[gdb/build, c++20] Fix build with gcc 10

With gcc 10 and -std=c++20, we run into the same problem as reported in commit
6feae66da1d ("[gdb/build, c++20] Handle deprecated std::allocator::construct").

The problem was fixed using:
...
-template<typename T, typename A = std::allocator<T>>
+template<typename T,
+         typename A
+#if __cplusplus >= 202002L
+         = std::pmr::polymorphic_allocator<T>
+#else
+         = std::allocator<T>
+#endif
+        >
...
but that doesn't work for gcc 10, because it defines __cplusplus differently:
...
 $ echo | g++-10 -E -dD -x c++ - -std=c++20 2>&1 | grep __cplusplus
 #define __cplusplus 201709L
 $ echo | g++-11 -E -dD -x c++ - -std=c++20 2>&1 | grep __cplusplus
 #define __cplusplus 202002L
...

Fix this by using the library feature test macro
__cpp_lib_polymorphic_allocator [1], which is undefined for c++17 and defined
for c++20:
...
 $ echo | g++-10 -E -dD -x c++ - -include memory_resource -std=c++17 2>&1 \
   | grep __cpp_lib_polymorphic_allocator
 $ echo | g++-10 -E -dD -x c++ - -include memory_resource -std=c++20 2>&1 \
   | grep __cpp_lib_polymorphic_allocator
 #define __cpp_lib_polymorphic_allocator 201902L
 $
...

A similar problem exists for commit 3173529d7de ("[gdb/guile, c++20] Work
around Werror=volatile in libguile.h").  Fix this by testing for 201709L
instead.

Tested on x86_64-linux, by building gdb with
{gcc 10, clang 17.0.6} x {-std=c++17, -std=c++20}.

PR build/32503
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=32503

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

[1] https://en.cppreference.com/w/cpp/feature_test#cpp_lib_polymorphic_allocator

1 files changed, 7 insertions, 1 deletions

diff --git a/gdb/guile/guile-internal.h b/gdb/guile/guile-internal.h
index 8d83b8b..1057b83 100644
--- a/gdb/guile/guile-internal.h
+++ b/gdb/guile/guile-internal.h
@@ -30,7 +30,13 @@
 #include "objfiles.h"
 #include "top.h"
 
-#if __cplusplus >= 202002L
+/* 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 .  */