assert-h, verify: Fix conflict with standard C++ header files on macOS

[Bruno Haible]

After strengthening the C++ tests of <assert.h>, I get compilation errors
on macOS 12.5, such as:

In file included from ../../gltests/test-assert-h-c++.cc:26:
In file included from 
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/v1/iostream:37:
In file included from 
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/v1/ios:214:
In file included from 
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/v1/__locale:15:
In file included from 
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/v1/string:520:
In file included from 
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/v1/__functional_base:26:
In file included from 
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/v1/utility:221:
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/v1/__utility/pair.h:411:5:
 error: storage class specified for a member declaration
    static_assert(_Ip < 2, "Index out of bounds in 
std::tuple_element<std::pair<T1, T2>>");
    ^
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/v1/__config:896:29:
 note: expanded from macro 'static_assert'
# define static_assert(...) _Static_assert(__VA_ARGS__)
                            ^
../gllib/assert.h:256:7: note: expanded from macro '_Static_assert'
      _GL_STATIC_ASSERT (__VA_ARGS__, "static assertion failed", -)
      ^
../gllib/assert.h:244:5: note: expanded from macro '_GL_STATIC_ASSERT'
    extern int (*_GL_GENSYM (_gl_static_assert_function) (void))               \
    ^

The problem is that _GL_STATIC_ASSERT expands to something that starts with
'extern', and that is apparently forbidden in C++ in member-declaration
position.

This patch fixes it.


2023-02-04  Bruno Haible  <bruno@clisp.org>

        assert-h, verify: Fix conflict with standard C++ header files on macOS.
        * lib/verify.h (_Static_assert): Don't redefine with clang ≥ 3.8.0
        in C++ mode.
        * tests/test-assert-h-c++.cc: Also check against conflict with the
        standard C++ header files.
        * tests/test-assert-h-c++2.cc: Likewise.

diff --git a/lib/verify.h b/lib/verify.h
index b63cb26432..8f786af7f5 100644
--- a/lib/verify.h
+++ b/lib/verify.h
@@ -222,7 +222,21 @@ template <int w>
 
 /* _GL_STATIC_ASSERT_H is defined if this code is copied into assert.h.  */
 #ifdef _GL_STATIC_ASSERT_H
-# if !defined _GL_HAVE__STATIC_ASSERT1 && !defined _Static_assert
+/* Define _Static_assert if needed.  */
+/* With clang ≥ 3.8.0 in C++ mode, _Static_assert already works and accepts
+   1 or 2 arguments.  We better don't override it, because clang's standard
+   C++ library uses static_assert inside classes in several places, and our
+   replacement via _GL_VERIFY does not work in these contexts.  */
+# if (defined __cplusplus && defined __clang__ \
+      && (4 <= __clang_major__ + (8 <= __clang_minor__)))
+#  if 5 <= __clang_major__
+/* Avoid "warning: 'static_assert' with no message is a C++17 extension".  */
+#   pragma clang diagnostic ignored "-Wc++17-extensions"
+#  else
+/* Avoid "warning: static_assert with no message is a C++1z extension".  */
+#   pragma clang diagnostic ignored "-Wc++1z-extensions"
+#  endif
+# elif !defined _GL_HAVE__STATIC_ASSERT1 && !defined _Static_assert
 #  if !defined _MSC_VER || defined __clang__
 #   define _Static_assert(...) \
       _GL_VERIFY (__VA_ARGS__, "static assertion failed", -)
@@ -233,6 +247,7 @@ template <int w>
       _GL_VERIFY ((R), "static assertion failed", -)
 #  endif
 # endif
+/* Define static_assert if needed.  */
 # if (!defined static_assert \
       && __STDC_VERSION__ < 202311 \
       && (!defined __cplusplus \
diff --git a/tests/test-assert-h-c++.cc b/tests/test-assert-h-c++.cc
index 4fa23bfeec..6b76565633 100644
--- a/tests/test-assert-h-c++.cc
+++ b/tests/test-assert-h-c++.cc
@@ -21,6 +21,10 @@
 
 #include <assert.h>
 
+/* Check against conflicts between <assert.h> and the C++ header files.  */
+#include <stddef.h>
+#include <iostream>
+
 
 int
 main ()
diff --git a/tests/test-assert-h-c++2.cc b/tests/test-assert-h-c++2.cc
index 3c19de325c..ffd7c8cf92 100644
--- a/tests/test-assert-h-c++2.cc
+++ b/tests/test-assert-h-c++2.cc
@@ -18,3 +18,7 @@
 #include <config.h>
 
 #include <cassert>
+
+/* Check against conflicts between <cassert> and other C++ header files.  */
+#include <stddef.h>
+#include <iostream>