Re: ../../gdbsupport/new-op.cc:137:1: error: ‘void operator delete [](void*, std::size_t)’ is a usual (non-placement) deallocation function in C++14 (or with -fsized-deallocation) [-Werror=c++14-compat]

[Simon Marchi <simon.marchi@polymtl.ca>]

On 2022-03-02 11:30, Andrew Burgess wrote:
> * 周春明(日月) via Gdb-patches <gdb-patches@sourceware.org> [2022-03-01 15:48:47 +0800]:
> 
>>
>> Hi GDB maintainers,
>>
>> I tried to build new GDB12, but encounter below error, anyone could tell me how to fix it? Thanks!
>>
>>  CXX new-op.o
>> ../../gdbsupport/new-op.cc:32:13: error: ‘void operator delete(void*, std::size_t)’ is a usual (non-placement) deallocation function in C++14 (or with -fsized-deallocation) [-Werror=c++14-compat]
>>  extern void operator delete (void *p, std::size_t) noexcept;
>>  ^
>> ../../gdbsupport/new-op.cc:33:13: error: ‘void operator delete [](void*, std::size_t)’ is a usual (non-placement) deallocation function in C++14 (or with -fsized-deallocation) [-Werror=c++14-compat]
>>  extern void operator delete[] (void *p, std::size_t) noexcept;
>>  ^
>> ../../gdbsupport/new-op.cc:119:1: error: ‘void operator delete(void*, std::size_t)’ is a usual (non-placement) deallocation function in C++14 (or with -fsized-deallocation) [-Werror=c++14-compat]
>>  operator delete (void *p, std::size_t) noexcept
>>  ^
>> ../../gdbsupport/new-op.cc:137:1: error: ‘void operator delete [](void*, std::size_t)’ is a usual (non-placement) deallocation function in C++14 (or with -fsized-deallocation) [-Werror=c++14-compat]
>>  operator delete[] (void *p, std::size_t) noexcept
> 
> I was able to reproduce this on Ubuntu 16.04.1 with their gcc 5.4.0.
> I was unable to easily rebuild GCC 5.4.0 on my current development
> machine to check if this is reproducible with upstream gcc, or is just
> something impacting Ubuntu.  However, you can configure like:
> 
>   ../src/configure ...configure-flags-here... CXXFLAGS="-Wno-error=c++14-compat"
> 
> to disable this warning/error which I believe should be fine.
> 
> For why this error is occurring, I'm honestly not 100% sure what the
> error is telling us.  I _think_ what it's saying is that the delete
> operator that we're declaring/defining conflicts with a "usual
> deallocation function", which is added in c++14 and means something
> specific.  I guess the idea is that maybe we're just randomly defining
> this version of delete for some reason, and then, if/when we move on
> to c++14 this function will get called unexpectedly by the language
> runtime in some situations.
> 
> As time moved on I think this warning was relaxed, possibly with this
> commit:
> 
>   https://gcc.gnu.org/legacy-ml/gcc-patches/2015-05/msg01883.html
> 
> All this makes me wonder if the usual deallocation functions are ever
> actually used, and indeed, I applied the patch below, and GDB still
> seems to build fine, so this might be an alternative approach.  Maybe
> we should commit this to master?
> 
> Thanks,
> Andrew
> 
> ---
> 
> diff --git a/gdbsupport/new-op.cc b/gdbsupport/new-op.cc
> index 1d066ba..4faa557 100644
> --- a/gdbsupport/new-op.cc
> +++ b/gdbsupport/new-op.cc
> @@ -27,11 +27,6 @@
>  #include "host-defs.h"
>  #include <new>
>  
> -/* These are declared in <new> starting C++14.  Add these here to enable
> -   compilation using C++11. */
> -extern void operator delete (void *p, std::size_t) noexcept;
> -extern void operator delete[] (void *p, std::size_t) noexcept;

The story of this is that ASan gave some alloc-dealloc mismatch warnings
if we didn't define these specific delete operators, so we defined them
in this commit:

  https://gitlab.com/gnutools/binutils-gdb/-/commit/5fff6115feae7aaa23c0ae8d144e1c8418ee2ee1

But for the variants of delete that are only introduced in C++14, it
meant adding functions without an equivalent declaration when building
in C++11, which produced some -Wmissing-declarations warnings.  So these
declarations were added in this commit:

  https://gitlab.com/gnutools/binutils-gdb/-/commit/b038b53f1ff4bf00ecdead1db23eddc4fd654305

The idea being that in C++11, these delete operators won't get used, but
there will be a declaration to avoid the warning, and in C++14 the
declarations will duplicate those found in headers, which is harmless.

And now here we are, we have a C++11 compiler that complains about
declaring these delete operators.

So we can't simply remove the declarations and / or definitions, we
would just re-introduce the problems fixed by these commits.

I think that a clean way to fix this would be to conditionally define
these delete operators based on the C++ version.  So we would remove the
declarations, as you do in your commit, but place the definitions under
an "#if __cplusplus >= xyz".

Simon