[gcc r15-4582] ginclude: stdalign.h should define __xxx_is_defined macros for C++

[Jonathan Wakely via Libstdc++-cvs]

https://gcc.gnu.org/g:5467f5b5ca6f2302573649b4a4f897793a9a6e7f

commit r15-4582-g5467f5b5ca6f2302573649b4a4f897793a9a6e7f
Author: Jonathan Wakely <jwak...@redhat.com>
Date:   Tue Oct 22 16:26:27 2024 +0100

    ginclude: stdalign.h should define __xxx_is_defined macros for C++
    
    The __alignas_is_defined macro has been required by C++ since C++11, and
    C++ Library DR 4036 clarified that __alignof_is_defined should be
    defined too. The whole <stdalign.h> header was deprecated for C++23 (see
    LWG 3827) and is likely to be removed for C++26 (see P3348), but we can
    deal with that later.
    
    The macros alignas and alignof should not be defined, as they're
    keywords in C++.
    
    gcc/ChangeLog:
    
            * ginclude/stdalign.h (__alignas_is_defined): Define for C++.
            (__alignof_is_defined): Likewise.
    
    libstdc++-v3/ChangeLog:
    
            * testsuite/18_support/headers/cstdalign/macros.cc: New test.

Diff:
---
 gcc/ginclude/stdalign.h                            |  6 ++++--
 .../18_support/headers/cstdalign/macros.cc         | 24 ++++++++++++++++++++++
 2 files changed, 28 insertions(+), 2 deletions(-)

diff --git a/gcc/ginclude/stdalign.h b/gcc/ginclude/stdalign.h
index 5f82f2d68f27..980b1bba4e2c 100644
--- a/gcc/ginclude/stdalign.h
+++ b/gcc/ginclude/stdalign.h
@@ -26,12 +26,14 @@ see the files COPYING3 and COPYING.RUNTIME respectively.  
If not, see
 #ifndef _STDALIGN_H
 #define _STDALIGN_H
 
-#if (!defined __cplusplus                                              \
-     && !(defined __STDC_VERSION__ && __STDC_VERSION__ > 201710L))
+#if !(defined __STDC_VERSION__ && __STDC_VERSION__ > 201710L)
 
+#ifndef __cplusplus
 #define alignas _Alignas
 #define alignof _Alignof
+#endif
 
+/* These are defined for C++, but deprecated in C++23.  */
 #define __alignas_is_defined 1
 #define __alignof_is_defined 1
 
diff --git a/libstdc++-v3/testsuite/18_support/headers/cstdalign/macros.cc 
b/libstdc++-v3/testsuite/18_support/headers/cstdalign/macros.cc
new file mode 100644
index 000000000000..c50c921cd59a
--- /dev/null
+++ b/libstdc++-v3/testsuite/18_support/headers/cstdalign/macros.cc
@@ -0,0 +1,24 @@
+// { dg-options "-D_GLIBCXX_USE_DEPRECATED=1 -Wno-deprecated" }
+// { dg-do preprocess { target c++11 } }
+
+#include <cstdalign>
+
+#ifndef __alignas_is_defined
+# error "The header <cstdalign> fails to define a macro named  
__alignas_is_defined"
+#elif __alignas_is_defined != 1
+# error "__alignas_is_defined is not defined to 1 in <cstdalign>"
+#endif
+
+#ifndef __alignof_is_defined
+# error "The header <cstdalign> fails to define a macro named 
__alignof_is_defined"
+#elif __alignof_is_defined != 1
+# error "__alignof_is_defined is not defined to 1 in <cstdalign>"
+#endif
+
+#ifdef alignas
+# error "The header <cstdalign> defines a macro named alignas"
+#endif
+
+#ifdef alignof
+# error "The header <cstdalign> defines a macro named alignof"
+#endif