[Bug gdb/29715] New: gdbsupport/default-init-alloc.h fails to compile as C++20, std::allocator::construct() removed

["andrew at ishiboo dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=29715

            Bug ID: 29715
           Summary: gdbsupport/default-init-alloc.h fails to compile as
                    C++20, std::allocator::construct() removed
           Product: gdb
           Version: HEAD
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: gdb
          Assignee: unassigned at sourceware dot org
          Reporter: andrew at ishiboo dot com
  Target Milestone: ---

When attempting to build GDB with `-std=c++20`, `default-init-alloc.h` fails to
compile:

../../gdbsupport/../gdbsupport/default-init-alloc.h: In instantiation of 'class
gdb::default_init_allocator<unsigned char, std::allocator<unsigned char> >':
/usr/include/c++/10/bits/stl_vector.h:87:21:   required from 'struct
std::_Vector_base<unsigned char, gdb::default_init_allocator<unsigned char,
std::allocator<unsigned char> > >'
/usr/include/c++/10/bits/stl_vector.h:389:11:   required from 'class
std::vector<unsigned char, gdb::default_init_allocator<unsigned char,
std::allocator<unsigned char> > >'
../../gdbsupport/common-utils.cc:439:25:   required from here
../../gdbsupport/../gdbsupport/default-init-alloc.h:52:12: error: 'construct'
has not been declared in 'std::allocator<unsigned char>'
   52 |   using A::construct;
      |            ^~~~~~~~~

This is because `std::allocator::construct()` was deprecated in C++17 and
removed in C++20. There does not appear to be a more fine-grain feature test
for this, so this class will need to be modified to use the existing code when
<= C++17 and a new implementation using `construct_at()` when >= C++20.

https://en.cppreference.com/w/cpp/memory/allocator/construct

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