GH-47207: [C++][C++23] Upgrade Arrow to be compatible with C++23

[BwL1289]

Rationale for this change

Arrow’s C++ codebase currently targets an older C++ standard. Moving to C++23 helps keep the project aligned with modern compiler defaults, enables new language features, and ensures continued compatibility with upcoming compiler versions.

During this upgrade, several forward declaration patterns used in the codebase became invalid under C++23 rules, requiring changes to maintain build correctness.

What changes are included in this PR?

This PR updates code to resolve forward declaration errors triggered by C++23 compliance checks. Specifically, it corrects invalid or outdated forward declarations that are rejected by C++23.

Applies the fixes in:

• cpp/src/arrow/acero/asof_join_node.cc
• cpp/src/arrow/filesystem/mockfs.cc
• cpp/src/arrow/flight/types.h

Are these changes tested?

Yes.

Are there any user-facing changes?

No.

• GitHub Issue: [C++] c++23 supported? #47207

[github-actions]

⚠️ GitHub issue #47207 has been automatically assigned in GitHub to PR creator.

[kou]

Could you rebase on main to use #47318 ?

[BwL1289]

@kou I tried rebasing just now but ran into merge conflicts I'm finding difficult to resolve. I think it's better if you do it.

[pitrou]

Perhaps we should start with C++20 as in #47330 ?

[BwL1289]

I disagree, but whatever @kou wants to do.

[kou]

Perhaps we should start with C++20 as in #47330 ?

The link should be #45445 ?

I disagree, but whatever @kou wants to do.

Based on the current C++23 CI job failure, we need to require C++20 to pass the failure:

https://github.com/apache/arrow/actions/runs/17113416247/job/48539553905#step:6:2347

FAILED: src/arrow/CMakeFiles/arrow_array.dir/array/array_nested.cc.o 
/usr/bin/ccache /usr/bin/c++ -DARROW_HAVE_RUNTIME_AVX2 -DARROW_HAVE_RUNTIME_AVX512 -DARROW_HAVE_RUNTIME_BMI2 -DARROW_HAVE_RUNTIME_SSE4_2 -DARROW_HAVE_SSE4_2 -I/build/cpp/src -I/arrow/cpp/src -I/arrow/cpp/src/generated -Wredundant-move -Wno-noexcept-type -Wno-self-move  -fdiagnostics-color=always  -Wall -Wno-conversion -Wno-sign-conversion -Wdate-time -Wimplicit-fallthrough -Wunused-result -fno-semantic-interposition -msse4.2  -g -Werror -O0 -ggdb -g1 -std=c++23 -fPIC -MD -MT src/arrow/CMakeFiles/arrow_array.dir/array/array_nested.cc.o -MF src/arrow/CMakeFiles/arrow_array.dir/array/array_nested.cc.o.d -o src/arrow/CMakeFiles/arrow_array.dir/array/array_nested.cc.o -c /arrow/cpp/src/arrow/array/array_nested.cc
/arrow/cpp/src/arrow/array/array_nested.cc: In member function 'const std::shared_ptr<arrow::Array>& arrow::StructArray::field(int) const':
/arrow/cpp/src/arrow/array/array_nested.cc:1080:51: error: 'std::shared_ptr<_Tp> std::atomic_load(const shared_ptr<_Tp>*) [with _Tp = arrow::Array]' is deprecated: use 'std::atomic<std::shared_ptr<T>>' instead [-Werror=deprecated-declarations]
 1080 |   std::shared_ptr<Array> result = std::atomic_load(&boxed_fields_[i]);
      |                                   ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~
In file included from /usr/include/c++/14/memory:81,
                 from /arrow/cpp/src/arrow/array/array_nested.h:24,
                 from /arrow/cpp/src/arrow/array/array_nested.cc:18:
/usr/include/c++/14/bits/shared_ptr_atomic.h:142:5: note: declared here
  142 |     atomic_load(const shared_ptr<_Tp>* __p)
      |     ^~~~~~~~~~~
/arrow/cpp/src/arrow/array/array_nested.cc:1089:22: error: 'void std::atomic_store(shared_ptr<_Tp>*, shared_ptr<_Tp>) [with _Tp = arrow::Array]' is deprecated: use 'std::atomic<std::shared_ptr<T>>' instead [-Werror=deprecated-declarations]
 1089 |     std::atomic_store(&boxed_fields_[i], std::move(result));
      |     ~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/shared_ptr_atomic.h:183:5: note: declared here
  183 |     atomic_store(shared_ptr<_Tp>* __p, shared_ptr<_Tp> __r)
      |     ^~~~~~~~~~~~
/arrow/cpp/src/arrow/array/array_nested.cc: In member function 'std::shared_ptr<arrow::Array> arrow::UnionArray::field(int) const':
/arrow/cpp/src/arrow/array/array_nested.cc:1360:51: error: 'std::shared_ptr<_Tp> std::atomic_load(const shared_ptr<_Tp>*) [with _Tp = arrow::Array]' is deprecated: use 'std::atomic<std::shared_ptr<T>>' instead [-Werror=deprecated-declarations]
 1360 |   std::shared_ptr<Array> result = std::atomic_load(&boxed_fields_[i]);
      |                                   ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/shared_ptr_atomic.h:142:5: note: declared here
  142 |     atomic_load(const shared_ptr<_Tp>* __p)
      |     ^~~~~~~~~~~
/arrow/cpp/src/arrow/array/array_nested.cc:1372:22: error: 'void std::atomic_store(shared_ptr<_Tp>*, shared_ptr<_Tp>) [with _Tp = arrow::Array]' is deprecated: use 'std::atomic<std::shared_ptr<T>>' instead [-Werror=deprecated-declarations]
 1372 |     std::atomic_store(&boxed_fields_[i], result);
      |     ~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/shared_ptr_atomic.h:183:5: note: declared here
  183 |     atomic_store(shared_ptr<_Tp>* __p, shared_ptr<_Tp> __r)
      |     ^~~~~~~~~~~~
cc1plus: all warnings being treated as errors

We can ignore the deprecated-declarations warnings but it may hide other deprecated warnings.

Anyway, this is stuck due to the rebase failure. We can't continue this.

[lidavidm]

Does it have to be a commit-by-commit rebase? Why not squash first, then rebase and force push?

[BwL1289]

@kou please see this PR: #47415. I will close this now.