[Bug build/32503] gdb: 'construct' has not been declared in 'std::allocator<unsigned char>

["cvs-commit at gcc dot gnu.org" <sourceware-bugzilla@sourceware.org>]

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

--- Comment #4 from Sourceware Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Tom de Vries <vries@sourceware.org>:

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=6f04937e1d7db190ca96ec08d8aeb25f0ec6e994

commit 6f04937e1d7db190ca96ec08d8aeb25f0ec6e994
Author: Tom de Vries <tdevries@suse.de>
Date:   Fri Jan 10 08:40:11 2025 +0100

    [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

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