Support C2X and C++17 static_assert

C2X and C++17 finally added support for a simple, single-argument
‘static_assert’ that implements what the Gnulib ‘verify’ macro was
doing back in 2005.  Implement static_assert on older platforms.
The only remaining advantage of ‘verify’ is a shorter name.
* doc/posix-headers/assert.texi (assert.h):
* doc/verify.texi (Compile-time Assertions):
Modernize for C2X and C++17.
* lib/verify.h (_GL_HAVE__STATIC_ASSERT1, _GL_HAVE_STATIC_ASSERT1):
New macros.
(_GL_HAVE__STATIC_ASSERT): Remove.
(_GL_HAVE__STATIC_ASSERT): Rely more heavily on __STDC_VERSION__.
(_GL_VERIFY_TRUE, _GL_VERIFY_TYPE): Remove 2nd arg, the diagnostic
string.  All callers changed.
(_GL_VERIFY): Require 3 or more args, of which only the first 2
are used.  All callers changed.
(_Static_assert): Allow either 1 or 2 args, and define if
!_GL_HAVE__STATIC_ASSERT1 instead of defining if
!_GL_HAVE__STATIC_ASSERT.
(static_assert): Define if !_GL_HAVE_STATIC_ASSERT1 instead
of defining if !_GL_HAVE_STATIC_ASSERT.
(verify_expr, verify): Don’t bother trying to copy the expression
into the diagnostic, since 1-argument static_assert doesn’t.
(verify): Prefer 1-argument _Static_assert if it works.
* m4/assert_h.m4 (gl_ASSERT_H): Check for 1-argument static_assert.

1 files changed, 13 insertions, 8 deletions

diff --git a/doc/posix-headers/assert.texi b/doc/posix-headers/assert.texi
index 785a07aa98..fa99d3b880 100644
--- a/doc/posix-headers/assert.texi
+++ b/doc/posix-headers/assert.texi
@@ -5,29 +5,34 @@ POSIX specification:@* @url{http://www.opengroup.org/onlinepubs/9699919799/based
 
 Gnulib module: assert-h
 
-See also the Gnulib module @code{assert}.
+See also the Gnulib modules @code{assert} and @code{verify}.
 
 Portability problems fixed by Gnulib:
 @itemize
 @item
-The C11 and C++11 @code{static_assert}, and the C11
-@code{_Static_assert}, are not supported by many platforms.
-For example, GCC versions before 4.6.0 do not support @code{_Static_assert},
-and G++ versions through at least 4.6.0 do not support @code{static_assert}.
+On older platforms @code{static_assert} and @code{_Static_assert} do
+not allow the second string-literal argument to be omitted.  For
+example, GCC versions before 9.1 do not support the single-argument
+@code{static_assert} that was standardized by C2X and C++17.
+@item
+Even-older platforms do not support @code{static_assert} or
+@code{_Static_assert} at all.  For example, GCC versions before 4.6 do
+not support @code{_Static_assert}, and G++ versions before 4.3 do not
+support @code{static_assert}, which was standardized by C11 and C++11.
 @end itemize
 
 Portability problems not fixed by Gnulib:
 @itemize
 @item
-C11 @code{_Static_assert} and C++11 @code{static_assert}
+C @code{_Static_assert} and C++ @code{static_assert}
 are keywords that can be used without including @code{<assert.h>}.
 The Gnulib substitutes are macros that require including @code{<assert.h>}.
 @item
-The C11 @code{static_assert} and @code{_Static_assert} can also
+The C @code{static_assert} and @code{_Static_assert} can also
 be used within a @code{struct} or @code{union} specifier, in place of
 an ordinary declaration of a member of the struct or union.  The
 Gnulib substitute can be used only as an ordinary declaration.
 @item
-In C99, @code{assert} can be applied to any scalar expression.
+In C99 and later, @code{assert} can be applied to any scalar expression.
 In C89, the argument to @code{assert} is of type @code{int}.
 @end itemize