[gdb/guile] Use SCM_DEBUG_TYPING_STRICTNESS 0

I build gdb with libguile v2.0.9, and ran into:
...
In file included from /usr/include/guile/2.0/libguile.h:56,
                 from ../../gdb/guile/guile-internal.h:30,
                 from ../../gdb/guile/scm-arch.c:26:
/usr/include/guile/2.0/libguile/inline.h: In function 'int scm_is_pair(SCM)':
/usr/include/guile/2.0/libguile/tags.h:97:53: error: \
  operation on '*0' may be undefined [-Werror=sequence-point]
 #   define SCM_UNPACK(x) ((scm_t_bits) (0? (*(SCM*)0=(x)): x))
                                            ~~~~~~~~~^~~~~
...

Fix this by using SCM_DEBUG_TYPING_STRICTNESS 0.

We were already using this for c++20 due to a Werror=volatile in SCM_UNPACK
when using libguile v2.0.10.

Tested on x86_64-linux.

Approved-By: Tom Tromey <tom@tromey.com>

1 files changed, 18 insertions, 11 deletions

diff --git a/gdb/guile/guile-internal.h b/gdb/guile/guile-internal.h
index ef7a1d3..090badd 100644
--- a/gdb/guile/guile-internal.h
+++ b/gdb/guile/guile-internal.h
@@ -30,18 +30,25 @@
 #include "objfiles.h"
 #include "top.h"
 
-/* GCC introduced C++20 support in GCC 8, using -std=c++2a (the name of the
-   C++20 standard before publishing) and __cplusplus 201709L.  In GCC 10,
-   -std=c++20 was added, but __cplusplus stayed at 201709L, and was only
-   changed to the standard 202002L in GCC 11.  Consequently, some C++20
-   features and restrictions need to be tested against the non-standard
-   201709L, otherwise the build with GCC 10 and -std=c++20 will break.  */
-#if __cplusplus >= 201709L
-/* Work around Werror=volatile in SCM_UNPACK for
-   SCM_DEBUG_TYPING_STRICTNESS == 1.  Reported upstream:
-   https://debbugs.gnu.org/cgi/bugreport.cgi?bug=65333 .  */
+/* For libguile v2.0.9 and SCM_DEBUG_TYPING_STRICTNESS == 1, SCM_UNPACK(x) is
+   defined as:
+
+     ((scm_t_bits) (0? (*(SCM*)0=(x)): x))
+
+   and for v2.0.10 it's defined as:
+
+     ((scm_t_bits) (0? (*(volatile SCM *)0=(x)): x))
+
+   The volatile was added to avoid a clang warning.
+
+   The latter form causes a Werror=volatile with C++20.
+   This was reported upstream (
+   https://debbugs.gnu.org/cgi/bugreport.cgi?bug=65333 ).
+
+   The former form causes a Werror=sequence-point with gcc 7-14.
+
+   Work around these problem by using SCM_DEBUG_TYPING_STRICTNESS == 0.  */
 #define SCM_DEBUG_TYPING_STRICTNESS 0
-#endif
 #include "libguile.h"
 
 struct block;