@@ -38,7 +38,8 @@ extern "C" {
    expand to code that computes the number of 1-bits of the local
    variable 'x' of type TYPE (an unsigned integer type) and return it
    from the current function.  */
-#if __GNUC__ > 3 || (__GNUC__ == 3 && __GNUC_MINOR__ >= 4)
+#if (__GNUC__ > 3 || (__GNUC__ == 3 && __GNUC_MINOR__ >= 4)) \
+    || (__clang_major__ >= 4)
 # define COUNT_ONE_BITS(GCC_BUILTIN, MSC_BUILTIN, TYPE) \
     return GCC_BUILTIN (x)
 #else
diff --git a/lib/dirent.in.h b/lib/dirent.in.h
index 6fa44f0d28d..23c4e055774 100644
--- a/lib/dirent.in.h
+++ b/lib/dirent.in.h
@@ -58,7 +58,7 @@ typedef struct gl_directory DIR;
 /* The __attribute__ feature is available in gcc versions 2.5 and later.
    The attribute __pure__ was added in gcc 2.96.  */
 #ifndef _GL_ATTRIBUTE_PURE
-# if __GNUC__ > 2 || (__GNUC__ == 2 && __GNUC_MINOR__ >= 96)
+# if __GNUC__ > 2 || (__GNUC__ == 2 && __GNUC_MINOR__ >= 96) || defined __clang__
 #  define _GL_ATTRIBUTE_PURE __attribute__ ((__pure__))
 # else
 #  define _GL_ATTRIBUTE_PURE /* empty */
diff --git a/lib/ignore-value.h b/lib/ignore-value.h
index 7a922268431..ec3288f0dfc 100644
--- a/lib/ignore-value.h
+++ b/lib/ignore-value.h
@@ -39,8 +39,9 @@
    versions 3.4 and newer have __attribute__ ((__warn_unused_result__))
    which may cause unwanted diagnostics in that case.  Use __typeof__
    and __extension__ to work around the problem, if the workaround is
-   known to be needed.  */
-#if 3 < __GNUC__ + (4 <= __GNUC_MINOR__)
+   known to be needed.
+   The workaround is not needed with clang.  */
+#if (3 < __GNUC__ + (4 <= __GNUC_MINOR__)) && !defined __clang__
 # define ignore_value(x) \
     (__extension__ ({ __typeof__ (x) __x = (x); (void) __x; }))
 #else
diff --git a/lib/intprops.h b/lib/intprops.h
index dfbcaae73e3..220f532e499 100644
--- a/lib/intprops.h
+++ b/lib/intprops.h
@@ -86,6 +86,7 @@
 /* Does the __typeof__ keyword work?  This could be done by
    'configure', but for now it's easier to do it by hand.  */
 #if (2 <= __GNUC__ \
+     || (4 <= __clang_major__) \
      || (1210 <= __IBMC__ && defined __IBM__TYPEOF__) \
      || (0x5110 <= __SUNPRO_C && !__STDC__))
 # define _GL_HAVE___TYPEOF__ 1
@@ -239,7 +240,7 @@
 #endif
 
 /* True if __builtin_add_overflow_p (A, B, C) works, and similarly for
-   __builtin_mul_overflow_p and __builtin_mul_overflow_p.  */
+   __builtin_sub_overflow_p and __builtin_mul_overflow_p.  */
 #define _GL_HAS_BUILTIN_OVERFLOW_P (7 <= __GNUC__)
 
 /* The _GL*_OVERFLOW macros have the same restrictions as the
diff --git a/lib/malloca.h b/lib/malloca.h
index cfcd4de4ad8..ccc485a6a4d 100644
--- a/lib/malloca.h
+++ b/lib/malloca.h
@@ -89,7 +89,7 @@ extern void freea (void *p);
 /* ------------------- Auxiliary, non-public definitions ------------------- */
 
 /* Determine the alignment of a type at compile time.  */
-#if defined __GNUC__ || defined __IBM__ALIGNOF__
+#if defined __GNUC__ || defined __clang__ || defined __IBM__ALIGNOF__
 # define sa_alignof __alignof__
 #elif defined __cplusplus
   template <class type> struct sa_alignof_helper { char __slot1; type __slot2; };
diff --git a/lib/regex_internal.h b/lib/regex_internal.h
index f6ebfb003e8..9a0c2ed97c8 100644
--- a/lib/regex_internal.h
+++ b/lib/regex_internal.h
@@ -841,10 +841,10 @@ re_string_elem_size_at (const re_string_t *pstr, Idx idx)
 #endif /* RE_ENABLE_I18N */
 
 #ifndef FALLTHROUGH
-# if __GNUC__ < 7
-#  define FALLTHROUGH ((void) 0)
-# else
+# if (__GNUC__ >= 7) || (__clang_major__ >= 10)
 #  define FALLTHROUGH __attribute__ ((__fallthrough__))
+# else
+#  define FALLTHROUGH ((void) 0)
 # endif
 #endif
 
diff --git a/lib/stdalign.in.h b/lib/stdalign.in.h
index cd786bed2cd..e4809b401f7 100644
--- a/lib/stdalign.in.h
+++ b/lib/stdalign.in.h
@@ -34,11 +34,12 @@
    requirement of a structure member (i.e., slot or field) that is of
    type TYPE, as an integer constant expression.
 
-   This differs from GCC's __alignof__ operator, which can yield a
-   better-performing alignment for an object of that type.  For
-   example, on x86 with GCC, __alignof__ (double) and __alignof__
-   (long long) are 8, whereas alignof (double) and alignof (long long)
-   are 4 unless the option '-malign-double' is used.
+   This differs from GCC's and clang's __alignof__ operator, which can
+   yield a better-performing alignment for an object of that type.  For
+   example, on x86 with GCC and on Linux/x86 with clang,
+   __alignof__ (double) and __alignof__ (long long) are 8, whereas
+   alignof (double) and alignof (long long) are 4 unless the option
+   '-malign-double' is used.
 
    The result cannot be used as a value for an 'enum' constant, if you
    want to be portable to HP-UX 10.20 cc and AIX 3.2.5 xlc.
@@ -55,7 +56,8 @@
 /* GCC releases before GCC 4.9 had a bug in _Alignof.  See GCC bug 52023
    <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=52023>.  */
 #if (!defined __STDC_VERSION__ || __STDC_VERSION__ < 201112 \
-     || (defined __GNUC__ && __GNUC__ < 4 + (__GNUC_MINOR__ < 9)))
+     || (defined __GNUC__ && __GNUC__ < 4 + (__GNUC_MINOR__ < 9) \
+         && !defined __clang__))
 # ifdef __cplusplus
 #  if 201103 <= __cplusplus
 #   define _Alignof(type) alignof (type)
@@ -102,8 +104,9 @@
 #  define _Alignas(a) alignas (a)
 # elif ((defined __APPLE__ && defined __MACH__                  \
          ? 4 < __GNUC__ + (1 <= __GNUC_MINOR__)                 \
-         : __GNUC__ && !defined __ibmxl__) \
-        || (__ia64 && (61200 <= __HP_cc || 61200 <= __HP_aCC)) \
+         : __GNUC__ && !defined __ibmxl__)                      \
+        || (4 <= __clang_major__)                               \
+        || (__ia64 && (61200 <= __HP_cc || 61200 <= __HP_aCC))  \
         || __ICC || 0x590 <= __SUNPRO_C || 0x0600 <= __xlC__)
 #  define _Alignas(a) __attribute__ ((__aligned__ (a)))
 # elif 1300 <= _MSC_VER
diff --git a/lib/stddef.in.h b/lib/stddef.in.h
index 2e50a1f01e8..87b46d53204 100644
--- a/lib/stddef.in.h
+++ b/lib/stddef.in.h
@@ -97,7 +97,7 @@
    and the C11 standard allows this.  Work around this problem by
    using __alignof__ (which returns 8 for double) rather than _Alignof
    (which returns 4), and align each union member accordingly.  */
-#   ifdef __GNUC__
+#   if defined __GNUC__ || (__clang_major__ >= 4)
 #    define _GL_STDDEF_ALIGNAS(type) \
        __attribute__ ((__aligned__ (__alignof__ (type))))
 #   else
diff --git a/lib/stdio.in.h b/lib/stdio.in.h
index 6c338dd6c0b..cbebc8462fd 100644
--- a/lib/stdio.in.h
+++ b/lib/stdio.in.h
@@ -63,7 +63,7 @@
    gnulib and libintl do '#define printf __printf__' when they override
    the 'printf' function.  */
 #ifndef _GL_ATTRIBUTE_FORMAT
-# if __GNUC__ > 2 || (__GNUC__ == 2 && __GNUC_MINOR__ >= 7)
+# if __GNUC__ > 2 || (__GNUC__ == 2 && __GNUC_MINOR__ >= 7) || defined __clang__
 #  define _GL_ATTRIBUTE_FORMAT(spec) __attribute__ ((__format__ spec))
 # else
 #  define _GL_ATTRIBUTE_FORMAT(spec) /* empty */
diff --git a/lib/stdlib.in.h b/lib/stdlib.in.h
index 59f9e6c71d1..5c598a275d1 100644
--- a/lib/stdlib.in.h
+++ b/lib/stdlib.in.h
@@ -102,7 +102,7 @@ struct random_data
 /* The __attribute__ feature is available in gcc versions 2.5 and later.
    The attribute __pure__ was added in gcc 2.96.  */
 #ifndef _GL_ATTRIBUTE_PURE
-# if __GNUC__ > 2 || (__GNUC__ == 2 && __GNUC_MINOR__ >= 96)
+# if __GNUC__ > 2 || (__GNUC__ == 2 && __GNUC_MINOR__ >= 96) || defined __clang__
 #  define _GL_ATTRIBUTE_PURE __attribute__ ((__pure__))
 # else
 #  define _GL_ATTRIBUTE_PURE /* empty */
diff --git a/lib/string.in.h b/lib/string.in.h
index aa9802791ee..c0c1a54f39d 100644
--- a/lib/string.in.h
+++ b/lib/string.in.h
@@ -55,7 +55,7 @@
 /* The __attribute__ feature is available in gcc versions 2.5 and later.
    The attribute __pure__ was added in gcc 2.96.  */
 #ifndef _GL_ATTRIBUTE_PURE
-# if __GNUC__ > 2 || (__GNUC__ == 2 && __GNUC_MINOR__ >= 96)
+# if __GNUC__ > 2 || (__GNUC__ == 2 && __GNUC_MINOR__ >= 96) || defined __clang__
 #  define _GL_ATTRIBUTE_PURE __attribute__ ((__pure__))
 # else
 #  define _GL_ATTRIBUTE_PURE /* empty */
@@ -329,7 +329,8 @@ _GL_WARN_ON_USE (stpncpy, "stpncpy is unportable - "
    GB18030 and the character to be searched is a digit.  */
 # undef strchr
 /* Assume strchr is always declared.  */
-_GL_WARN_ON_USE_CXX (strchr, const char *, (const char *, int),
+_GL_WARN_ON_USE_CXX (strchr,
+                     const char *, char *, (const char *, int),
                      "strchr cannot work correctly on character strings "
                      "in some multibyte locales - "
                      "use mbschr if you care about internationalization");
@@ -524,7 +525,8 @@ _GL_CXXALIASWARN (strpbrk);
    locale encoding is GB18030 and one of the characters to be searched is a
    digit.  */
 #  undef strpbrk
-_GL_WARN_ON_USE_CXX (strpbrk, const char *, (const char *, const char *),
+_GL_WARN_ON_USE_CXX (strpbrk,
+                     const char *, char *, (const char *, const char *),
                      "strpbrk cannot work correctly on character strings "
                      "in multibyte locales - "
                      "use mbspbrk if you care about internationalization");
@@ -532,7 +534,8 @@ _GL_WARN_ON_USE_CXX (strpbrk, const char *, (const char *, const char *),
 #elif defined GNULIB_POSIXCHECK
 # undef strpbrk
 # if HAVE_RAW_DECL_STRPBRK
-_GL_WARN_ON_USE_CXX (strpbrk, const char *, (const char *, const char *),
+_GL_WARN_ON_USE_CXX (strpbrk,
+                     const char *, char *, (const char *, const char *),
                      "strpbrk is unportable - "
                      "use gnulib module strpbrk for portability");
 # endif
@@ -553,7 +556,8 @@ _GL_WARN_ON_USE (strspn, "strspn cannot work correctly on character strings "
    GB18030 and the character to be searched is a digit.  */
 # undef strrchr
 /* Assume strrchr is always declared.  */
-_GL_WARN_ON_USE_CXX (strrchr, const char *, (const char *, int),
+_GL_WARN_ON_USE_CXX (strrchr,
+                     const char *, char *, (const char *, int),
                      "strrchr cannot work correctly on character strings "
                      "in some multibyte locales - "
                      "use mbsrchr if you care about internationalization");
diff --git a/lib/verify.h b/lib/verify.h
index f1097612704..58172f3cb7f 100644
--- a/lib/verify.h
+++ b/lib/verify.h
@@ -233,6 +233,13 @@ template <int w>
 
 /* @assert.h omit start@  */
 
+#if defined __has_builtin
+/* <https://clang.llvm.org/docs/LanguageExtensions.html#builtin-functions> */
+# define _GL_HAS_BUILTIN_ASSUME __has_builtin (__builtin_assume)
+#else
+# define _GL_HAS_BUILTIN_ASSUME 0
+#endif
+
 #if 3 < __GNUC__ + (3 < __GNUC_MINOR__ + (4 <= __GNUC_PATCHLEVEL__))
 # define _GL_HAS_BUILTIN_TRAP 1
 #elif defined __has_builtin
@@ -294,7 +301,9 @@ template <int w>
    diagnostics, performance can suffer if R uses hard-to-optimize
    features such as function calls not inlined by the compiler.  */
 
-#if _GL_HAS_BUILTIN_UNREACHABLE
+#if _GL_HAS_BUILTIN_ASSUME
+# define assume(R) __builtin_assume (R)
+#elif _GL_HAS_BUILTIN_UNREACHABLE
 # define assume(R) ((R) ? (void) 0 : __builtin_unreachable ())
 #elif 1200 <= _MSC_VER
 # define assume(R) __assume (R)
diff --git a/lib/warn-on-use.h b/lib/warn-on-use.h
index 23c10fdd122..3f728d1a9dc 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) \
@@ -99,27 +106,33 @@ _GL_WARN_EXTERN_C int _gl_warn_on_use
 # endif
 #endif
 
-/* _GL_WARN_ON_USE_CXX (function, rettype, parameters_and_attributes, "string")
-   is like _GL_WARN_ON_USE (function, "string"), except that in C++ mode the
+/* _GL_WARN_ON_USE_CXX (function, rettype_gcc, rettype_clang, parameters_and_attributes, "message")
+   is like _GL_WARN_ON_USE (function, "message"), except that in C++ mode the
    function is declared with the given prototype, consisting of return type,
    parameters, and attributes.
    This variant is useful for overloaded functions in C++. _GL_WARN_ON_USE does
    not work in this case.  */
 #ifndef _GL_WARN_ON_USE_CXX
 # if !defined __cplusplus
-#  define _GL_WARN_ON_USE_CXX(function,rettype,parameters_and_attributes,msg) \
+#  define _GL_WARN_ON_USE_CXX(function,rettype_gcc,rettype_clang,parameters_and_attributes,msg) \
      _GL_WARN_ON_USE (function, msg)
 # else
 #  if 4 < __GNUC__ || (__GNUC__ == 4 && 3 <= __GNUC_MINOR__)
-#   define _GL_WARN_ON_USE_CXX(function,rettype,parameters_and_attributes,msg) \
-extern rettype function parameters_and_attributes \
-     __attribute__ ((__warning__ (msg)))
+/* A compiler attribute is available in gcc versions 4.3.0 and later.  */
+#   define _GL_WARN_ON_USE_CXX(function,rettype_gcc,rettype_clang,parameters_and_attributes,msg) \
+extern rettype_gcc 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_gcc,rettype_clang,parameters_and_attributes,msg) \
+extern rettype_clang function parameters_and_attributes \
+  __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) \
-extern rettype function parameters_and_attributes
+#   define _GL_WARN_ON_USE_CXX(function,rettype_gcc,rettype_clang,parameters_and_attributes,msg) \
+extern rettype_gcc function parameters_and_attributes
 #  else /* Unsupported.  */
-#   define _GL_WARN_ON_USE_CXX(function,rettype,parameters_and_attributes,msg) \
+#   define _GL_WARN_ON_USE_CXX(function,rettype_gcc,rettype_clang,parameters_and_attributes,msg) \
 _GL_WARN_EXTERN_C int _gl_warn_on_use
 #  endif
 # endif
diff --git a/m4/gnulib-common.m4 b/m4/gnulib-common.m4
index 50acc0a474b..03da2287d48 100644
--- a/m4/gnulib-common.m4
+++ b/m4/gnulib-common.m4
@@ -1,4 +1,4 @@
-# gnulib-common.m4 serial 53
+# gnulib-common.m4 serial 55
 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,
@@ -45,7 +45,7 @@ AC_DEFUN([gl_COMMON_BODY], [
                 ? 6000000 <= __apple_build_version__ \
                 : 3 < __clang_major__ + (5 <= __clang_minor__))))
    /* _Noreturn works as-is.  */
-# elif _GL_GNUC_PREREQ (2, 8) || 0x5110 <= __SUNPRO_C
+# elif _GL_GNUC_PREREQ (2, 8) || defined __clang__ || 0x5110 <= __SUNPRO_C
 #  define _Noreturn __attribute__ ((__noreturn__))
 # elif 1200 <= (defined _MSC_VER ? _MSC_VER : 0)
 #  define _Noreturn __declspec (noreturn)
@@ -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)
diff --git a/m4/stddef_h.m4 b/m4/stddef_h.m4
index 6bcfadb74ef..d8bc8ff64e4 100644
--- a/m4/stddef_h.m4
+++ b/m4/stddef_h.m4
@@ -1,5 +1,5 @@
 dnl A placeholder for <stddef.h>, for platforms that have issues.
-# stddef_h.m4 serial 6
+# stddef_h.m4 serial 7
 dnl Copyright (C) 2009-2020 Free Software Foundation, Inc.
 dnl This file is free software; the Free Software Foundation
 dnl gives unlimited permission to copy and/or distribute it,
@@ -19,7 +19,7 @@ AC_DEFUN([gl_STDDEF_H],
        [AC_LANG_PROGRAM(
           [[#include <stddef.h>
             unsigned int s = sizeof (max_align_t);
-            #if defined __GNUC__ || defined __IBM__ALIGNOF__
+            #if defined __GNUC__ || defined __clang__ || defined __IBM__ALIGNOF__
             int check1[2 * (__alignof__ (double) <= __alignof__ (max_align_t)) - 1];
             int check2[2 * (__alignof__ (long double) <= __alignof__ (max_align_t)) - 1];
             #endif
diff --git a/m4/stdint.m4 b/m4/stdint.m4
index 29ad826d8ea..e0fa8a51fb3 100644
--- a/m4/stdint.m4
+++ b/m4/stdint.m4
@@ -1,4 +1,4 @@
-# stdint.m4 serial 54
+# stdint.m4 serial 55
 dnl Copyright (C) 2001-2020 Free Software Foundation, Inc.
 dnl This file is free software; the Free Software Foundation
 dnl gives unlimited permission to copy and/or distribute it,
@@ -152,7 +152,7 @@ uintmax_t j = UINTMAX_MAX;
 /* Check that SIZE_MAX has the correct type, if possible.  */
 #if 201112 <= __STDC_VERSION__
 int k = _Generic (SIZE_MAX, size_t: 0);
-#elif (2 <= __GNUC__ || defined __IBM__TYPEOF__ \
+#elif (2 <= __GNUC__ || 4 <= __clang_major__ || defined __IBM__TYPEOF__ \
        || (0x5110 <= __SUNPRO_C && !__STDC__))
 extern size_t k;
 extern __typeof__ (SIZE_MAX) k;